My speedo has recently gone haywire. Not sure if its the sensor or the guage cluster cause my laptop died. I will be cruising along at around 1500 rpms in 6th and my speedo will bounce all over the place. Even when Im stopped it will read between 0 and 20 or 30 mph. And my car seems to run really bad which is why I'm asking. It seems to run really rough and then it suddenly smooths out and runs fine but it seems to follow the speedo. Is my mind just plainyg tricks on me or what?

 

Your main answer is NO. If you'de have said your tach was acting funny, your problem would be a little less complicated. But you said your speedo is acting up. The signal for the speedo is isolated from other functions ALTHOUGH the signal does go through the PCM. The VSS puts out a sine wave to the PCM which inturn puts out pulses to the instrument cluster and cruise control module.

The first question I have is, does the cruise control act up or drop out when this problem occurs?

If your 2 problems are occuring simultaneously, the only thing I could think of is perhaps a bad ground to the PCM. You obviously want to check all the grounds at the battery and the passenger fender. There are also 2 other points to check carefully for ground. Directly next to the ICM on the left side of the engine. This ground point is critical for the PCM. The other is located right above the starter.

Let us know what you find.

Dave

 

The cruise control acts very wacky. It started doing it about amonth ago on my way to Florida. Just dropped riht out of cruise and then tried to pick up speed again. Not a cool ride or feeling that far away from home. Ill check my grounds. Thanks

 

Finally got it scanned and the datamaster reads 0 mph but the speedo is all over. Cluster problem right? What could I check?

 

The VSS will have more effect on driveability on an auto car. You don't give any transmission info.

 

Its an M6