Speedometer inaccurate

[Airplanedoc]

Let me start by saying I did a search for similar issues and found several but not a fix. I have a 1996 GMC Sierra 1500 5.7 2WD and my speedometer reads 15 to 20 miles higher than actual speed with a scanner I read live data and the computer is seeing the correct speed from the VSS. I had the instrument cluster checked and was told the speedometer is accurate, could the problem be the computer? The wiring schematic shows the computer sending the signal to the instrument cluster.

 

[Erik the Awful]

I had the instrument cluster checked and was told the speedometer is accurate

Somebody's blowing smoke. "Go away, I don't want to work on it."

 

[Airplanedoc]

They replaced the printed circuit board in the cluster just in case it was an intermittent problem and repaired my erratic oil pressure indication so I tend to believe them. I'm thinking about ordering an exchange vehicle control module.

 

[scott2093]

my speedometer reads 15 to 20 miles higher than actual speed with a scanner I read live data and the computer is seeing the correct speed from the VSS

This is also saying scanner is correct to gps or radar and dash isn't?

 

[Airplanedoc]

That’s correct is scanner and gps are within a mile of each other

 

[scott2093]

hmm..Not familiar with the 96.... If it were my 93, I'd say for sure it was the speedo...
Looking quick yours also has a vehicle speed sensor calibrator module...whatever that is....
is that what you were referring to?

Here's where I found the 96 manuals...

88-98 Service Manuals

Bought a 96 service manual on a cd, and uploaded it to my computer there are 2 books with a combined storage of 750MB. I took a quick gander through the pages on the forum, and haven't seen a service manual for the 96. how and where do I upload the book 1 and 2 pdf files to help out for 96'ers...

www.gmt400.com

 

[Airplanedoc]

Looking at the wiring schematic on Prodemand I don't see that my truck has a speed sensor calibrator module.

 

[scott2093]

Looking at the wiring schematic on Prodemand I don't see that my truck has a speed sensor calibrator module.

Not sure... Maybe that is the Vehicle control module you speak of? There's supposed to be a vcm section 8A 20-24 in the one of the 2 linked manuals but of course it's missing for some reason. But it seems like it's the one vcm you speak of. Really don't know without digging more...Maybe I missed it...

Looks like there is a black/white engine ground wire on it (circuit 451)...Guess you make sure it has a good ground to block...