Questions and Answers

Dying Cadillac DeVille Q. I have a 1986 Cadillac DeVille, 8 cylinder, P/S, P/B, A/C ,Cruise Control, Power Windows. The problem that I am having is that whenever I am driving it and I hit a bump the speedometer falls down to 0 and then when I have to stop at a signal it dies on me.

I have had a new speedometer cable put on and that didn't correct the problem. So far no one can figure it out. When the speedometer falls down to 0 the next time I hit a bump it will start working again. I know this sounds really crazy, but when it is really hot outside, it seems to do it a lot. It doesn't do it all the time but a lot here lately. The service soon light came on yesterday too. I am just lost on trying to figure it out. Do you have any ideals what it could be?

The car has not turned over once yet but it is getting close to 100,000 miles. I do not know what size engine it is. Could it possible be something with the transmission or maybe a sensor.

Thanks,
Lynne

A. The speedometer system is a little complicated in that car so trying to diagnose the problem will be difficult.

The fact that the "Service Engine Soon" light came on indicates that the computer has detected a fault, stored a Diagnostic Trouble Code (DTC) and is alerting you that there is a problem. It should be an easy matter of pulling up the DTC and use that as a starting point for the troubleshooting procedure.

There is a Vehicle Speed Sensor Signal Buffer that amplifies the speed signal before it goes to the Electronic Control Unit (ECU) and Body Control Module (BCM). It is possible the problem is with the buffer since both the engine and speedometer are affected.

Additional Information provided courtesy of ALLDATA