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Q&A for DS4 timing thread

What sets top dead center reference? Is it the crank position sensor or is one optic sensor position different?

How is it timed mechanically so to speak?
 
Good question, as the strange crank\cam\IP timing procedure seems to cause much confusion - we get the strange version because the DB2 was designed for an all-gear timing train, where the IP shaft rotates CW - our timing chain results in CCW shaft rotation - yep, that's right: while there is one reversible camring for the DB2, there are two camrings for the DS4 IP, one for CW operation, one for CCW operation - DS4 timing is that critical

Now, as you know, the Top Dead Center point in a Diesel engine is same as a spark-infested engine, referenced to crankshaft position when piston #1 is at TDC

TDC in the DS4 injection pump is the #1 slot in the lo-res 8-slot cylinder timing track of the encoder disk - the #1 slot is wider than the other 7 to allow for TDCO compensation

- setting camshaft timing calls for the cam-sprocket mark to be at 6 o'clock in order to line up with the crank-sprocket mark at 12 o'clock
- setting IP timing calls for installing the IP drive-gear on the camshaft with it's mark at 6 o'clock, then rotate the crank 360* where the drive-gear mark is at 12 o'clock, install the IP driven-gear with it's mark at 6 o'clock to line up with the drive-gear mark at 12

Now, again rotate crankshaft 360* so the crank mark at 12 and cam mark at 6 are now re-aligned at TDC, and IP driven gear is now at 12 - crank and cam and IP OS encoder disk are now all aligned to crankshaft TDC position

PCM references all IP timing to crankshaft TDC via the Crank Position Sensor, down on the timing cover - IP TDC is referenced to the widest slot, which is #1 in the cylinder-timing track - as the timing chain "stretches" and the cam and IP become retarded, PCM compares OS encoder disk TDC to crank TDC and will compensate for the slowly-retarding encoder disk

The 8 slot lo-res track on the OS encoder disk is for cylinder timing, the 512 hi-res is for increased timing resolution during the 360* crank rotation as required for precise control of each injection event

Also, my bad: note the major correction in number of timing tracks on the OS encoder disk - there are only two, 512 slot hi-res and 8 slot lo-res - the 64 number I misteakenly referred to is the hi-res\lo-res ratio - if anyone copied the DS4 thread, run it thru again, as that faux pas is corrected.

For those whose truck crashed and burned because the PCM could not find that third track.............sue me!!!!!!!!!!
 
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OK. I never fully understood base timing vs TDCO. I kinda understand it as the tDCO being the range the IP can compensate the advance/ retard of the BAse timing ?? Or is that over simplified.
 
Top Dead Center Offset, a negative number, seems a strange concept, as why would anyone design a system where advanced IP timing would result in better performance, then arbitrarily let PCM take away, or retard, some of that advance?

(+) means advance, (-) means retard in timing numbers

It would seem to engender the theory that if some is good, more is better - "ain't no engineers gonna slow me down!"

Ain't 'zactly what happens, folks

When the fuel is turned up in the mechanical DB2, you don't just get more fuel at WOT, you also get more fuel at idle - also tends to make the throttle more sensitive - then, because the window for the injection event becomes narrower as rpm increases and the mechanical injection systems can only allow a limited range of advance, those systems are limited to the lower rpm ranges - and, as we all know, torque is low-rpm power, horsepower is hi-rpm power: more combustion events per unit of time = more BTU = more power - that's how you get 1500hp out of a 2Litre engine at 20,000rpm

Same happens with the electronic versions, butcept we got TDCO, which not only compensates to retain oem throttle sensitivity, but also compensates thruout the rpm range as the window for the injection event becomes increasingly narrower as rpm increases - it is used there to decrease the timing required for the pumping event, in order to get the relatively slow-moving fuel up to pressure and into the chamber in time for the combustion event - the DS4 is still a mechanical pump, like the DB2, but with advantage of electronic control - TDCO adds an offset delay to the 512 hi-res timing count such that the pumping plunger rollers are further up the ramps as pumping\injection-event timing begins, creating higher pressure at that point so that injection pressure is attained sooner, getting the fuel into the cylinder sooner so the injection event and combustion event can occur within the available window - no possible way to do any of that with the DB2

Thats the beauty of letting a computer handle the chores - PCM thinks at 12 million cycles per second at 650rpm or 6000rpm - rpm is based on 60 second time period, so divide that by 60 and the number of cylinders firing per crank rotation to find how many injections are required per second, then compare that to 12mhz - PCM is just a tad quicker, if my dyslexic calculations are correct

Note: there is a method of mechanically turning up the DS4, called "the Bump", "Optic Bump", and etc - it is accomplished by advancing the mechanical position of the Optic Sensor on the camring, resulting in similar performance to advancing timing and doing TDCO LEARN - PCM doesn't know about the advanced position, so fuel rate is increased, advance is increased, performance increases - butcept, because PCM doesn't know about this little trick, an unwanted result is the accompanying increase in throttle sensitivity, to which some drivers cannot become accustomed
 
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Refer to the DS4 thread up in sticky - the copy-link function is not working, here

Post #1 is theory and background, or the long slow pitch - post #2 and #3 is practical implementation, or the fast curve-ball
 
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I am interested in changing my TDCO to -1.5 .


That is the recommended advance(retard?) spot vs stock position for cold weather climates as far as I know.

What is required for me to do this? OBD1 seems easy for scanner. OBD2 doesn't. A scantool is required, which one? I don't trust any garage, and don't have time to raod trip the pig to Texas!
 
So, ASUUMING my TDCo Is stock value (-0.5) which is 3.5* base at idle, moving it 2 MM to the right (clockwise) will gain it 5* base, at 2.5* / mm movement. Resulting in desired 8.5 base timing at idle.

Then doing a KOKO procedure it will set TDCO at -1.5?
 
I am contemplating on doing the same as matt, however all i need is a gmtdscan, due to me being obd-1. How noticeable is the difference from stock to turned up a little?? I heard it can make cold weather starting better? The engine clatters more? So with a gmtdscan all i have to do is keep moving the ip and when the computer says -1.9 sumpin its all good?
 
As the OBD2 post states, Matt, if the ESO is exactly vertical, do a KOKO to make sure TDCO starts off at -0.5* - move the IP 2mm away from the turbo, lock it down, do KOKO and result will be within spec - just be sure to start off at exactly vertical - any problem, return it to vertical, do KOKO, you'll be back where you started

DS - you will need Gmtdscan Tech to do the OBD1 procedure - Gmtdscan is just a reader - I set mine at -1.94* as suggested for max power, but the loud rattle at 50* one cool autumn morn convinced me the sky was falling, whereupon I set it back to -1.5* - less agony, but still great performance increase - it's still at -1.5*, 4-5yrs later, and the sky never even budged - go figger...................

Remember: ECT above 180*F or neither procedure will be reliable
 
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Note: there is a method of mechanically turning up the DS4, called "the Bump", "Optic Bump", and etc - it is accomplished by advancing the mechanical position of the Optic Sensor on the camring, resulting in similar performance to advancing timing and doing TDCO LEARN - PCM doesn't know about the advanced position, so fuel rate is increased, advance is increased, performance increases - butcept, because PCM doesn't know about this little trick, an unwanted result is the accompanying increase in throttle sensitivity, to which some drivers cannot become accustomed
Click to expand...


and you are "anti-bump" correct?.....Just because of the throttle sensitivity? Have you bumped before? Is the performance advantage the same? (besides the touchy pedal)

And wasn't it you who came up with this whole TDCO modification in response to TDG's bump in the first place? :postwhore: - Back in 2004?

Did I get that right?
 
Gmtd what is the coldest temps get around your parts?? I regularly start in conditions around 0 degrees F...
 
It can drop into the 20's here in dead of Winter, mostly above 30's - generally 40's nite, warming up to 80's daytime, sometimes '90's - extremely harsh conditions, eh
 
Correct, Louis - I would choose TDCO over Bump, right, for the stock throttle sensitivity, but there are those who like bumping it - but, there are some ruff streets here that require Cruise to prevent jerking the cross'n'bearing joints outta the 'shafts, even with normal APP sensitivity, and a stiff-riding dooley doesn't help atall on those

Actually, I had been laboring thru the TDCO\timing thing with absolutely no help from S or GM, and some stuff seemed to fit, other puzzling stuff giving me a headache - so I had bought several DS4's (and DB2's) for up close and personal scrutiny, then when TDG described the mechanical functions of the Bump, the rest of the puzzle fell into place - plus, we get normal throttle, bumpers get squirrely - iirc, he later put a scanner on the Bump to find that his timing had effectively advanced +5*, just what we get with 2mm IP rotation

Also, as I recall, TDG is even now somewhere up there in the Great White North, bumping his heart out.............
 
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Not long after I bought my truck it set a code 88, still ran OK but annoyed me.
I got GMTDScan Tech and went to set timing. I noticed there were scribe marks on the IP and it was about 2mm to the drivers side. I lined them up and set the TDCOffset but could never achieve more than -.9* offset. I discovered that idle fuel rate was around 5mm3 so I debumped it slightly so now my idle fuel rate is about 6.5mm3. After that I was able to set TDCOffset to -1.5*
Maybe if I had turned the pump even more to the pass. side I could have achieved -1.5* TDCO without debumping it.

Leo
 
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