Confused with Speed?

[NoDak]

Alright fellas, we finally have the '99 6.5L Suburban (see signature) running and driving. But the indicated speed (speedometer) is about 1/2 of what it should be.

We have the ecm and tranny out of a '96 3/4 ton 6.5L pickup. The transfer case came from a '98 3/4 ton 4L80E (not sure if Diesel or Gas?). Installed a new Gear Vendor.

We calibrated our VSSB according to tire size and gearing, Gear Vendors did same. If I swap between the two, I get an appropriate change in indicated speed for the .78 GV OD ratio. So, we came up with similar calibrations, in seperate states.

I'm just confused with reading 1/2 the actual vehicle speed? My "divide ratio" for 662 revs/mile (245-75-16), 3.73 axle ratio and I assume a 40 tooth sensor... gives 24.693. If I understand this correctly, I'd have to double the divide ratio to get the correct speed. I don't see anything larger than a 36.594 divide ratio, thus I can't fix my problem with the VSSB.

Oh yeah, I tried a new VSS just to be sure that wasn't messed up.

What might I be missing?

 

[bison]

Missing nuttin, my 89 had that problem since brandnew.No one was ever able to fix it.
New ECM,VS buffer and dash cluster,still registers 99% of the time 1/2 the speed and 1/2 the mls traveled.
Milage sucks big time,but towns are closer together:hihi:

 

[BudTX]

I have no idea how to address your problem.

On a lighter note, this fellow came up with an idea that might work:

 

[NoDak]

Now that did make me laugh!

But I'm still hoping someone has a clue!?

Mileage really sucks cause you can't get into road gear when the ecm thinks you are only going 40 mph when you are really going 80 mph!! Let's see, if I could get to 55 mph (110 mph actual) you think she'd shift into lock up overdrive? Yikes!! I'm not patient, but I'm also not crazy.

After thinking a bit about this, I'm not sure if I'd have to double the divide-by ratio or 1/2 it. Either way, I'd be off the scale.

Is it possible the transfer case is machined a bit off and the sensor is too far away and doesn't see all the pulses?

Anybody!? Help?

 

[bison]

Live with it,life is to short to worry about these trivial things

 

[great white]

Well, I'll talk through it a bit and see if it gives you any ideas;

It should be a 40 PPM ring.

That's what the vssb reads.

The signal then goes out to the PCM and the speedo.

If the speedometer and PCM are seeing the same signal (seems to be from your description of no lockup and wrong speedo), you can assume those are not the problem.

What is the shifting like? If the PCM thinks the speed is reading lower than actual, it's probably feeling like it's "hanging" in gears longer.

That would seem to leave the downstream items; VSSB, the sensor, the reluctor ring or the wiring as the problem.

How did you calibrate your VSSB? Jumper wires? DIP switches?

If you're using a chart off the 'net to come up with the jumper settings, be aware there's a couple ones out there that have the settings reversed which gives incorrect readings. I r4emeber my speedo being very low with one I tried.

If you have DIP switches installed, head out with a GPS and try different settings until you get it correct. That could take a loooong while though, there's lots of switches.

I'd do all this with the gear vendors in a 1:1 selection to eliminate that as a variable. Then frig with it after getting the speedo working in a stock configuration.

 

[deejaaa]

i have the same prob, only different. after i installed my GV, mine is off by 20 when i hit 70 mph. before it kicks in the speed is dead on. tried to figure mine out but i know how fast i'm going by using the gps. it's annoying.

 

[buddy]

Well, is the VSS located in the normal place on the tranny or does the gear vendors replace that transfer case? Any chance its not actually seeing all the teeth correctly now? Or does the GV have the teeth now and they didnt put the right amount of teeth? If the VSS sees your stock tranny and the GV multiples the shaft speed from there then I dont see how it could ever be right when the GV is operating.

Your rolling circumference is 95.2" so your VSSB should probably be set up for the 0.7734 jumper settings which would be
1 0 1 0 1 1 1. Jumper 1 of 7 are both in the middle, and if you have the VSSB with the jumper edge facing you, 1-7 is the 7 on the right and the inverse would be 0 1 0 1 0 0 0 would be starting in the middle and going left.

The whole thing strung together reading it left to right, the jumpers need to be
0 0 0 1 0 1 0 1 0 1 0 1 1 1. Which would be
off off off on off on off on off on off on on on

Is that what you set it for?

I attached a quick reference sheet I made for modding the VSSB, mostly compliments of tbichips.com/drac

 

[NoDak]

Update...

1. We calibrated the "main" vssb using the 6.2 6.5 Diesel Page Volume I or II. I'm fairly certain this info is good as we used this to calibrate my nephew's '99 1 ton dually and it was perfect the first time. Not to say we couldn't make a mistake, but not likely one due to bad info.

2. If our calibration is the problem, the GV calibration is similarly wrong. Again, the GV calibration yeilds a .78 offset compared to our calibration.

3. The GV is out of the picture as we can't get to 45 mph minimum.

4. I unplugged the 4wd shift control module (electric shift, not auto 4wd) and went for spin. No change!

5. I just figured out that I have a '97 thru '99 vintage 4wd shift control module vs. the '96 p/n?? Not sure if that matters yet?

Stay tuned!

 

[great white]

Youir sig says 5.7 "donor truck".

What electronic parts were swapped over from it?

IT: VSSB, cluster, wiring, etc......

 

[buddy]

Check the jumpers on the VSSB, or even take a pic and post it.

Can you post the equation that you are using for the calibration?

 

[NoDak]

Youir sig says 5.7 "donor truck".

What electronic parts were swapped over from it?

IT: VSSB, cluster, wiring, etc......

VSSB, ECM (PCM), cluster and tranny came from '96 3/4 ton 4wd ext cab short box 6.5L with 4L80E.

Engine harnes, dash harness, tranny/transfer harnesses were all purchased new. Dash harness was actuall for an auto 4wd and we had to convert to push button 4wd.

Four wheel drive shift control module and connectors were taken from used '98 1/2 ton. As mentioned above, I've now discovered a difference between the '96 module p/n vs. p/n for '97 to '99.

Transfer case was used and taken from 3/4 ton 6.5L with 4L80E.

I purchased 1999 vintage wire harnesses simply because the vehicle was a '99. Not sure this was a good idear?

 

[NoDak]

Check the jumpers on the VSSB, or even take a pic and post it.

Can you post the equation that you are using for the calibration?

I'll work on that. How the heck do I attach a pdf file or jpg file?

 

[buddy]

If you go to "Go Advanced" then there is a little paperclip button to manage attachments, which allows you to browse your computer for a file. If its something you had to pay for from the page, then perhaps just this part or just the equation. You can crop a jpg in Paint or with Microsoft image editor if you have recent Microsoft Office version.

 

[NoDak]

Both methods come up with the same result for the right hand seven spots. But what about the left hand seven spots when I use your method buddy? The method you posted appears to have 1/2 as many choices? I used 30.47 in diameter, 95.724 in circumference or 661.9 revs/mile for my 245-75-16 tires.

1. Per "The Diesel Page 6.5L Turbo Diesel (Volume I)"...

Divide Ratio = (661.9 revs/mile x 3.73 ratio x 40 teeth per rev)/(4,000 pulses per mile) = 24.693 therefore the closest choice is 24.689

which reads 0001010 1010111

2. Per buddy's above posted method

Input Ratio = (63360 in/mile x 3.73 ratio x 40 teeth/rev)/(95.724 inches x 128,000 units?) = .7715 therefore the closest choice is .77344

which reads ??????? 1010111

Either way, I'll have to go take a snap shot of what I have, eh?

 

[94K30]

silly question, but does that change when you are in low range?

 

[buddy]

The jumpers on the left are exactly opposite of the ones on the right. That is shown on the picture on first page. And you came up with same as my post above yours. So it really just matters if the jumpers were placed correctly, and its important that solder did not flow and connect a jumper that was supposed to be off.

 

[NoDak]

I reviewed post #8 again as I obviously missed something. I'm still confused how the info I have has one less than twice as many calibration options. But I do see alot of the options that are inverted/opposite as your info shows buddy.

As for the gear vendor? The GV is behind the transfer case and the stock vss. You simply remove the output cone and vss, install the GV, then stick your vss back in the GV looking at the stock pin wheel. You can only activate the overdrive in 2-wheel drive as you are only shifting the rear output shaft and not the front.

So, to make the GV read the correct speed, they use your stock vssb in direct drive. When the GV is shifted to overdrive (.78) then they switch over to the GV provided (parallel) vssb. Which, I'll assume is calibrated exactly as we are attempting to do with 3.73 x .78, everything else the same.

For our troubleshooting, I think we'll leave 'er in direct until we at least hit the side of the barn, eh? Again, if the GV was screwing us up, we'd be .78 off, not 50% off. Also, we swapped either vssb into the "stock" position to see what would happen. At a given rpm, our stock calibrated vssb gave us 10 mph and at the same engine speed we got about 8 mph with the GV vssb in the stock (direct drive) position.

Perhaps, now you may understand my reluctance to suspect the vssb. Oh yeah, I've been fooled before, so if fooled again, it would be too easy to fix (recalibrate two vssb's and rock and roll?).

Just for the record, when the 4wd controller is connected, we can get it to shift into either 4-low, 4-high or 2-high. Yeah, another obvious question, but that would reveal that we do have the same gear ratio in the both end right!? Also indicates the GV is in direct.

 

[buddy]

I am guessing that the parallel GV VSSB is calibrated for about 2.91 gear ratio to account for the 0.78:1 ratio following the 4L80Es 0.75:1 OD ratio. That simple enough.

Its 1-7 jumpers are likely 1 1 0 1 1 0 1

Although how do you run these in parallel? Is there a plug and play setup, where you plug in the inputs and there are outputs to both of these VSSB modules? If so, I would simply remove this splitter box from the circuit and your speedo may be restored. And you may need a new one of these. Something is picking off half of the pulses, unless some of that is simply used as an effective voltage and something is essentially splitting the voltage in two, which would need to be some resistance in series to the VSSB or to the speedo/PCM.

The sheet I made doesnt have all of the options on it, just ones that 6.5 guys would need. I cut off the top and bottom of the calibration values. All of the info can be found at tbichips.com/drac

It would seem as though it would have been simpler to have an extension harness that relocated the VSS to the output of the GV unit, with just one VSSB then.

 

[NoDak]

Well, I thought the GV switched from one vssb to the other. I also see we are off by a couple divide ratios, perhaps we used a different number for tire revs per mile way back when we started this deal. My vssb is set to 25.297 rather than 24.750 as I expected. Not really significant. But take a look at photos, the GV vssb is a whole different animal, perhaps it can be programmed.