Seedometer 2x actual speed?

[Luminated]

Got two 90 Euros: unit "A" had a blown engine but good trani, unit "B" good engine poor trani. Took the good engine and trani and put them back in unit "A". So unit "A" has its original trani but different engine and all its components.

Just as a safety thing took it to my GM dealer for a once over and scope tuning on their $25,000 electronics and a "Technician" who knew what he was doing . They gave it a clean bill, except for the speedometer. It reads twice the actual road speed. Yes, I took the speed pickup out of both tranies and tried both, both gave me the same reading. Dealer was stumped and now wants to change the EMC (or PMC apparently they're both the same-who knew?) Anyway no guarantee but the bill and that seems a little steep for an experiment. Besides wouldn't the electronics pick up anything wrong with the EMC?

Anyone got any ideas? And direct me to a good code reader for this car, please!

[richtazz]

Most retail code readers for OBDI cars are nothing more than a fancy jumper wire molded in a block of plastic and a book of codes. Professional scan tools like the Snap-on MT-2500 are the only way to scan real time info on these cars. These tools are a little pricey for most DIY's. The next option would be a program and adapter set-up like Auto Tap, which require a PC (a laptop would be best) to operate. As far as the speedo issue, since you reused the original trans, this is strange. I would inspect the wires from the VSS to see if something got pinched, causing a signal issue. If the wires check ok, try disconnecting the battery, then touch the positive and negative cables together for 30 seconds or so. This will erase any possible residual memory in the ECM. Reconnect everything and see if it corrects itself.

[Blue Bowtie]

AFAIK, a '90 did not use an output from the ECM for the speedometer signal like most newer vehicles, but used a separate buffered output from a VSS module or a direct VSS input to the speedo head. It still used a vacuum operated cruise control servo and may not have had a 4K PPM VSS input to that.

I could be mistaken about this - Memory is a dangerous thing upon which to rely.

You can also check the VSS output with a two-channel oscilloscope. I'm not sure if the '90 used an optical or a PM tachogenerator type VSS, since that was close to the time when they changed.