99-02 Speedometer Calibration?

[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.