So wierd speedo/computer glitch

[Michael Kikendall]

Ok gents, had to remake an account, as I was deployed for the last year and couldn't get into my old account..
Got a new to me Tahoe to replace my old 95 C1500....
Truck is a 99 Tahoe (Last of the GMT400 body style woot woot)
Its the weird year without the VSS buffer module...

So speedo is reading EXTREMELY fast compared to GPS... IE- 60 on GPS (Verified) is 80 on the speedo.. Hooked up my new ELM327 and Dash command today to figure out WTF the computer is seeing.. Computer is reading 60 MPH, just like the GPS.. Why would the speedo read exceptionally high, when the VSS is reading the correct speed? Also- Tach is reading almost 300-400 RPM higher than the computer, and the engine temp gauge is almost 50 degrees cooler than the computer... Anyone have an idea?

Truck is a 1999 Tahoe, 2door, 4wd with the L31 5.7 Vortec engine, 4L60E transmission and the NP1 (I think... automatic 4wd, 2hi 4hi 4lo). Ive went over the electrical system, grounds were scrubbed, scratched, polished, and tightened as well.

Bad steppers maybe?

 

[RawbDidIt]

What did you check with the electrical? The only common denominator I see between the three issues you mentioned is the electrical, and if you've eliminated the computer as the source I'd take a look at the instrument cluster. If it were my truck, I'd start there.

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[Michael Kikendall]

As far as electrical, I checked all the connections, from the transfer case VSS sensors up to the ECU. Thought those might have been an issue since I had to repair the 4wd wiring harness that is on the same loom last week from one of the previous owners having something getting wrapped around the harness and ripping it up. Checked the wiring for obvious frays/shorts/damage under the dash, and to make sure all the grounds in and outside of the cab I could find were clean and tight.
Looks like Im gonna go junk hard hopping for a new cluster to test.. Those things are like gold around here though, seems like every vehicle I find has them jerked out when I find it.

 

[RawbDidIt]

I mean, one could buy one on amazon, test it and return it if it doesn't fix the problem...

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