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FYI: TDCO - Timing the DS4 Injection Pump

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gmctd

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DS4 Injection Pump

The DS4-831-XXXX Diesel injection pump is a rotary Distributor-type Solenoid-activated 4 pumping-plunger 8 cylinder 0.31" plunger-diameter model - the four remaining digits denote Stanadyne final calibration specs - it is a constant-beginning variable-ending pump design with three modes controlled by the Fuel Solenoid valve: fill - pump - spill

Diesel fuel is the sole lubricant and coolant in the DS4

The IP body and fuel distributor is attached to and rotatable on the engine timing cover and can be positioned within a limited arc wrt 360* and crankshaft Top Dead Center to establish Base injection timing for the fuel distributor - PCM can rotate the internal camring\OS within the limits of the IP body to vary injection timing in a positive direction (advanced) from that Base - the internal fuel distributor rotor, pumping plunger assembly and encoder disk on the IP shaft rotate thru that 360* when the crankshaft spins - the four pumping plunger rollers operate in parallel on the 8 internal ramps on the camring, pumping the 1850psi hi-pressure required to "pop" the injectors during each injection event - the distributor rotor directs the hi-pressure fuel to each cylinder port around the fuel distributor, timed to crank rotation and the piston in each selected cylinder being at some point approaching Top Dead Center - PCM controls the FUEL SOLENOID via the Fuel Solenoid Driver to close the valve that enables the pumping event and opens it to end the injection event - it is a constant-beginning variable-ending pumping\injection event

The oem lift pump is a solid-state electronically-controlled solenoid-actuated shuttle-type fuel pump with two one-way valves - it is pump-thru when failed type - the electronic control regulates pressure according to flow, shutting down at spec'ed pressure when flow slows, as at idle, resuming pumping as flow increases, as at take-off and driving, but always maintaining spec'ed pressure - it is called a lift pump because it lifts the fuel from the source to the IP, also a xfer pump because it transfers fuel from the source to the IP - just something else to argue with the parts guy about..............

Filtered inlet fuel from the lift pump enters the IP at the top front thru the Engine Shut Off solenoid valve, thru a filter screen into the housing pressure regulator then into the internal transfer pump, which is powerful enuff to pull fuel from the tank if the oem lift pump should fail - housing pressure is regulated to ~25psi at idle by bypassing xfer pump output back into the input, increasing to upwards of 125psi as engine rpm increases in order to deliver the fuel within the limits set by the ever-narrowing injection window

Not all lift pumps are flow-thru when fail - those will leave you dead on the side of the road, stranded, wishing you'd listened to the sophisticated all-knowing Moderators on DTR, eh.........

Now, picture a camshaft, with the lobe ramps rising and falling around the base diameter - comparitively, the CAMRING is a ~2.750" diameter machined collar ~0.75" wide x ~0.425" thick, hardened\ground smooth on the outside diameter surface, with 8 hardened\ground ramps rising and falling around the inside diameter surface, much like an inverted camshaft - the valleys between the ramps, closest to the camring outside diameter, are the base diameter, from where pumping begins - the peaks of the ramps are closest to the physical center of the camring - as the IP shaft rotates, centrifugal force combined with fuel at internal IP housing pressure pushes the pumping plungers outward in their bores against the rollers, forcing the rollers against the camring ramps - the rollers ride inward and outward on the ramps as the IP shaft spins, driving the pumping plungers which intake fresh charge of fuel on the outward stroke, pumping the fuel charge on the inward stroke - the four plungers operate in parallel, pumping into a central chamber, thence into the distributor rotor and timed into each of the 8 distributor ports to the cylinders as the crankshaft rotates

As the pumping plunger roller rides down the camring ramp from outermost plunger position to innermost plunger position, PCM closes the Fuel Solenoid valve, allowing fuel pressure to build in the injector pipe connected to the injector until calibrated injector spring pressure is reached, which at that time passes fuel thru the injector nozzle into the cylinder - PCM opens the FSol upon reaching the required metered volume (idle to fwot), releasing excess fuel and pressure back into the IP reservoir\accumulator, ending the injection event - the rollers reach the ramp peaks and over and down the opposite side of the ramps, heading for the valleys - residual fuel pressure in the injector pipe is prevented from dropping to 0psi by an external one-way delivery valve at each port on the fuel distributor, thereby reducing pumping time during subsequent injection events for each cylinder - the DS4 (and it's ancestor, the DB2) builds injection pressure for each cylinder individually

The odd shapes of the ~0.250"o.d.\0.093"i.d. injector pipes are resulted from making each pipe the exact same length as the others, such that injection timing is identical for all cylinders

System injection pressure to each cylinder is set by the calibration of the injector springs - oem is ~1825psi min, ~2050-2275psi max - when that pressure is reached and the injector "pops", the pumping plunger rollers on the ramps stop building pressure and start producing volume at that pressure - therefore, injection pressure at any rpm idle~fwot is fixed, only the injected volume is variable - the FSol, under PCM management, meters injected volume

PCM manages idle on a per-cylinder basis, measuring angular velocity of the OS timing tracks in the IP to determine if crank is speeding up or slowing down, metering less or more fuel to compensate - this is how idle is maintained under varying loads, such as ac on, turning on the lights, shifting into any gear range from P\N, etc - there is no "fast-idle" like in a gasser (patooie!), which burns unnecessary fuel (gasoline, after all, is a fuel - just not our favorite fuel!) - therefore, Cylinder Imbalance codes occur only at idle - if Cyl Imbal codes keep occuring after being cleared, the injectors may not be quite what they seem, and will require removal for flow-bench testing - also, if injectors are upgraded, PCM has no way to compensate for higher-flow injectors, particularly if injector flowrates are not evenly balanced or matched - only the expensive ones are.............

DS4 Timing

BASE timing is the most retarded position of the camring\Optic Sensor wrt crank TDC, limited to and set by the physical position of the IP on the timing cover

ACTUAL timing is the measured position of the camring\OS while the engine is running, and is variable to match rpm and loading requirements

DESIRED timing is the amount of advance wrt Base that PCM has determined as required to match rpm and loading requirements

DESIRED and ACTUAL timing should always match within a degree or so

Top Dead Center Offset is a calculated value derived from the total advance available and limited by the physical position of the IP on the timing cover wrt crankshaft TDC - this factor allows a range of rpm and timing not easily manageable in the old mechanical injection systems, where cranking-in advance and turning-up the fuel affected operation across the rpm band, idle to fwot - total available range for TDCO is +2.50* to -2.50*, probably for IP testing\calibration purposes, not useable in-vehicle - PCM will DTC for TDCO over-range at some value greater than -2.02* up to -2.20*

PCM measures injection timing via the Optic Sensor which is attached to the camring, controlling any required injection advance with the Timing Stepper Motor, which moves the advance piston, rotating the camring\OS - the OS reads timing pulses thru the OS encoder disk attached to the IP driveshaft along with the pumping plunger assembly and fuel distributor rotor - this timing disk, oft mistakenly referred to as the tone-wheel, has two separate tracks of timing windows, or slots, circling the disk at separate diameters -

- outer-most diameter is the Hi-res track, with 512 slots in 360*, 64\cylinder, a binary number corresponding to available memory locations, used for precision timing determination wrt crank TDC
- inner-most diameter is the lo-res cylinder-timing track with 8 slots, one for each cylinder, which PCM compares to crank TDC to determine start of injection cycle for each cylinder - #1 slot is more than twice wider than the other 7

- 512 / 8cyl = 64 hi-res slots per cylinder
512 hi-res slots / 360deg = 1.422 slots\deg or 0.703deg\slot

- #1 lo-res TDC slot is 10 hi-res slots wide to allow for TDCO
10/1.422 = 7.03deg /2 for crank angle

- #2 thru #8 lo-res TDC slots are 4 hi-res slots wide
4/1.422 = 2.817deg /2 for crank angle

PCM has precise control of the injection event by the 22* rotation of the camring\OS around the encoder disk, which is fixed to the IP shaft which is geared to the camshaft which is timing-chained to the crankshaft - 22* internal advance results in 11* at the crank, as the camshaft-driven IP rotates at 1/2 crankshaft speed

So, let's begin solving the mysteries of Timing and TDCO - procedures were performed with a 2004\current TECH2:

NOTE: Engine Coolant Temperature is critical, here, as Timing and TDCO procedures do not function if ECT is below ~172*F, altho some pcware scantools will command the functions below that limit - TECH2 will not command the functions unless ECT is above ~172*F, which is where PCM is no longer calling for cold-start advance\fast-idle - in cooler climes, Intake Air Temperature also affects cold-start advance and fast-idle, so even if ECT is at 195*F operating temps, if IAT is below ~80*F due to the cold air, the timing procedures will be skewed - if temps are normal and idle rpm is ~695-750rpm, depending on manual or automatic trans, reliable DS4 timing procedures can be initiated, with repeatable results

+3.5* oem or +8.5* performance Base timing is resulted from the locked-down-by-three-12point-nuts physical position of the IP on the timing cover - PCM cannot change or alter BASE timing, but can DESIRE timing that is advanced wrt Base timing

When TIME SET is commanded PCM first sets idle rpm to 1000rpm to prevent the engine dying during timing procedures, disables DESIRED timing advance, using 0 advance to move the camring\OS to it's most retarded position, then wratchets the TSM to rotate the camring\OS between the physical limits set by the current location of the IP, comparing that range of movement to crank TDC - thus, PCM is determining actual IP position wrt TDC and measuring the available range of advance in degrees at that position - camring\OS rotation is limited to 22* total by the design of the advance piston, which gives 11* total advance at crankshaft - minimum timing position with 0* advance is actual BASE timing, or +3.5*BTDC crank, with total advance at that position of 14.5*BTDC - rotating the IP ~2mm in the advance direction adds +5* for +8.5* actual BASE timing, with max advance of +19.5*BTDC - PCM never commands, and the IP cannot respond to, negative advance, or retard, below actual BASE timing - 'nuther words, it cannot go below actual spec'ed BASE timing value .......unless the IP was mistakenly retarded to some degree less than oem-spec'ed minimum of +3.5*

Once PCM determines IP position in degrees wrt crank *BTDC during TIMESET, it wratchets the TSM to move the camring\OS into BASE timing position, again, determined by the physical position of the IP - that position is actual BASE timing with 0* desired advance - OBD1 can read this number at any position, OBD2 fudges the +3.5* number at any position in order to fake-out any enquiring minds, in accordance with the FSM blurb on timing alteration resulting in nil performance gains - when TIMESET is exited, PCM resumes normal injection timing operation - now, you're setup do TDCO LEARN

Next, when TDCO LEARN is commanded (and here's where OBD2 KOKO procedure comes from), PCM once again dials in 1000rpm, sets DESIRED (advance) to 0 to fully retard the camring\OS for starting position, wratchets the TSM thru min to max timing, prolly verifying what was measured in TIMESET, then moves the camring\OS back to actual BASE timing position with 0 advance - PCM then computes a TDCO value based on that measured range - now, altho PCM has disabled displayed DESIRED timing function and TSM response in TDCO LEARN, if engine rpm is increased PCM still calls for, or DESIRES, timing advance as required to match the increased rpm - this 'hidden' advance is where the range of TDCO values for each actual base timing setting comes from: PCM continually recalculates TDCO based on the varying advance required for the varying rpm as if that were the actual BASE timing - so, you get a range of values for each actual BASE timing position: at +8.5*BTDC you can get -1.5* to -1.94* to -2.20* TDCO while in TDCO LEARN merely by increasing engine rpm until you see the desired value, then exit TDCO LEARN to lock that value into PCM memory

NOTE: While total available range for TDCO is +2.50* to -2.50*, PCM will DTC between -2.02* to -2.20*, whereupon you must stop, attempt to clear the DTC, then start over from the beginning, as that particular TDCO DTC will seldom clear at this stage - it's a real catch-22, so Beware, Caution, Mise' en Garde and Achtung, here, kiddies

To clear your OBD1 DTC's, put a wire jumper into pins A/B in the DLC connector under the dash, turn on the key, push brake and APP to the floor and hold for 1 minute or so. Turn off key, release pedals, then turn on key again. Should have cleared all codes, you should get a flashing 12. (courtesy of JiFaire)

OBD2 requires a scantool to clear DTC's


OBD2 KOKO procedure takes advantage of the positional advance remeasuring (TIMESET) that TDCO LEARN goes thru prior to calculating the correct TDCO value - thus, you don't need TIMESET in OBD2

You also do not need TIMESET in OBD1 unless you move the IP - merely resetting TDCO within each possible range of values for each BASE timing setting can be accomplished with TDCO LEARN, as BASE timing has not been altered
Once again, always do a scan to determine the current parameters before attempting to reset those parameters - if you don't know where you're starting from, how're you gonna know where you're adjusting to??????????

And, that's how it really works, folks, word up.................

postscript:

NOTE: engine stumble\hesitation will result during the period when PCM wratchets OS timing 22* between minimum and maximum limits to determine IP position - this is normal

If the Engine Shut Off solenoid is 90* vertical to the horizontal plane across the valve-covers, this should be within a RCH of +3.5* Base timing

A - Stanadyne flowbench procedures for testing the DS4 are not comparably compatible with functional operation required by in-vehicle systems

B - If the EFI values are already correct when doing TIMESET and TDCO LEARN, no change in operation will be observed - if the IP is moved, several things change noticably, beginning with the 1000rpm idle which, hopefully, prevents engine dying during incorrectly performed procedures

C - Aftermarket tunes, incl chips\eproms, seldom respond like the oem version, particularly in timing functions - don't be surprised if your results are skewed or even non-functional with your a\m chip
- the Heath tunes have normal oem functionality with scantools (courtesy of turbovanman)

I compared procedures using a new TECH2 and GMTDScan Tech to find no difference, except that I prefer GMTDScan Tech as more user-friendly - same with Autoenginuity
__________________
 
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gmctd

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OBD1 timing procedures for oem factory tune* - results may vary with aftermarket tunes
- Bill Heath's tunes respond normally with scantools (courtesy of member Turbovanman)

All DTC's must be cleared prior to performing Timing and TDCO procedures

To clear your OBD1 DTC's, put a wire jumper into pins A/B in the DLC connector under the dash, turn on the key, push brake and APP to the floor and hold for 1 minute or so. Turn off key, release pedals, then turn on key again. Should have cleared all codes, you should get a flashing 12. (courtesy of member JiFaire)

I compared procedures using a new TECH2 and GMTDScan Tech to find no difference, except that I prefer GMTDScan Tech as more user-friendly

Standard procedure

When installing a replacement DS4**, or resetting timing, start with the Engine Shut Off solenoid in 90* vertical to horizontal plane across valve-covers - this should within a RCH of +3.5* Base timing

Start the engine, do an initial scan on engine parameters for 180*F ECT, idle speed 695~750rpm, ACTUAL IP timing +8.5*BTDC, DESIRED IP timing +8.5*BTDC, TDCO -0.5*

ECT must be at operating temperature ~180*F or timing procedures are invalid - I prefer 195*F

Scribe a line across the IP and timing cover flange

Scribe a second line across the timing cover flange, 2mm to the right of the first line - 1mm = 2.5*

Loosen the 3 12pt nuts on the IP flange, rotate the IP, aligning the IP flange line to the 2mm line for a +5* advance, secure the nuts

Start the engine, scan again verifying at least +180*F ECT, ACT\DES +8.5*, idle 695-750rpm

Command TIMESET, rpm will increase, engine will hesitate and stumble as PCM determines new IP position, wait ~5 seconds, exit TIMESET

Command TDCO LEARN, engine rpm will again increase, and hesitate\stumble, will self-exit when new value is learned

Scan again for +8.5*BTDC ACT\DES IP timing, TDCO at -1.5*

NOTE: engine stumble\hesitation is resulted during the period when PCM wratchets OS timing 22* between minimum and maximum limits to determine IP position

----------------------------------------------------------------------------------

If initial scanned TDCO value is elevated to some level greater than -0.5*, this may indicate BASE timing is not +3.5*BTDC

In this case Command TIMESET, exit; command TDCO LEARN, rescan TDCO - if same value, you will have to punt: either

- move IP to exacty vertical and do the standard setup

- or move IP 1mm to right, do TIMESET and TDCO LEARN, scan for new value
repeat at smaller increments until desired TDCO value is attained - PCM will DTC for TDCO over-range at some value greater than -2.02* for OBD1

*Again, aftermarket tunes seldom comply, but TDCO values at idle always represent Base timing, if you interpolate after the initial scan:

-0.5* = +3.5*

-1.5* = +8.5*

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** Alternately, having scribed the line across the IP\timing cover, remove the old IP, accurately measure the position of the scribed line to either end of the upper slot - transfer that dimension to the replacement IP, scribe a line, match that line to the timing cover line when installing the IP, and you should be within a RCH of the original timing, no timing procedures necessary - the IP machining tolerances are that close

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Last edited:

gmctd

Diesels, Anonymous
Messages
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Location
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OBD2 timing procedures for oem factory tune* - results may vary with aftermarket tunes
- Bill Heath's tunes respond normally to scantools

All DTC's must be cleared prior to performing Timing and TDCO procedures - OBD2 DTC's require a scantool

I compared procedures using a new TECH2 and Autoenginuity to find no difference, except that I prefer Autoenginuity as more user-friendly

Standard procedure

When installing a replacement DS4**, or resetting timing, start with the Engine Shut Off solenoid in 90* vertical to horizontal plane across valve-covers - this should be within a RCH of +3.5* Base timing

Start the engine, do an initial scan on engine parameters for 180*F ECT, idle speed 695~750rpm, ACTUAL IP timing +8.5*BTDC, DESIRED IP timing +8.5*BTDC, TDCO -0.5*

ECT must be at operating temperature ~180*F or timing procedures are invalid - I prefer 195*F

Do KOKO* to verify timing, rescan, shut the engine off

Scribe a line across the IP and timing cover flange

Scribe a second line across the timing cover flange, 2mm to the right of the first line - 1mm = 2.5*

Loosen the 3 12pt nuts on the IP flange, rotate the IP, aligning the IP flange line to the 2mm line for a +5* advance, secure the nuts



*Do the KOKO procedure (courtesy of member OregonHorseTug)
  1. Mash pedal to floor and hold it there
  2. Key ON 45 seconds
  3. Key OFF
  4. Release pedal
  5. Wait 30 seconds
  6. Turn key and start engine after glow cycle
As engine fires up, engine rpm will increase, engine will hesitate and stumble as PCM determines newly-adjusted IP position, then will smooth out to normal idle after new TDCO value is calculated

NOTE: engine stumble\hesitation is resulted during the period when PCM wratchets OS timing 22* between minimum and maximum limits to determine IP position

Scan again verifying at least +180*F ECT, ACT\DES +8.5*, idle 695-750rpm, new TDCO value should be -1.5* to match newly-adjusted +8.5*Base timing

----------------------------------------------------------------------------------

You still need a scantool to verify initial\current\final parameters in OBD2, but timing can be altered simply by using the KOKO procedure, rather than commanding the scantool procedures - TIMESET has been eliminated as an OBD2 procedure, tho you can still use the command - TECH2 has retained the TIMESET command, as the factory scantool is used for both OBD1 and OBD2

----------------------------------------------------------------------------------------

If initial scanned TDCO value is elevated to some level greater than -0.5*, this may indicate BASE timing is not +3.5*BTDC

In this case command TDCO LEARN or KOKO, then rescan TDCO - if same elevated value, or any value other than -0.5*, you will have to punt: either

- move IP to exacty vertical and do the standard setup

- or move IP 1mm to right, do TDCO LEARN or KOKO, scan for new value
repeat at smaller increments until desired TDCO value is attained - PCM will DTC for TDCO over-range at some value over -2.20* in OBD2

*Again, aftermarket tunes seldom comply, but TDCO values at idle always represent Base timing, if you interpolate after the initial scan:

-0.5* = +3.5*

-1.5* = +8.5*

----------------------------------------------------------------------------------------

** Alternately, having scribed the line across the IP\timing cover, remove the old IP, accurately measure the position of the scribed line to either end of the upper slot - transfer that dimension to the replacement IP, scribe a line, match that line to the timing cover line when installing the IP, and you should be within a RCH of the original timing, no timing procedures necessary - the IP machining tolerances are that close

----------------------------------------------------------------------------------------
 
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gmctd

Diesels, Anonymous
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FYI: feel free to ask questions in the Q&A thread down in the main forum - I'd like to not dilute this thread any more than necessary - significant topics will be appended, here, with reference (credit or blame) to the topic starter or query-meister - we can take this down to sub-atomic particle level, if required.................

http://thetruckstop.us/forum/showthread.php?t=12059

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Nerd-speak:

Electronic Fuel Injection (EFI) - fuel injection controlled by a computer, with sensors, modules, and solenoids

Common Rail Diesel (CRD) - EFI at super-high pressure, ~20,000psi, injectors are usually solenoids

Analog = infinitely variable signal, voltage can be negative or positive, frequency from DC to LIGHT

Digital = signal is either on or off - signal can be negative going, as from +5v to 0v, or can be positive going, as from 0v to +5v - usually ground-based, 0v, upper level is supply-rail dependent, frequency from DC to Light

Alternating Current (AC) - analog signal crossing a baseline, 0v in AC power circuits, with positive-going and negative-going component - in analog signal circuits, baseline can be shifted to some DC level by the circuitry that generates the signal (output), or reads the signal (input) - usually applied to 60hz ac power generation, positive-going component is equal to negative-going component across the 0v baseline - due to the physical properties of magnets and iron cores, and the electrical properties of copper conductors and coils, ac power voltage generation results in a sine wave, a complex waveform of sine and cosine calculations which cannot be accurately measured with analog meters or digital meters, so the displayed voltage is not even the Root Mean Squared voltage, which would allow calculation of the Peak (and Peak-to-Peak) voltage - high-end DVM's usually have a switchable RMS converter for accurate ac power-line voltage display - it would be switched off when measuring other analog waveforms

Direct Current - unchanging voltage level, like a battery

Thus, we have:

DC - steady voltage state, either polarity, not rising\falling, no frequency component

Analog - infinitely variable, either polarity, can be "alternating", can have frequency component

Digital - steady state, either on or off (ones and zeros), switchable between ones and zeros, can have frequency component

AC - analog, varying with positive and negative voltage content, sinusoidal waveform in power generation, has frequency component - often applied to varying analog signal waveforms without sinusoidal content

In elctronics, Discrete is transistors'n'diodes'n'capacitors'n'resistors'n'tr ansformers'n'fuses'n'stuff - can have discrete digital and discrete analog circuitry

Integrated Circuit (IC) is some of that discrete stuff scrunched together on a teeny silicon circuit board, usually in an *injection-molded epoxy package - can have IC digital and IC analog circuitry

Transistor-Transistor-Logic (TTL) is mostly direct-coupled bipolar transistors with a few-odd diodes thrown in as token representation

Bipolar transistors are NPN or PNP doped junctions, usually silicon but sometimes germanium flavored, are lo-impedance current devices, transfer function results in odd harmonics, single polarity drivers either pull up to the rail, or pull down to 0

Field Effect Transistors (FET) are N or P doped, are hi-impedance voltage devices, operate similar to electron tubes, transfer function results in even harmonics

Complementary Metal Oxide Silicon (CMOS) devices are hi-impedance FET circuits, direct-coupled circuitry is similar to Complementary Output Pair (COP) hi-fi power amps, where N device pulls signal up, P device pulls signal down (and verse vica), very sensitive to static discharge

Metal Oxide Silicon FET's (MOS-FET) are discrete and IC devices, usually for power circuitry, very sensitive to static discharge

Analog is infinitely variable

Analog control is infinitely variable

Digital is on or off

Digital control is on or off

Digital logic is multiple on\off circuits parallel-arranged in binary bits'n'bytes

One form of computer-synthesis is like analog logic

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Sensors

A magnetic pickup sensor with a coil around a magnet will give an analog signal something like a sine-wave when the tooth sweeps by, as in the speed sensor for the speedometer, and the two speed sensors in the 4L80E - those are 2-wire twisted-pair because of the low-voltage ac signal, and go thru an amplifer\squaring circuit in the PCM and VSS module for conversion from analog ac to digital wave suitable for PCM logic circuitry

A Hall-effect device (magnetic-type) sensor sets a digital off till the tooth sweeps by the pickup, then an abrupt rise to on then abruptly back off - it is a square pulse, as from the Crankshaft Position Sensor (and Cam Sensor in gassers (patooie!)) - this type sensor has 3-wires, one for +5v power, one for ground, and one for digital signal to PCM logic circuitry (PCM is +12v powered, but has internal +5v regulators for logic and sensor reference)

In an Optic Sensor a similar square on-off pulse occurs when the encoder-disk window sweeps between the Infra-Red diode transmitter and the IR-sensitive photo-transistor receiver - this type sensor will usually have 3-4-wires, +5v power for the diode, ground for the diode and transistor, digital signal output

A varying frequency creates a varying voltage which drives a meter to indicate varying crankshaft rpm - the analog tachometer in these trucks operates off one phase of the 3-phase winding in the alternator, which is a variable-frequency 3ph ac generator with solid-state rectifier diodes and 400hz regulator

The speedometer works similar, but with varying freqency from a sensor on the transmission or transfer case output shaft

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PCM signals

PCM has a 12.5mhz clock with 512k eprom, receives signals from several sensors, analog signals into op-amp circuitry, digital signals into digital circuitry, and sends digital control signals to several solenoids and modules - 6.5 PCM does not do any analog control

A digitally-synthesized signal is how a digital computer with ones'n'zeros can make a varying analog signal - similar to the on-screen display of curves on the old b-w, cga, vga monitors, which was a series of digital steps that formed coarse curves and circles - the more and finer the digital steps the smoother the curve

The 6.5 EFI PCM is too primitive for computer synthesis - the nearest thing to c\s in these trucks is the stereo\cd\dvd\mp3\serious radio - also the radar detector - then a cell phone - blackberry - pacemaker - XBox - Wii - Buck Rogers space toy.................

Here are the modules, sensors, solenoids, and signal-types used by the PCM - passive sensors are part of a 'nuther separate circuit, do not generate output without external influence - active sensors are solid-state devices requiring power and ground to generate a signal on a third wire:

IAT, ECT sensors (to PCM) are passive temperature-variable resistors in a +5v ref voltage-divider circuit in the PCM - an analog-to-digital converter in PCM changes the variable analog voltage to 16-bit binary digital 1-2-4-8 1-2-4-8 1-2-4-8 1-2-4-8

Boost and EGR sensors (to PCM) are active analog-voltage pressure transducers using +5v ref from PCM - the analog voltage varies as pressure changes - an analog-to-digital converter in PCM changes the variable analog voltage to 16-bit binary digital

Crank Position Sensor, Optic Sensor are active - pulses increase as rpm increases - to PCM:
5v_ ''''''''''''''''''''__ ''''''''''''''''''''''''''' __ '''''''''''''''''''''
0v_________| |___________| |________


NOTE: PCM can use clock ticks to turn on electronic switches for any length of time, as below:
5v_ ''''''''''''''''''' __ '''''''''''''''''''''''''''' ______ '''''''''''''''''''''
0v_________|||___________||||||||________


Wastegate Solenoid drive, pulse width modulated - WG duty cycle runs ~66%, decreasing to reduce Boost, increasing to raise Boost - also EGR solenoids - from PCM:
~10%duty cycle
12v_____''''''_ ''''''''''''''''''''''''''''''_''''''''''''''''''''''
0v________||___________||_________

~50% duty cycle
12v_____''''''_____ ''''''''''''''''''''_____ '''''''''''
0v________|||||||_______|||||||____

~100% duty cycle
12v_____''''''_______________________
0v________|||||||||||||||||||||||||||


Fuel Inject signal from PCM to FSD module - pulse width increases to increase fuel rate - (Note:any of these three signal examples may be inverted - I've slept some since '03\'04) :
5v_ ''''''''''''''''''''__ ''''''''''''''''''''''''''' __ '''''''''''''''''''''
0v_________| |___________| |________

Fuel Solenoid Driver module to FS coil - pulse width increases to increase fuel rate - from PCM:
Idle
12v___''''''''''''''''''''''''''__ ''''''''''''''''''''''''''' __ '''''''''''''''''''''
0v______ _______| |___________| |________

FWOT
12v___ '''''''''''''''''''''''''____________ '''''''' __________
0v_____ ________|..................|___|

:
Timing Stepper Motor drive, 2 phase - each pulse steps motor ~5* - TSM reverses direction when lower signal leads upper signal - from PCM:
5v_____''''''____'''''''''''____'''''''''''''____
0v_______|.....|___|.....|____|.....|_____
5v_______''''''''''''____'''''''''''''____'''''''' '''''____
0v___________|.....|____|.....|____|.....|_____


VSS (to PCM) after analog input from magnetic pickup has been squared - output frequency varies with vehicle speed - mag pickup is passive, becoming 'active' as "tone-wheel" tooth approaches sensor: a real conundrum :
5v_____ ''''_'''''''_''''''_''''''_''''''_'''''''_''''''_' ''''''_''''''_
0v_____ _| |_| |_| |_| |_| |_| |_| |_| |_| |


Speedo, Tach drive from VSS, PCM - active - frequency varies with vehicle speed and engine rpm - to speedo, tach:
5v_____ ''''_'''''''_''''''_''''''_''''''_'''''''_''''''_' ''''''_''''''_
0v_____ _| |_| |_| |_| |_| |_| |_| |_| |_| |


4L80E = PCM uses pwm at ~30hz to apply TCC clutch, PCM uses prm to vary line pressure
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On-going collation -

Cranking the engine CAUSES THE PUMPING PLUNGERS TO MOVE IN THEIR BORES IN THE ROTOR, PUSHING FUEL INTO THE CENTRAL ROTOR PASSAGE TOWARDS THE OPEN FS METERING VALVE WHICH IS SPILLING THE FUEL, PREVENTING ANY PRESSURE RISE UNTIL PCM FIRES THE FSD TO CLOSE THE VALVE)

And that fuel, if PMD were dead, would only go through the valve because FS METERING VALVE) would be forever open? = (CORRECT) How does the open valve prevent the fuel from being "distributed" to injectors. = ( FUEL SPILLED BY THE OPEN METERING VALVE CANNOT GO THRU THE SELECTED PORT TO THE INJECTOR - VALVE CLOSES, SEALING ROTOR PASSAGE FROM SPILL CHAMBER, FUEL IN ROTOR PASSAGE GOES TO SELECTED DISTRIBUTOR PASSAGE - PRESSURE RISES THRU SELECTED PORT TO INJECTOR)

the plunger is IN ROTOR PUMP BLOCK) and valve is in the (ROTOR - spill chamber is IN THE DISTRIBUTOR - END OF ROTOR AND METERING VALVE is in THE SPILL CHAMBER IN THE DISTRIBUTOR, MINUS FS SOLENOID, ARMATURE, AND ONE-WAY PORT VALVES -THAT SPRING-LOADED ROD OPENS THE VALVE WHEN FS DRIVE-CURRENT IS REMOVED - METERING VALVE OPENS THE ROTOR PASSAGE TO THE SPILL CHAMBER, FUEL PASSES BACK THRU TO THE PUMPING PLUNGERS - ADDITIONAL FUEL AT HOUSING PRESSURE ENTERS THE PLUNGERS THRU THE CHARGING ANNULUS PASSAGE IN THE DISTRIBUTOR HEAD)

How does fuel injection change at constant pulse width = (CLOSURE DURATION IS CONSTANT, hopefully - HOLDING\METERING PULSE WIDTH IS VARIABLE PER DEMAND). Does the valve have to fully seat to begin injection? = (MUST BE FULLY SEATED TO BEGIN PUMPING\PRESSURE RISE WHICH POPS INJECTOR - PUMPING\INJECTION EVENT BEGINS WHEN PCM RECEIVES FS CLOSURE SIGNAL, INDICATING VALVE IS FULLY SEATED - PCM OPENS VALVE TO END INJECTION, SPILLING REMAINDER OF FUEL MOVED AS ROLLERS HEAD UP REMAINDER OF RAMPS TO THE PEAKS - THEN OVER TO THE DOWN-RAMPS TOWARD THE VALLEYS TO BEGIN AGAIN)
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that is the fuel metering valve, of which one inherent function is spill

GMTDScan is a wunnerful pcware scantool, but that and a DVM does not tell the whole story - Closure Time pulse from the FSD module tells the PCM to start timing the metering pulse, which varies in width according to demand fuelrate - overall pw timing includes CT

LIMP MODE fixes idle rpm, locks TSM timing, limits max fuel rate by limiting the metering pulse width, locks out turbo management, locks out Cruise, limits the 4L80E to two lower ranges - this results when CPS or OS failure occurs, CT is too long or erratic\intermittent, etc - these limitations resut in long-start condition, requiring long cranking periods before engine will fire

Secondary "limp modes" occur when certain parameters are exceeded, such as high Boost, where PCM will set DTC, reducing and limiting fuel rate and limiting turbo control until restart - however, PCM is too primitive to reduce fuel rate when Boost is too low for demand conditions, thus the black smoke with reduced power when wg system fails - that scenario is not a limp mode

This is all digital, based on the 'clock' pulses from the OS on the 512-slot track on the encoder disk, and sync'ed by the clock pulses from the 8-slot cylinder track and the CPS - PCM has 64 counts per cylinder with which to work it's magic - the 64 counts are spread across TDC of each cylinder, some BTDC, some ATDC, allowing precise management of the injection\combustion event

Any perceived "voltage variation" is due to frequency and ELI - however, low battery voltage at the FSD, be it from grounding, resistive connection, or failing battery system, will increase CT due to slower FS reaction time, thus affecting injection event timing - DTC36: there's ole ELI, at it again................
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Remember, in digital signals, the signal is driven to the upper rail, in this case 5v, but is also driven to the lower rail, 0v, so DVM must integrate and average the upper voltage with 0v - you will get different readings in DCV and ACV ranges

An oscilloscope is the desirable tool for investigating computer-controlled systems, as most everything is digital: solenoids, valves, switches, position sensors, etc - temperature and pressure sensors are mostly analog, with variable resistances and variable voltages representing response to change - these can be read with a 'scope or DVM

With inductors in the system, inductive kick may be a usable part of the signal, but not even seen by a DVM - inductive kick can be as high as 2400v in a 12v system - the FS coil gives between 100-180v of inductive kick when power is applied, which is why the PNP driver transistors are hi-voltage rated, in addition to hi-current rated, designed to pass a lot of current thru the FS coil

The waveform at question * resembles the Fuel Injection Pulse Width at idle, and at question ** resembles the waveform at FWOT, but on a much higher voltage, as the drive voltage is just under +12v battery voltage due to solid state forward voltage drop of the bipolar drivers and the steering diode in the emitter circuits - on-time to off-time is determined by demand fuelrate - the max pulse width is never as wide as the duration for the rollers to ride comletely up the ramps
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PCM sends a digital Fuel Inject control pulse to the FSD, which is (a) a power driver that applies battery current to the FS solenoid coil via the collectors of the paralleled PNP driver transistors, and (b) a comparator circuit that monitors that current for the changes that result when the flux density changes as armature\plunger begins moving, continues forward motion, and stops as the valve seats - the resultant digital Closure-Time pulse is sent to PCM which starts counting clock ticks for injection pulse width timing - PCM has been counting ticks since sending the injection pulse to the FSD, so CT is included in Injection Pulse Width timing - FS is merely a coil, responding to applied power - power on is a 1, coil energized, power off is a 0, coil deenergized
 
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gmctd

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So, let's talk a little bit about Limp Mode - and just fyi, kiddies: Sildenafil citrate won't even help here...........

To wit: the much-dreaded oft-feared Limp Mode, or, more formally, Limp Home Mode, is resulted when PCM senses over-limit, under-limit, intermittent or erratic response from, or loss of, a sensor(s) - symptoms are CEL, very obvious loss of power, limited APP input, 1st and shift to 3rd gear only, no turbo, no TCC lockup, no cruise, no nuttin' - but the engine will get you to a desired destination, albeit, not in the style to which you have become accustomed

tbc..............
 
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