P0500 Speed Sensor Malfunction (1997 Mercury Tracer)

I'm at the end of my rope on this one and any help would be greatly appreciated. I have a 97 Mercury Tracer with a P0500 (Vehicle Speed Sensor Malfunction) code being thrown. The speedometer & odometer do not work at all but other dash gauges do work. I first noticed this problem during my first drive in the spring after the car sat idle through an upstate New York winter (very cold).

I did the obvious and replaced the speed sensor but that did not fix it. I then thought that maybe the speedometer in the dash itself was bad. So I pulled another dash for this model at the local junkyard and installed it but had the same result. (No speedometer/odometer function, but all other gauges worked). Then thinking the wiring might be faulty, I disconnected the speed sensor connector cable and attached 2 long cables directly from the speed sensor directly to the VSS signal and ground pins on the computer. But I still had the code and no speedometer/odometer functionality.

I looked at the signal going into the computer with an oscilloscope while driving and I can clearly see an AC sine wave that gets larger and skinner/tighter as you speed up and the opposite when you slow down. So it looks to me like the signal into the computer is good. The electrical schematics make it appear that the VSS signal goes both to the computer and to the dash. And I'm getting a failure indication at both locations (dash gauge does nothing and computer throws code)

Any thoughts on what else I could try? I'm almost ready to try a new computer, but since everything else on the car runs fine I'm not sure that is the problem. Albuquerque has installed speeding cameras at intersections so I really need to know how fast I'm going. (I've already got one camera ticket)

Thanks all.

I'd try hardwiring the sensor wires directly to the pcm and then out to the cluster, if it still doesn't work then the pcm is probably bad! assuming the speed sensor that you installed is good that is!

From what you described the speedometer should work but of course it doesn't, so I would start fom the beginning.

You know you have a pulse signal from the speed sensor to the computer. Do you have the pulse signal to the speedo? The signal should be from the same color wire as the signal to the computer (white w/black) at the cluster.

Also check for constant batt power to the green wire to the speedo circuit at the back of the instrument cluster and battery power with the key in the run position at the black/yellow wire to the speedo & lastly a good ground from the black wire to the speedo. If all of those signals are good and the signal from the speed sensor is good, your speedometer should work.

Keep us informed

might be worth calling a dealer to see how common a problem that was for those cars or pcms

Thanks for everyones input. I verified previously that the speed sensor signal at the dash dircuit is good. I will try the direct hardwiring thing since my only thoughts left are wiring or the computer. If direct wiring doesn't work, I'll try a used computer I've found on eBay for $25. Though since I can see the VSS signal at the computer and the dash, I'm leaning toward the computer being the only part of the loop left that is not working.

I'll keep you posted.

I hope your not on the wrong track. You said the VSS signal was good at the cluster therefore the speedometer should work even if there is a problem at the computer.

Both items are looking for the same speed sensor signal but if you have the signal to the cluster and the speedometer still doesn't work is it possible that your not reading things properly and perhaps you are not getting the correct signal? Just a thought since I've made mistakes like that before and ended up in the wrong direction.

I know you said that you used a scope to check the signal and had a changing waveform but I usually have good luck checking the signal with a voltmeter and look for an ac voltage signal.

Looking forward to your results!!

Well, after much effort, I have found the problem. I went back and rechecked all the wiring resistance to make sure everything checked out. The VSS+ signal at the speed sensor was correctly connected to both the computer VSS+ input pin as well as the VSS+ input pin on the dash. Same goes for VSS- at the speed sensor, computer, and the dash. Also, both the VSS+ and - signals were not shorted to power or ground.

However, Ford's electrical schematic showed that these VSS- signals should eventually connect to ground (i.e. negative battery terminal). But I could not see that with an ohm meter. So sure enough, I tried manually connecting a wire from the VSS- input pin on the computer to a nearby ground on the cigarette lighter and the speedometer worked and the check engine light is no more.

I then looked at Ford's ground schematics and it is clear as day that these three VSS- signals at the speed sensor, dash, and computer are all connected together and then go through a male-to-female connector under the hood before they reach the negative terminal on the battery. All other ground pins on the computer go through a different ground path/connector to reach the negative battery terminal. Apparently I have a bad ground connection somewhere on that first ground cable. If I can't find the fault (I'm guessing it's corrosion or a crack at the connector) I'll just "hardwire" a short to the cigarette lighter ground since that seemed to fix it during my testing. Hope this helps someone in the future that is wrestling with another quirky P0500 code on a Mercury Tracer or a Ford Escort.