Digital speedometer reading half of what it should.

[R Crusoe]

I have a 1989 Pontiac Grand Prix SE with 5-speed manual transaxle. Has anyone experienced digital speedometer reading half of what it should be reading? I've changed out the VSS but that did not fix the problem. I swapped out instrument cluster with analog cluster and still only half of speed indicated. No codes registered. Changed out ECM w/o any improvement. Any help out there?

[RobertISaar]

did this issue pop up more or less instantly?

 

also, since you swapped out the ECM, which MEMCAL is in the current one? should be a 4 letter code on it, i'll lookup and make sure you have what you're supposed to. and is it more or less exactly half of what it should display?

 

 

 

otherwise, the only possible things(in order of where the VSS signal travels) that could be causing this:

 

VSS

VSS wiring

ECM

MEMCAL

wiring from ECM to cluster

cluster

 

since you've swapped in a different cluster with the same results and ECM and VSS, there aren't a lot of possibilities left....

 

and i can't fathom why someone would possibly do so, but perhaps check to make sure the signal from ECM to cluster is on the right pin.... the 1227727 ECM has a couple of different VSS outputs, the 4000 pulse per mile signal is the one that's normally used by nearly everything, but there is also a 2000 pulse per mile signal pin, which would cut the displayed speed in half... no idea why someone would do that, but perhaps it's worth checking if it has always been that way in your possesion. B8 is the 4000 PPM signal pin. B16 is one of the 2000 PPM signal pins, B14 is the other.

[R Crusoe]

Thanks for the reply. Issue with speedometer popped up spontaneously. Original equipment MEMCAL is in the current ECM. I've had the car since 1991 and nothing was changed that would affect the speedo prior to it registering wrong speeds. Did change out the VSS without any change. GM dealer could not find the cause either. Perhaps I need to removed the connector to the VSS and clean it out with CRC.