1999 C5 Corvette no speedometer

[Matthew Jeschke]

About to cry. I have my hot rod stored for years. The bleeping battery boiled and leaked on wiring. I've got most of it all fixed up. However, the speedometer will not register. It passes self check, and scanner shows a good VSS signal to PCM. However, the signal is not making it from the PCM to the instrument cluster.

I'm absolutely baffled by wiring diagrams though. They make NO sense. I have the GM factory service manual. I'd presume the vehicle speed signal (from PCM to cruise, radio, speedometer, etc) would be somewhere in the vehicle to probe without taking the dash apart. it's obviously used by the cruise control, yet service manual shows the signal nowhere on cruise control diagram.

I did find a connector under the hood that's supposed to have the signal run through it before entering the cabin... However, the connector pins don't align up in the actual diagram, so no way for signal to be the pin diagram shows.

I really need to somehow energize the wire and probe it for a break. I've never done that before. Any tips???

I thought I'd buy All Data subscription for the vehicle... thinking there must be an update to the electrical diagrams, only to find same diagrams ug.

Here's a video of my current trouble...

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[Matthew Jeschke]

It's official I'm stupid. Those are TWO separate connectors. I thought they were two sides of same connector.

Still any tips to diagnose Vehicle Speed Signal are greatly appreciated.

 

[exp500]

Oscilloscope is easiest, frequency meter next possibility. Good Luck! I enjoy following your projects.

 

[Matthew Jeschke]

Thanks @exp500

I have a miniature scope. Would be perfect If I could find out how to tap the line in the footwell.

 

[exp500]

I have made many "universal" connectors from Upullit cutoffs for diagnostic purposes. The most difficult one was a Cruise control box cable for 89-99. I had a bad run on boxes for awhile.

 

[Matthew Jeschke]

Great tip! Wouldn't be a bad idea to add a diagnostic connector in the cabin when I find the wire under the dash in. I'm hoping it runs through the electrical center... Seems that I diagnose this further, the PCM runs serial data (with vehicle speed in the data stream) and a standard vehicle speed signal *sigh* PITA. The cruise must use the serial data, along with Speed Volume Controller (radio) etc. Only the cluster appears to use the vehicle speed signal.

Years ago I tore apart, I mean tore apart, my Tahoe Vehicle Speed Signal. All the harnesses it ran through I opened up. Traced the signal, scoped the wires, drove with an oscope instead of a cluster, etc. etc. etc. I'm very surprised to see this 1999 Corvette uses a different setup for vehicle speed! I don't get it... it's like it's much newer, fancier, but not necessarily better in my opinion. All the new vehicles must use that serial data instead of the old vehicle speed signal pulses.

I had been posting on a corvette forum but I think too many corvette owners only know how to wash them haha. They tried helping but nowhere near as helpful as on here.

 

[Kenneth L Nielsen]

There are a few good smart guys on CF. Did you post in tech - C5 Diag will probably chime in . If it were me I would scan it just like you to confirm the signal is being sent, now to figure out if it is wiring or the cluster. Or a corroded connector. Did you go through all of the DIC codes carefully? Search the CF archives?

 

[Matthew Jeschke]

I think at this point my case would be greatly improved if there was a VSS signal line in the passenger footwell I could probe. Then I'd just put it all back together and probe that line while driving with my oscope. Otherwise, I hate to take the dash apart :/

I know the gromet is there on the passenger side where the wire goes through... I just don't know if it breaks out to the BCM. wiring diagrams I see say no but hard to believe as I think it locks the doors when I hit a certain speed.

 

[Matthew Jeschke]

Posted on Corvette Forum. This has been one of an issue. I think I found a fault. None of the ground pins are connected to chassis ground for the PCM.

I went back into the Factory Service Manual. It says the PCM is grounded through S120 then to G120. I can only find one additional reference to S120 as:

Engine harness, approximately 15.0 cm (5.9 in) from the main harness in the branch leading to the Powertrain Control Module (PCM) connectors (CKT451)

I can find no further reference, diagrams, NADA for those two points, S120 & G120. I thought I'd found and tested all the grounds... I have no clue where these are located.

 

[Matthew Jeschke]

I finally removed the instrument cluster. It was much easier than I thought it would be. Getting ready to probe the signals on the instrument cluster connector.

I think maybe worth noting that the cruise control doesn't work either. I think cruise control is done by the throttle controller w/ VSS Signal as well? I'd assume this VSS setup is somewhat common on newer vehicles? It's MUCH different than my 2001 Tahoe I also took apart and studied.

@Kenneth L Nielsen I'm not showing any DTC or MIL codes. I have searched