I have a 1990 3.1 with about 60K original miles. Injectors replaced at 40K ran great, no problems. About a year ago, the Speedometer and A/C quit working at the same time. Now, intermittently, the Speedometer will work and the A/C works at the same time, but no speedo and no A/C. I can't figure out what to check or where to start. ECM? Vehicle Speed Sensor?

Further, the car doesn't perform well, but when the Speedo and the A/c work, the car runs fine and idles perfectly, but when the speedo and A/C are not working the idle is high (about 1000-1200 rpm) and the low speed performance is sluggish along with poor mileage. The transmission won't go into OD either.

HELP

 

Check the grounds on the backs of the cylinder heads and clean if necessary. Only common link between all of those systems is the ECM. May also help to pick up a used one from a J/Y (J-body) and swap. If the swap doesn't work, then at least you'll have a spare.

Also check for any codes that may have popped up.

 

Thanks for the advice. I will try the grounds and clean them. Where is the ECM located? I would like to try removing it, cleaning the contacts and trying it again.

 

ECM is on a white bracket under the passenger's side of the dash. Just slides out, but I've heard there's Velcro on it so it may take a bit of force to pull out if it's the stock unit. You'll need to remove the panel down by the passenger's feet to get at it (not the one attached to the front of the dash, the big one down by the feet with the light and grill in it for the blower motor).

 

I'd start with the ground on the back of the head. I forgot to hook that back up after my motor swap and took me 2 weeks to track down all the wires to find it. Once I hooked it backup my speedo worked great again.