99-02 Speedometer Calibration?

[Matthew Jeschke]

I am surfing the web and can find info to calibrate the speedometer for bigger (or smaller) tires on newer trucks but not the 2002 and older. Apparently the newer ones use a shop scan tool / programmer and can set it. However, I've no clue on the 2002 or older.

How do I calibrate my speedometer for my 2001 Tahoe?

This is a a bit of a follow up on my thread where I'm swapping in a HD cluster with transmission temperature gauge. Linked below:

https://www.tahoeyukonforum.com/threads/2001-transmission-temp-cluster-from-hd.119263/#post-1453647

I bought a rebuilt gauge with trans temp but they didn't bother to program the speedometer for my truck

 

[Matthew Jeschke]

I busted out my old tuning software. Haven't used this stuff in about 18 years. I had to update the software but amazingly it still works. I'm not for sure why the cluster is affecting the speed readout. I'm assuming the parameters are set in the ECU which I never changed?

 

[Matthew Jeschke]

This is beyond weird. This is a copy of the current program loaded to the PCM. Why would it have two conflicting calibrations? I've never tuned this truck. I am not sure about former owner but as far as I know I have stock tire size? 265/70/16

From figures above these two should be equal

2,537.9881 (trans revs/mile) * 40 (VSS pulses/rev) = 101,519.524 (VSS pulses/mile)

Column 2 says 97,277 (VSS pulses/mile)

I don't understand how that could be. Conflicting data came down from the computer?

Based off my tire size 265/70/16 - 659 tire revs/mile

Gears are 3.73 trans revs : 1 tire rev

I'm going to assume the computer uses column 2: 97,277 (VSS pulses/mile)

97,277 (VSS pulses/mile) * (1 rev transmission / 40 VSS pulses) * (1 tire rev / 3.73 rev transmission) = 651 tire revs / mile

651 computer / 659 actual = 0.987

My GPS Measured speed was 69 MPH
My speedometer registered speed was 60 MPH

That's 0.869

Is there anyway to check the Vehicle Speed Sensor? It obviously is reading, but do they get dirty or something? It's as though it's missing 5 of the 40 counts per revolution. That or there's issue in the air core motor control inside the cluster.

 

[drakon543]

alot of vehicles if you gps check the speedometer reading you will generally be off a mile or 2 atleast. then factor in different size tires can be a big change when you wouldn't think there should be such a big difference. 245 -> 265 equats to aprox 5mph off in my tahoe and a similar change in my civic size was about 6mph

 

[Matthew Jeschke]

Weird, when I replicate math the PCM is doing I get precisely the right speed. However, after changing the cluster I now get the wrong speed. Calibration is stored in the computer, not in the cluster.

Sent from my SM-A505U using Tapatalk

 

[drakon543]

well your looking at perfect world exact math vs whatever the vss is sending vs whats actually making it to the cluster too. which is where the typical couple mph off as a typical comes into play also. you could technically have a slight issue with the vss, the wires(interference also), a slight bad connection at the computer throwing the reading off, or the cluster rebuild was done poorly. im unsure of any vss test that would work correctly. follow the wires for the vss make sure its not damaged or laying on anything that could add any interference. check the plug where it connects to the computer maybe some contact cleaner and a follow up application of dialectic grease to the plug. continue from there and check the wires to the cluster. open the cluster up and take a look and see if anything looks wrong. maybe something got installed a little off and the cluster won't know that so it could be throwing what its actually showing. if you have the original cluster put that back in and see if the problem is corrected. besides the extra temp gauge is everything else exactly the same or did you get a cluster with a different style speedo? i dont know anything about tuning when ive ever done speedo correction it was on a handheld with the easy mode correction. just trying to give you some options of where to start looking without just throwing parts at it. also your op asked about different tire sizes vs the speedo. regardless to the math your getting if you changed the tire size that needs to be corrected in the tune im not sure where.

 

[Matthew Jeschke]

Thanks Despite burning hundreds of dollars in what I thought would be an affordable temp gauge, is a fun opportunity to refresh with tuning haha I pulled out my old dusty HP Tuner module. Plugged it in, it now has a KILLER awesome scan / logging tool.

- My next step is to read exactly what speed the computer thinks I'm going and compare that to GPS.

I can read directly what the computer thinks the speed is (essentially it's VSS calculation's result for speed). I'll compare that to the cluster. If it doesn't match then it's unfortunately the cluster.

- Speedometer rebuild guy says I can send it back for a replacement however

I'm a bit leery about sending back to the shop that rebuilt it, although they said they'd fix or exchange for no charge. Reason is, I had to re-rebuild most of the cluster when I got it. The cluster they sent me was all cracked up. Granted cracks would have been hidden in trim, but doesn't really meet my quality standards. Especially when I paid $200 for a programmed cluster you can buy all day long for $100 un-programmed. Additionally, two air core motors were bad

I luckily had the $80 junkyard cluster I destroyed trying to transfer memory chips / odometer and parts from it got the $200 one working.

This morning I swamped the air core motor on the speedometer and got the speedometer's accuracy to go from 85% to 90% of GPS monitored. I have one more air core motor not sure if that will improve it anymore. I additionally, have anther gauge cluster motherboard, only I destroyed the daughter board trying to swap chips. I think the board still works only the odometer is toast on it as the chip I broke on daughter board controls the odometer. I may try that and see what speed it comes in at as well.

 

[Matthew Jeschke]

I tested output from the ECU computer. It was spot on with the GPS.

After I tested the replacement air core motor a bit more it was also 85% of actual speed same as previous air core motor it had.

Sheet. I think this cluster is defective

 

[Matthew Jeschke]

If this doesn't get weirder. I have a second cluster that only the gauges operate on. I put it in the truck and test drove it. Same result. It's also 85% of actual speed.

I wonder if there's a difference between the factory Tahoe w/o temp gauge and the 2500 HD with Temp gauge? Something that would case the speedometers to register differently.

Wiring diagram:

 

[Matthew Jeschke]

Did some more diagnosing:

- PCM to the cluster connection 0.1 ohms
- Certain VSS to PCM is good as that reports accurate speed.
- Oscilloscope to measure waveform on VSS of cluster compared to GPS measured velocity. 56hrz 5V square wave at 50MPH and 28hrz at 25 MPH
- Tested another cluster with same results

Anybody know what the ideal signal from PCM to cluster / VSS pin looks like? I want to validate if my PCM is generating the right signal.

 

[Matthew Jeschke]

I found another / easier place to probe for PCM to Speedometer / VSS line. The line comes in and is spliced through the Body Junction Block. From there it is distributed to the various computers in the vehicle.

 

[Matthew Jeschke]

I'm dreading this... But I found a short on pins A3 through A8 on the gauge connector. It goes away when the BCM is unplugged. I don't know if that's normal? Seems a bit odd.

 

[Matthew Jeschke]

Found my issue, now to fix it :| The short is behind and above the steering column. There's a wire bundle there. I had fished a line through that area to run my light bar. When I wiggle those wires the short goes away. I was able to drive it while the short was gone and it read speed correctly. However, short seems to have come back *sigh* will have to take the dash apart to get at that.

 

[wjburken]

Found my issue, now to fix it :| The short is behind and above the steering column. There's a wire bundle there. I had fished a line through that area to run my light bar. When I wiggle those wires the short goes away. I was able to drive it while the short was gone and it read speed correctly. However, short seems to have come back *sigh* will have to take the dash apart to get at that.

Glad you found the cause. At least now you know your thinking was right and you weren’t out in left field.

 

[Matthew Jeschke]

Thanks... I'm saving my notes here mainly for me. But maybe someday somebody else will have a shorted out speedo too haha.

I just spent about 3 hours digging through the dash. Yuck... I pulled the vent buttons out on left of steering wheel, just as I did that the short went away. I spent hours wiggling harnesses and couldn't get it to come back. My latest theory is that I had unplugged the onstar connector C2. Additionally, I have the cable that feeds data to the rear view mirror from OnStar unplugged too. I think it was almost acting as an antenna, intermittent connection to the cable feeding the instrument cluster somehow. Those lines run very close to each other. I plugged back in the onstar module and hopefully the issue doesn't return. I cannot for the life of me think what else it would be, minus completely tear down of the dash and wiring harness *sigh*

 

[HiHoeSilver]

Matthew, your determination and resourcefulness with this truck is LEGENDARY! You have documented some really amazing info. Hats off, buddy.

 

[Matthew Jeschke]

Haha thanks, I am worried this one maybe beyond my abilities. I feel like I'm waging a one man war (although forum does help so maybe not quite so). I currently have my dash and it's harness pulled apart. The underlying issue continues to elude me.

I found some peculiar things checking connections just now. The first is documented in photo below. There's a short between low reference (5v reference?) from BCM and the VSS line from PCM going into the BCM. I disconnected the VSS line into the PCM but the issue with speedometer persists

Another odd thing I observed before disconnecting VSS from the BCM. With both PCM and BCM connected I got 2 volts on VSS to ground.

I disconnected the PCM. That turned into a full 5 volts.

I immediately knew it was a short to one of the computers. I unplugged my
- Radio
- Cruise Control
- Lastly the BCM

Somehow the 5V power line for the sensors / computers in the truck is getting shorted to the VSS line. I believe this is taking place in the dash somewhere.

I cannot seem to find electrical diagrams on the 5v computer power supply system in the truck. I have all data diy for the truck, however, more than half my time trying to fix this has been searching it. It's almost impossible to find or search on that thing.

Additionally, not sure if this is normal. I observed 1.6k ohms VSS (for cluster) to ground. This was on the output of the PCM. Not sure what's normal to see there. I just noted it.

 

[Matthew Jeschke]

Okay so this gets even more intense. I found issues on the under hood part of the C100 Connector.

- Variable Effort Sterring Control Circuit:

I found B to L had 10 Ohm load. This is Variable Effort Steering Control circuit. After researching more 10-15 Ohms indicates healthy EVO for variable steering assist. http://my.cardone.com/techdocs/PT 20-0024.pdf

- Oil Pressure Sensor Signal:

This is shorted to A/T Shift Lock Control Switch, pint R to J on Mail / engine compartment side of Connector 100. Voltage and Fuel Level gauges twitch when oil pressure sensor is connected. Once disconnected they stop twitching.

I need to find more information on the Steering column lock circuit. Oil pressure circuit is super simple. It goes through the fire wall and into the cluster. The sensor essentially goes straight to the cluster.

 

[Matthew Jeschke]

I bet I have over 100 hours into this and am about ready to scream. I've tore into a huge part of the harness under the hood as well as under the dash. A massive part of the truck is taken apart. I bought a function generator to try and emulate VSS directly into the cluster but I have no idea what VSS is supposed to look like going to the cluster. I tried to wire a scope up to the truck and drive it but there was no way to record the scope output. I had to try and hold a cell phone in front of the scope to record what the scope was saying while yelling out the speed the truck is going so I could figure it out. Only to find out I can barely see what the cell phone recorded.

I googled for hours and read as much of all data diy / mechanics manual as I could.

I just spent the last several hours swamping air core / gauge motors around wondering if one of the motors is bad. I also used an oscilloscope, function generator, and DC power supply to compare the trip odometer reading to the speedometer with a stopwatch. This sucks. I have no clue, but my truck is in like a million parts and I still cannot figure out why the speedometer does not match a GPS and the PCM.

The company I bought the cluster from took my known good / original cluster. So I cannot easily swap it in to see if the problem is the cluster or the truck. Also I destroyed it and another cluster trying to swap odometer readings. *sigh* I'm on the verge of pushing the truck out in the street and lighting it on fire.

 

[Matthew Jeschke]

I figured it out and feel really stupid. Was up till midnight last night in frustration. I went and looked my donor cluster and my rebuilt cluster. One is 120MPH the other 100MPH speedo. I had switched the faces because the company that sold me the rebuilt one sent it with a cracked face. Turns out it was the scale on the speedometer itself that was off. I have to fix the crack in the rebuilt cluster and put the original face back with that circuit board.