Car is a 89 IROC, 350 TPI, 700R4.

I posted a question about the Autometer speedo the other day in the General Tech forum. I also contacted Autometer who suggested I call speedometersolutions.com. They told me they make an electric adapter that screws in where the factory cable does and it sends a signal to the computer so it thinks everything is normal. Also he said that the Autometer Speed would work correctly off of it.

My only thing is he's saying the computer uses the speedo/VSS as part of the engine management stuff. Is this true or is he just trying to sell me the adapter they make. The speedo was broken on my 86 and I never had a minutes trouble with it as far as drivability or fuel economy. If it doesn't have anything to do with engine management I can just get the 90-92 electronic speed sender. He said the computer in the 89 uses a 2 pulse setup and the 90-92's send a 4 pulse signal. If the Autometer speedo can work with either why does it have to connect to the computer at all. What does the computer do with the vss signal? If it doesn't do anything with the engine why does it matter as long as it works?

 

If you have an automatic transmission, you will lose torque converter lockup feature without a speedo signal into the ECM. There may be other issues as well, but that's the glaring one that stuck out when I thought about this.