I have been reading the forum for some time now and wonder if anyone can help me out - or rather help the UK garage where my STS 98 Cadillac is sitting at the moment!
I'll try to keep a long story short - I live in the UK and have a right hand drive cadillac which had an auto transmission problem...No Reverse Gear!!!!!
I paid a "recommended" garage £3700 !!!!!! to sort it out - it's now on it's 4th visit because they are struggling to repair it. Basically they have now fitted the auto gearbox 4 times and everything seems fine in the gearbox except when driving at 70mph the speedo is erratic (displays any number between 70 - 105 in about 1 second!) . Also... the cruise control does not work at any speed!
They have now replaced the electronic speed device in an attempt to fix the problem but this has not cured the faults.
I had a similar problem with my Speedo. Replaced the 20-dollar Speed Sensor that mounts on top of the Transmission tail-housing (accessed through the passenger-side wheel well) and everything was fine.
Automobile(s): White Diamond '03 DHS (with floor shift)
Re: Erratic speedo and no cruise control
The sensor Rock is talking about is the VSS (Vehicle Speed Sensor). The input speed sensor however, is a $20 sensor, $2000 deep in the trans. That said, I am not sure which would cause your problem but, I would venture to guess that it is one of them. A FSM would be invaluable.
Check for codes to see what comes up. Just as an example, P0502/3 are VSS codes. I had both of these, and replaced my VSS a few days ago, with similar symtoms (my speedo would sometimes just drop all the way to 0 though).