Speedometer does not work?

FRAWG85

MEMBER
1994 GMC SIERRA
5.7L
V8
4WD
AUTOMATIC
260,000 MILES

I have changed ignition switch (I was told they can go bad and are on the same circuit), VSS in tail shaft of transmission, VSSB (3 different tries), and have 2 different instrument clusters, I’ve run continuity test on all the wires everything checks out good I even replaced the ECM/PCM. I am at a loss what am I missing? Any help would be greatly appreciated.

5 Replies

FRAWG85

MEMBER
4 POSTS

Issue isn’t how fast I’m going I could care less but it keeps the transmission in 2nd gear and won’t let it shift throws it in "limp mode".

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Monday, March 13th, 2023 AT 4:20 AM

KEN L

MASTER CERTIFIED MECHANIC
47,513 POSTS

There is a speedometer buffer module just behind the instrument cluster. Here is the location so you can change it out to fix the problem. Check out the images (below). Please let us know what happens.

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Monday, March 13th, 2023 AT 11:40 AM

FRAWG85

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4 POSTS

Yes, I’ve changed that out already with 3 different modules with the same numbers on it QDM followed by some numbers it’s about the size of a deck of cards a white plastic box.

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Monday, March 13th, 2023 AT 11:52 AM

FRAWG85

MEMBER
4 POSTS

It’s behind the glove box on my truck next to the brain box.

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Monday, March 13th, 2023 AT 11:54 AM

KEN L

MASTER CERTIFIED MECHANIC
47,513 POSTS

Okay, let's check the signal out of the tail housing, remove the connector and use a voltage meter set to AC millivolts. Then raise the rear to the truck and put it on jackstands to allow the rear wheel to spin, then have a helper put the truck in drive while the engine is idling. The voltmeter should show a reading here are two guides to help us with the test:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

and

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

Please go over these guides and report back.