Hello all,
I have read through a ton of posts regarding problems with speedometer, but most of them seem to include the abs light and brake light being on. Not so with this vehicle.
history:
Sitting in the Sonic drive through and the van cuts off. instrument cluster gauges are all at zero, don't move at all when I turn on the ignition. The Sonic staff push me into a parking spot and I notice that when I turn on the ignition the trip meter blinks for a few seconds and then it displays "No Bus". Check for codes, none were recorded. After about 1/2 hour I am able to start the car right up, so I drive it to my long-time mechanic which is only about a mile away. Of course, trying to service the car when it isn't acting up is not possible, so I drive away and in about a mile the car dies again. Same symptoms as before. I have it towed back to the mechanic and he works on it and says the PCM has died. He replaces it with a remanufactured part and all seems OK until the vehicle has run for about 1/2 hour and then the speedometer starts jumping around. Sometimes the speedometer drops to zero and stays there. Other gauges are unaffected. Apart from that, the engine doesn't cut off, seems to drive OK, shifting normal and overdrive is OK. Decided to try doing some things on my own before I go back to the mechanic. Replaced the OSS on the transmission first. When I tried driving it after that, after it warmed up the Speedo went crazy the check engine light came on and the P0500 code was recorded. At this point I figured I might as well replace the ABS sensor on the top of the differential. Cleared the P0500 code by pulling the negative battery cable and then tried a test drive. Once the car ran at highway speed for around 45 minutes the speedometer went crazy again, but no check engine or codes recorded. I have never had the ABS or Brake light stay on, as many folks have. For what it is worth, when the speedo is acting up, I can turn on the cruise control, but I can't set it to hold a speed. I have gone from stem to stern checking ground wires and re-seating them. Battery voltage shows as 12.19. Do I need to have the mechanic try another PCM? Any help would be greatly appreciated!!

 

Change the sensor on the back left-side of the tranny. That's the one that controls the speedo and ultimately the cruise control.

 

Thanks for the reply, as I explained in the original post, I have already replaced the Output Speed Sensor on the transmission and the ABS sensor on the differential.

 

Need to verify you are getting 5 volt reference signal to the sensors. Would be really nice if you could plug into it with a scanner, and see if you are getting data from either/both sensors.

 

Okay missed that, which tends to happen with run-on paragraphs. I see that you got code P0500, which should be investigated further. For example, even though the sensor might be new and good, the wiring or connector to it could have an issue, or the gear that drives it could be stripped and turning intermittently.

 

Thanks for the replies! After I got the p0500 I replaced the ABS sensor on the differential. I then reset the PCM and drove the car at highway speeds for nearly 2 hours. After about 45 minutes the speedo started to act up again. This has consistently been a warmup problem. it doesn't start until the car has been at normal operating temperature for 30 to 45 minutes. I only have gotten the P0500 and the check engine light once, since replacing the ABS sensor I have had no codes or warning lights.

This is a 2001, so the sensors are not gear driven, they are magnetic (hall effect?) sensors.

Since the sensors only drop out after the car has run for quite a while, it will be tough to get the 5 volt check done. If I get the speedo to drop out completely, do I just leave it running and pull the two plugs? Then see if I get 5 volts from one of the pins to ground?

This is a different problem from the initial problem, which was "no bus" and a dead car. Am wondering if the "remanufactured" replacement PCM might have a thermal problem.

Will borrow my brother's code reader and see what it can show me.

Thanks again to all!

 

Wouldn't surprise me if the PCM wasn't a problem.......

 

update on this problem, new symptoms, it is now more than a speedometer problem.
To recap, the initial problem was an intermittent "no bus" error with the accompanying dead instrument cluster, the vehicle would not start. My mechanic replaced the PCM with a "remanufactured" unit.
The problem then became a speedometer problem, but only after the car was fully warmed up, having been driven for 30 or more minutes.
It is still a warm-up problem for the speedometer, but there are now symptoms which begin as soon as you start driving.
Once the car is warmed up, the speedometer starts to swing wildly and eventually drops to zero and stays there.
We now have the P0500 code back and check engine light back on.
Unlike everyone else with speedometer problems, we have NO ABS light or brake light.
The van now seems to have a problem shifting, when you take off from a cold start it may jump back and forth between gears when it reaches a shift point, then settle into the higher gear. Of course, it doesn't do it every time. I can manually switch in and out of overdrive at any time, whether the speedo is working or not.
The van is also stuttering intermittently while driving, but very infrequently. I have been going around 40-50 mph when it happens. I don't think it is losing power and it doesn't feel like it is transmission related. I am thinking it may be the ABS randomly pulsing the brakes as was noted in another person's thread.
The OSS and the ABS sensors have been replaced, both were replacements purchased from Autozone.
With the vehicle cold and after the speedometer has stopped registering, I checked the voltage going to the two replaced sensors:
The ABS sensor on the differential has about the same voltage across the two terminals whether it is cold or after the speedo has stopped registering. I get around 5 volts when the car is just started and 4.86 volts across the two terminals after the speedometer has died. A reading from each plug terminal to chassis ground, whether cold or with a dead speedo, yields 0.00 volts.
For the OSS sensor on the transmission, reading from each terminal individually to chassis ground, I get 2.7 volts when the car is first started and 2.608 volts after the speedometer has died. I get 0.00 volts between the two terminals on the OSS plug, whether the car is cold or after the speedometer has died.
Any suggestions for other tests I could perform, or for what part may be flaking out, would be greatly appreciated.
Thanks to all

 

Do you have access to a scanner that can read live data? Would be interesting to see what the PCM is seeing from those two sensors, and if they readings actually agree with reality.

 

Gents, I'm having the above issues; ABS light, Brake light, speedo stopped working, no cruise and odo stopped as well. Other than that, the van runs just fine. I'm getting P0500 code and replaced the diff sensor with no luck. Based on the thread above, I'm pretty sure it must be the speed sensor in the tranny. I have the 3.9L motor with the 3 speed tranny. Can someone point me to where the sensor is?