tylerwerrin

Hi guys,

My speedometer isn't working and I'm wondering if there are any additional checks I can run. Right now, I have a 1994 Camaro tune for M6 -- I'm trying to see why I'm not getting any signal at the speedometer.

When i scanned the PCM i pulled code 97, which is VSS output circuit not giving me 4000ppm. I was reading at 0mph with the key on, and the engine off. I checked the voltage at the Green/White wire (output wire) in the cab and it read .75v -- I had read on a previous forum entry that it is supposed to read between 6.5-7.5v. I checked the Green/white wire at B8 on the PCM and got a voltage reading of 11.8v -- weird, huh?

Does that mean the VSS is outputting a faulty signal (indicating faulty VSS or connection at the VSS)? Why else would my output wire be so high?

What else can I check? What are the voltages supposed to be at the VSS itself? perhaps I can track the problem down from there.

Any help would be very appreciated!

Tyler

Injuneer

The VSS has two wires. Purple is the ground from the PCM. The PCM supplies a 5V reference between the yellow and the purple wires. The reluctor wheel rotating near the magnet in the sensor causes the sensor to output a sine wave, which I believe varies from 0 - 5V.

The PCM takes that signal and processes it (based on programmed rear axle ratio and tire diameter) to produce the 4,000 PPM signal that is sent to the speedo, and to the cruise control. Does your cruise control work?

According to the pinout chart, pin B8 should read 6.5V key on/engine off, and 7.5V engine running.

DTC 97 sets when the PCM detects the wrong voltage on the green/white wire for 26 seconds.

I would first verify that you are getting the correct signal from the VSS to the PCM. Most likely it is OK. If you weren't you would have DTC 24.

Then verify the voltage on the green/white wire. Since the wire connects to two devices, perhaps there is a short to 12V somewhere between the PCM, the speedo and the cruise control.

tylerwerrin

I don't have the cruise control devices set up, so I have no way of testing them. I'm not getting the other DTC -- I haven't gotten one yet.

When i tested the voltage at B8 (i pulled back a little bit of the wire at the PCM) i tested it and got a voltage of 11.8 (which is the battery's voltage).

I haven't tried it with the engine on yet -- but either way, the voltage is incorrect. Inside the car, the green/white wire which feeds the speedo was reading .75v -- at the very least i've got a short in this wire as it should read the same as what it reads at the PCM (11.8)

I can check the wires at the VSS and confirm but like you said I think the VSS itself is OK (i'm not getting any other codes). Judging from the VSS code 97 troubleshooting guide, C220 branches off to C200 which feeds the cruise control. If I just run a new wire from B8 straight to the speedo, I should get something, right? Or do i HAVE to have cruise control hooked up?

Injuneer

You do not have to have the cruise control hooked up. Pull the harness connector off the PCM and check the green/white wire for voltage. That will tell you if you have a short to 12V somewhere in the wiring harness.

tylerwerrin

I checked the voltage -- when I first tried it, i got 11v at the green/white wire however when I checked at the circuit in the cab, i got 70mV. I figured that there was a short in the circuit somewhere along it's path so I decided to run a new wire from the B8 terminal straight into the cab to the speedometer.

I checked the voltage with key on, and now i'm getting 16.6mV! Wtf! I haven't tried it with the car on yet, just the key on, engine off.

I'll try putting the old wire back into the B8 pin on the harness. But I don't know why I'm getting such odd readings. Could this be the smoking gun for the VSS? if so, why aren't I getting a code 24?

tylerwerrin

Hey Fred --

You said pull the PCM connector off to test the voltage -- wouldn't that mess with the reading because the connector wouldn't be connected to 12v? I tested the wire by pulling back a little of the wire's skin and probed with my mutimeter -- is this the wrong way to do this?

I tried running a whole new wire from the B8 socket to the cab/speedo and again I got funky reads. However, I'm still not getting a VSS code -- i'm gonna squirm under the car and verify the VSS is attached securely, but i'm not sure what else it could be...

tylerwerrin

UPDATE:

So, here's what I've done so far:

Checked voltage at the PCM (B8) -- had odd floating readings, but when I found a good ground it stayed at .26v.

Ran a new wire directly from B8 directly to my speedometer and checked the voltage -- still at .26.

Checked the voltage at the VSS harness connector with Key On -- came back with 4.9v -- which is what I should come back with. OK.

Since i had another VSS, i figured I would just swap it out and see what happens. I swapped in the new VSS, voltage readings at the PCM still remained the same (.26v)

According to shbox's pinouts, I should be getting 6.5v with key off at B8 and 7.5v with the engine running. I'm getting neither of these. The speedometer still is displaying nothing.

I can try swapping in a different PCM but I don't think it will really make a difference. Other than doing that, is there anything else I can do? I am at a bit of a loss here.

Injuneer

This probably isn't the best forum to post this on. As you can see.... no one posts here and no one looks here. I've been trying to get the forum eliminated, because it no longer fulfills it's original purpose. I only drop by once in a while since I'm supposed to be the forum moderator, and I have to move the posts that get put here by mistake.

If you want, I'll move this to either "General 67-02 F-Body Tech", or "Computer......".

tylerwerrin

alright, thanks Fred -- as long as it helps to solve the problem i don't mind where the post is.

Injuneer

Moved to "General 67-02.....", and I'll copy it to "Computer....."