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.

 

Does your car have cruise control. If yes, does it work. If it does that says vss is good. The factory manual wants the tech to connect a signal generator to the circuit to test the pcm and speedometer itself. I don't suspose you have one? There is 2 other checks you can do. First check there is 12 volts with key on on the purple wire at the vss. A test light will work if no volt meter available. Should be battery voltage or light the test light. 2nd test is to test between purple and yellow wire at vss and at pcm A31 and A32 yellow and purple wires again. This time you need a volt meter as you are susposed to get more than 4 volts with key on. If on the first test you don't get batt voltage on the purple wire, then the wire or pcm circuit is bad. Also check the yellow wire is good to pcm. Hope this helps some.

 

If you get the proper vss readings and the speedo still doesn't work, it is a very common problem for electronic speedometers to have the circuit board go bad. My 90 camaro dd, I've replaced the circuit board in the speedo 3 times in the 8 years I've had the car. Circuit board is not available as a spare part unless you know someone in the speedo repair business. If bad you'll have to get another cluster and either put that in or take the circuit board out.

 

I got 4.9 volts at the VSS connector plug with Key On. It looks like whatever signal is coming from the VSS is fine -- i don't have a signal tester though or an oscilliscope but I have to assume for now that whatever is coming out of the VSS is going to the PCM.

The problem seems to be lying with the output wire that goes to the speedometer itself. the voltage isn't right, and the PCM is detecting this. I don't know why it is not reading correctly anymore, as it worked when I had the 4l60e in the car, and I didn't change any of the wiring.

Some people have mentioned that it could be there is no termination to the wire so the voltage is going wonky -- but wouldn't that stay at battery voltage instead of .25v? very strange!

And nope, no cruise control --

The engine harness is from a 1996 Caprice, the engine is a 1994 camaro lt1, if that helps anything.

I'm using the old Jaguar Xj6 speedometer,which is connected to an in-line signal converter. I don't think the speedometer has gone bad -- it worked fine before. Do you mean the circuit board in the PCM?

 

I was talking about the speedometer circuit boad inside the instrument cluster. I'm sure the jag speedo has some kind of circuit board in it, but as said, you can't get a replacement and would have to get a cluster or speedo if its seperate. One thing you need to know about voltage, if its wrong it doesn't mean for sure the pcm is sending the wrong voltage. A faulty electrical component on the circuit board of the speedo can cause the wrong voltage also. All the times my circuit board has failed on my 90 camaro, it was without warning, no bouncing needle, or wrong readings. The last time I used it it worked find and the next time I drove it no speedo. Too bad you don't live in Florida, you could use my signal generator and see for sure if the speedo works or no.

 

Tyler did some test on my A4 today:
Pin 'B8' at the PCM end.. key/on eng/off my DVM reads reads 8.6 volts to chassis ground, verified it's a clean 8.6V DC with my scope. With Key/on Eng/on in neutral it reads 10.6V Dc to chassis ground, again verified with scope, it had a very small insignificant .2V noise ripple.. With key/on/eng/on left wheel jacked up in gear at 10 MPH the line is a 10.2 V to ground square wave with frequency changing with speed (4K per mile pulse), put the DVM in AC mode between B8 and chassis ground and it reads 11.2V AC at 10 MPH. .. Are you sure the box between your PCM and speedometer that B8 (drk grn/wht wire) feeds is set up OK.
As far as VSS key/on eng/off A32 yellow wire reads 0.06 to chassis ground, verified with scope, ppl wire reads 10.6V DC to chassis ground, verified with scope.key/on eng/on in gear at 10MPH yellow reads +/- 15V AC (30v peek to peek), ppl wire reads +12v DC with a little ripple, verified with scope.

 

Thanks for the info bob... Looks like whatever I have going on is messed up. The box never got messed around with after I set it up with the original caprice 5.7 and the 4l60e. I tried running a new wire straight from the B8 connector (took out the old pin) and ran the wire straight to the input wire for the converter box -- when i probed the wire at the PCM i got .26v.

I've got a spare PCM, I could try swapping it out -- but it seems wierd that this would be the only circuit in the PCM to go bad. If i have a bad connection AFTER my converter box (ruffled a wire or something) could that cause my odd voltage? Because there is no termination at the wire? That will be my next step.

Thanks again for doing that detective work, it's very helpful!

PS I didn't probe the VSS wires at the PCM, just at the VSS sensor itself. When i probed both (stuck my + probe in the + side of the connector and the - side in the neg) i got 4.9v, which i think is within spec -- the VSS looks for a 5V signal i believe.

Oh and Cocoblo, I have another jag cluster -- the instruments are seperate guages -- it's probably fine but thats another good place to check.

 

Tyler just something additional.. In general any driver circuit requires a termination, so when the PCM drives a circuit to 0 volts (active) there is a resistor tied to a higher voltage potential at the remote end of the line to keep the circuit at a non 0 potential (inactive) level otherwise the circuit will float which is not proper design. The PCM checks for the inactive level when it is not activating the circuit, and if the voltage is not correct a "circuit" failure is indicated. Important is DTC's that are called circuit failures are used to detect open or shorted lines to the remote device/sensor/injectorsr/solenoid/etc. The PCM uses this approach for Injectors , tranny solenoids, ICM, SES indicator and many other remote circuits. Its a way for the PCM to verify wires and circuits are not open or shorted.
My guess is in a stock setup the speedometer cluster circuitry is supposed to keep the B8 pin at 10 volts when the 4k pulses per mile driver transistor in the PCM is not pulsing to 0 volts active. However the PCM is not seeing 10V when the line is supposed to be inactive . So either the PCM is keeping the circuit at a lower potential (4k signal) and is always on, or the box you have between the PCM and the Jag speedometer is not providing the voltage to drive the line up when the PCM knows it's s/b turned off..

 

success! It turned out to be... drumroll...

The wire which powered the converter box had been pulled just SLIGHTLY from it's "quick splice" (the wire was spliced with one of those plastic things to a hot ignition wire under the dash) causing the problem. Once i fixed the wire, the voltage read 10~ volts with key on! Hooray!

I drove the car around a bit today -- the speedo is reading about 2x as fast as the car is going -- what settings should I play with in TunerCATS to get it to read correctly? Right now, it has an M6 tune, which is 17ppr (i've got an M29 t56). I thought the PCM outputs 4000ppr no matter what?

 

Tyler, I don’t know if you’re on here anymore, I was reading a post you had with your vss. I have identical readings and issues except I’m using a painless harness and I don’t have a converter box. What does the converter box look like and was your green wire from the ecu running to it and then the converter box ran to the speedometer?

 

Man glad to see progress on this saga..I do not like those push in then crimp splices.The PCM looks at VSS pulses per tailshaft revolution, tire size and rear end gearing and then converts all the numbers into 4K per mile pulses. That way they can use a common speedometer for any drive train setup, PCM does all the work. All three variables are inputted during the TunerCat speedometer utility. Is the speed in the PCM (Freescan or TunerCat) correct or also reading high ?? If PCM is correct then it's pointing to the converter box. Also is it exactly 2X as fast or could it be 1.5X . I'm still hung up on that '93 M6 you have. Quote from jagsthatrun.com "An 11 tooth reluctor ring is used on 1993 LT1 engines with the Borg-Warner wide ratio (3.35 First gear) 6-speed transmission." Not sure how you would identify the '93 M6 wide ratio 1st gear ??

If your tired of working on this..you could just trick the setup by entering a 'customized' setup in tunercat for your car, EG: use tire revolutions per mile radial button in the speedometer utility and see if you can set it to 1620 which is close to double the normal 810 for a typical stock tire, that should halve your speed.. A GPS helps get the speed accurate as you tweak the numbers...

 

Hi Bob,

Yeah, its good to finally at least have the gauge clusters all operational (sorta). The T56 that I have is the M29 -- i verified it by looking at the tag on the side of the transmission. That means I have the standard 17 tooth reluctor ring -- everything is basically the same as the 94+ t56's, only i think my t56 doesn't have as high of a torque rating.

As for the 1.5x or 2x as fast reading of the speedo, that I'm not sure about -- I was puttering around the neighborhood in 3rd and the speedo was reading 55 -- I know I wasn't going 55!

my laptop's battery isn't the best so i can't really drive around with the laptop connected to the car unfortunately to verify MPH -- but I'll give it a shot. I tried adjusting the converter box, but nothing really changed. Honestly at this point i wouldn't mind just "tricking" the PCM into giving me the proper speed. I'll see what I can do. Thanks again for all your help!

 

Sorry to revive this thread, but I am having VSS and shifting problems with my 95 z28. When I test the voltage of the VSS purple wire I get 0.024 DC volts. When I test between the yellow and purple VSS wire I get no reading. I've tested at the VSS and PCM with the same results, and I've tried a different PCM with the same exact results.

 

You need to get an obd 1 scanner that does real time sensor data. Doing a running sensor scan will tell you what mph the pcm is seeing from the vss. You can compare the pcm vss mph to what speedometer is showing.

If pcm is seeing correct mph from vss, and speedometer mph is off, either circuit board inspeedometer or wiring between pcm & speedometer is bad.

If pcm is receiving incorrect mph from vss, then vss, wiring from vss to pcm is bad.

 

I did a test on the dark green/white VSS output wire and it was around 9.11 volts with the key on engine not running. According to shbox pin a31 A (red) plug is a ground. Does the little black box behind passenger side front kick panel have anything to do with the transmission VSS or shifting?

 

I tested a new VSS and an old VSS with a fluke 17b+ DMM on DC volts in hertz mode and both are operating good it seems. Is there another wire coming from the PCM/speedometer to have the car shift besides the dark green/white wire? Or can I do the same DMM test in hertz on the dark green/white wire?