1999 Chevy Express jumpy speedometer

Tiny
PEETEEROCK
  • MEMBER
  • 1999 CHEVROLET EXPRESS
  • V8
  • 2WD
  • AUTOMATIC
  • 60,000 MILES
In low gears, my speedometer will show much higher than actual speed. At highway speeds it works well. The transmission shifts too early because it thinks that it is going at a higher speed than it actually is.

My mechanic replaced the speed sensor. Same problem, so he put the old speed sensor back in.

After several weeks of this problem, it started working properly again, but only for about a week. Now, it is back to the same problem.

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Friday, April 11th, 2008 AT 5:37 PM

3 Replies

Tiny
JACK42
  • MEMBER
The signal goes from the VSS to the VCM (vehicle control module), then out to the EBCM (electronic brake control module) Cruise control module and dash. You might need to find a shop that does electrical, this may take some pin-to -pin testing to determine where the signal is getting messed up. Does anything else strange happen when the speedo messes up?
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Monday, April 14th, 2008 AT 3:34 PM
Tiny
PEETEEROCK
  • MEMBER
Hi Jack- thanks for your reply.

The transmission shifts at the wrong time when the speedometer is showing higher than the actual speed. Also, after about five minutes, the anti-lock brake warning light will come on. Otherwise, everything runs as it should.

The shop I took it to is an electrical shop, supposedly. They said because it is not the speed sensor, it is probably the instrument cluster. But, they didn't even test drive it after they replaced the speed sensor- they just said "here you go, you're done". I don't want to sink a bunch of $$ into an instrument cluster only to find out that is not the problem.
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Monday, April 14th, 2008 AT 3:53 PM
Tiny
JACK42
  • MEMBER
If you cant find a shop that is willing to do the troubleshooting, you may just wind up at the dealer. SInce the ABS light is coming on it could be a problem with the EBCM
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Tuesday, April 15th, 2008 AT 4:05 PM

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