I am looking at a really clean '92 coupe to purchase.Everything seems to work well, and it has been well cared for by the original owner. The only issue I can find is with the speedometer.When the car is started and still stationary, the speedo display constantly cycles random speeds, anywhere from 1 MPH to 80 MPH. It only does this while the car is sitting still, once the car moves, it immediately goes to 0, and registers normally at any road speed. So, to sum up, it works normally when the car is moving, acts up when the car is not moving. Any ideas on what could be causing this?

 

Well I am going to take a stab at this one and give you my opinion. I would start with plugging in a diagnostic tool such as Auto x-ray or datamaster. If it reads fine, then I would be looking at the speedometer head at that point or grounds. If not, then I would look at the adapter on the transmission. The 92 is the same as the 93, it uses a gear drive but that is converted to an electrical signal and sent to the computer and speedometer. That adapter-converter is not very much, I think it was $30-40 dollars. I may be way off here but at least I moved you back to the top!!

 

Hey 93,

I am inclined to agree with the idea of the speed sensor (mechanical to electrical signal converter).I ended up buying the car, and I'm very happy with it so far. Thanks for your input!

Jan

 

I'm trying to track down a similar problem in my '87 C4. After the car warms up, the speedo drops to zero and all functions requiring vehicle speed info stop working.

I tested the VSS by spinning it with a drill. Mesuring with the frequency counter scale on my DMM, I got a frequency reading that varied with the rate of rotation. This is exactly the signal that the Instrument cluster is looking for to convert to MPH. If I disconnect the VSS (brand new and good) midway between the VSS and cluster, I'm able to put a DMM across the VSS output. A resistance measurement gives a value of about 400 Ohms, proper for a working VSS.

Now the peculiar part. With the VSS installed and that connector undone, the engine wouldn't start, but, with the starter turning over the engine, the frequency reading from the VSS becomes a mess of random numbers instead of the steady zero it should be. Somehow, a spurious signal is being injected into the VSS output circuit that overwhelms the normal signal from the VSS. My tachometer works even when the speedo goes to the zero mode, but could a faulty tachometer filter have an effect on the VSS circuit? Are there any other filters that might stop working as soon as the temperature rises? Are there any functions timed by the ECM that might be a clue? For instance, the MAF burnoff function involves the timed switching of a circuit to ground. Are there other timed sequences?

I suppose I could rewire from the VSS to the cluster, but should I shield the circuit? What other problems might be present that I haven't identified yet?

 

I will be watching the responses to your issue. I just bought an '87 Coupe. I have no speedo/odo and a code 24. When I checked the connector at the VSS, it crumbled in my hand. I put on a new connector, plugged it in, and now I have a speedo for the first few seconds of driving, then nothing. I am assuming at this point that I need a new VSS. I'll wait to see what others say about your problem. Thanks! Ed

 

I've done a few more tests and may be narrowing in on a solution I don't want to deal with. Earlier, I had directly measured the output of the VSS at the connector (C237?) under the dash using the Hertz scale on my DMM. I got a very erratic signal, jumping around, and often stabilizing at a frequency about 1/4 engine RPM. Thinking I had an electrical filter problem, I spliced wires directly to the VSS, bypassing all the chassis wiring. This eliminated the erratic readings, probably from bypassing the engine compartment, but I'm getting no output from the VSS. The VSS is brand new and the large green gear was replaced by my mechanic when replaced the clutch. My mechanic was looking for the problem with the VSS. He discovered the original gear was broken, but could find nothing wrong with the rig gear. Since the speedometer would work for several miles, he tested it and thought he had fixed the problem. It stopped working after 15 miles, and would again work each time the car started cold. I replaced the VSS and replaced the wiring connector recently, and initially the speedometer behavior remained the same, but now it doesn't work even when the car is cold. There is a possibility the new VSS is bad, but the old one tests good, so that is easily dealt with.

The real problem appears to be the gears driving the VSS. The mechanic found enough damage to the green driven gear to replace it, but found no comparable damage to the drive gear. Given that the drive gear is inside the O.D. unit, he probably couldn't test much more than whether it was tight on the shaft and intact. There is a diagram in the FSM showing the drive gear and a clip to keep it positioned on the shaft. The protruding part of the clip appears to index the gear on output shaft. To keep it from rotating, there is a tang on the clip indexed into the shaft. The only way the drive gear could rotate independent of the shaft is if the tang is missing, or the tang is out of the index hole in the shaft. Can the clip be replaced or the gear repositioned through the VSS opening?

I also have concerns whether the gears are actually meshing. The green driven gear can slide on the D-shaft of the VSS, but has no spring element to hold it in contact with the drive gear.

Given the difficulty of checking all this on a convertible with no lift available, I'm hoping some listers have experience they can share. If I knew the exact problem, I could take it back to the mechanic to do the work.

 

My son just proposed an interesting solution to avoid pulling the drive train to replace faulty speedo gears. He suspects attaching a neodymium magnet to the drive shaft in close proximity to a home wound 400 Ohm coil wound on a bolt would generate the signal that the instrument cluster is looking for. A hose clamp would hold the magnet in place and allow balancing the assembly. Building the coil on a threaded carriage bolt would allow easily locating it with a proper gap. Any thoughts?

 

I had my VSS out and while trying to determine where trans fluid was leaking I disassembled it.

I put the halves back together and didn't have any readout which was strange because I had previously had readout, it was just leaking. Anyhow, long story short: I indexed the halves of the VSS 180* out which caused the drive gear to slide on the output shaft d/t the misalignment. Now this probably isn't your problem because you didn't disassemble your VSS but it has to do with your question about the gears. Remember the outside is indexed so it can only be on one way...it was the inner portion that I fubar'd. The drive gear was spinning over the retainer and then locked.

Next time you have it apart you could check the mesh of the gears with trans ***'y lube (probably could use vaseline since check ***** can be retained with it but double check that).

I guess now that there is no output, my thought is that the VSS is either the wrong one and the offset of the shaft that the driven gear rides on is not allowing that gear to make contact or the drive gear had slipped previously.

 

I did get output from the replacement VSS initially for the usual few miles, but that has now gone away, and it never works. When I go after the gears I'll evaluate bot units I now have using the drill and frequency meter.

I've gone over every reference I can find in the FSM. It appears the O.D. unit has to be removed and torn down to remove the output shaft. Although the output shaft seal can be replaced from the back end, there doesn't appear to be clearance to pass a new speedo drive gear through the same opening. I hope somebody tells me I'm wrong.

The drive gear appeared undamaged when the damaged green driven gear was replaced. The clip securing the drive gear to the output shaft looks suspiciously like a shear pin. If the tang got knocked out of the hole, it probably wouldn't take too many revolutions to wear it to a nubbin. If the coefficient of expansion of the gear is greater than the shaft, it would explain why it holds cold, but slips hot. From the detailed drawings in the FSM, I'm going to machine a piece of 3/16" stock so it tapers to 1/16" with a 1/8" tang. If I can line everything up, this might work to relock the drive gear. Meanwhile, I'l use digital calipers to measure the range of gear locations of the assembled unit to see if gear mesh is always guaranteed.

I didnt quite follow your comments about the drive gear.

 

Did you ever solve this problem? I have the exact same thing happening also on a 92? I have extensively chased this problem with no positive results. Thanks