98TAProject

I have searched the forums and have come up with a blank no one seems to have solved this problem that I can find. The speedometer in my 1998 Transam gets progressively more incorrect the faster I go at 45 it displays 51 at 70 it displays 83 but the PCM displays the correct speed when I connect my reader and look at the live data feed. I replaced the VSS and it did nothing so do I have to replace the entire gauge cluster or is there something else I should try first the car is bone stock factory rear end and tires my dad has owned it since new so I know no previous owners have messed with it.

Any help would be greatly appreciated!!

also the pcm live feed of the rpm and the gauge are about 500 rpm off at idle.

badformulaLS1

iTrader: (1)

What size tires are you running front and back?

98TAProject

245/50r16 97h

1 FMF

iTrader: (1)

sounds like you either got a bad stepper motor in the gauge cluster, that's what makes the needle move, or something else is bad in the cluster. i don't know about your year, but my 2006 gmc truck fell under the stepper motor recall. that was for something like 2002-6 years of all gm vehicles. i was able to replace all mine at home with a soldering iron for $60. not sure if it can be done on your year cluster. and there are places you can send clusters to get repaired. google gm cluster repair and stepper motor recall.
this is where i got my parts from: http://gm-stepper-motors.weebly.com/
you might want to call or email a few cluster repair shops and see if they know anything, they might say something like those years were common for having <your problem>...

1 FMF

iTrader: (1)

and i think with a gm tech2 scanner you can command the gauge cluster needles to read a certain value for diagnostics to pinpoint if the problem is inside the cluster.

98TAProject

that's what i was thinking but if the stepper motor was going bad wouldn't it show i am going slower than i really am not faster..

roninsonic

iTrader: (3)

that actually makes sense... it is off by a certain PERCENTAGE, So the faster the actual speed, the greater the discrepancy of indicated speed.

98TAProject

what feeds the RPM to the gauge cluster and could that be throwing off the speed if the cluster or pcm is reading the wrong RPM

1 FMF

iTrader: (1)

the pcm sends all the info as "class 2 serial data" to the instrument cluster. the cluster is it's own computer getting a serial data feed from the pcm, it reads the data then the processor or asic in the cluster tells each gauge needle where to point.
there is not a single wire going to the cluster that is for rpm, a wire for engine temp and so on. so if you have a scan tool and can read real time rpm and speedo values from the OBD2 port and those are correct but your cluster is off then most likely you need to replace the cluster. like i said, u might be able to further diagnose it with a gm scan tool to be sure it's in the cluster- i believe the cluster has it's own diagnostic codes. depending on how you go about it through the dealer you might get the $100 diagnostic waived or put towards the cost of a new cluster.

98TAProject

Just an update incase anyone else has this problem I replaced the gauge cluster with one pulled from a junk yard and all problems went away so it was a dieing cluster.