My odometer quit working check engine light came on and the car won't run unless I keep the gas pedal at 1500 rpm's. The odometer started working briefly and everything returned to normal but quit again and same symptoms. 5 speed transmission. I"m thinking the VSS went bad.

 

The one thing I can absolutely say with certainty is ; Your VSS malfunctioning will not affect the running of the engine in the way you describe.

You have something evil going on with your electrics/electronics for sure, but whatever it is, it ain't the VSS.

 

The next day I read the trouble code. It was code 51 which is "airbag deployed' . then I checked the connections on the VSS and they were good. While doing so I left the battery disconnected for about 45 minutes. The car started and ran just fine. I took it out to visit a transmission shop and hit some rough pavement on I-95 instantly got a check engine and the speedo stopped working and it felt like the engine timing was off. Looks like I might have a bad ground somewhere so tomorrow I will check all the sensor locations and see if I can find anything. I am worried that perhaps the air bag clock spring in the steering column might be bad. Any thoughts? Anybody ever have one of the SIR sensors or clock spring go bad?

 

Code 51 is a bad prom memcal. But can be the ecm not reading the prom and in limp home mode.
the Vss would be a 24 and cause stalling on deceleration or rapid stopping.

 

Well that makes sense considering the way the car was running? Would it be responsible for the spedo/odo not working also?
The memcal is the removable chip from the computer?

 

Not sure what your working with but if 90-92 1227730 tpi or 3.1l if the ecm messes up it is a Vss buffer and could cause no output to the speedometer. The best guess would be a bad ecm or ecm ground over a prom loosing data. All f body ecms for thirdgen are removable and will transfer to a new ecm same service number or rebuilt superseded.

 

It's a 5.7 tpi. so you're saying there is a bad ground within the ecm and the whole ecm including prom/memcal needs to be replaced?

 

I would check the grounds behind the passenger side cylinder head. If that’s ok and you still have code 51 I’d start with ecm replacement.

 

Thanks. I have a '91 Formula 5.7 so after I check the grounds and restart the '90 Formula if the same condition exists I'll swap out my 91 ecm to see if things change. really appreciate the help, really.

 

The memcal chip should probably say aujp on it . These very rarely fail but if it has a piggyback adapter like Hypertech I e see those go bad or cause connection issues.

 

I pulled the ECM yesterday and everything was as you described. I took the ECM out of my '91 Formula and put the memcal into my ' 90 and took it for a ride. About 3 miles into the trip the symptoms returned. ( I'm guessing that when the ECM reached a certain temperature it malfunctioned). Back to my garage and installed the complete ECM and memcal from my '91 into my '90. It took about a minute for the symptoms to disappear and after 20 or so miles it runs fine. Looks like I need a new ECM.
Again, thanks for your help.