Speedometer does not work?

1994 GMC SIERRA
260,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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[email protected]
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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.
Mar 12, 2023 at 7:15 PM
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[email protected]
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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".
Mar 13, 2023 at 4:20 AM
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STRAILER
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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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Mar 13, 2023 at 11:40 AM
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[email protected]
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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.
Mar 13, 2023 at 11:52 AM
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[email protected]
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It’s behind the glove box on my truck next to the brain box.
Mar 13, 2023 at 11:54 AM
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STRAILER
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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.
Mar 14, 2023 at 9:21 AM