I have a 1990 Deville 4.5L. When I bought this car I was told by the dealer that I had to fix the PRNDL (I trust the dealer, he's family). How do you test for the PRNDL? And how do you fix it? I know the code is E91 but how do you run the diagnostics to find the code? It affects the automatic door locks(they don't open or lock when shifting to park or drive) and the clock on the radio stays on when the car has been turned on then shut off. I have to shut off the clock on the radio by pressing the door button on the outside of the car. It also bangs on the gear when i shift into reverse. Any help?
The PRNDL switch (once known as a neutral safety switch) is located on the steering column near the point where it joins the dash. It's a common failure item.
The OBD won't give you anything other than the E91 code if there's a problem with it. To access OBD, turn the key on and press and hold OFF and WARMER simultaneously on the CCP until all LEDs light, then release. Trouble codes will display in the Fuel Data Center, ending with the display .7.0 You can exit OBD by pressing AUTO or turning off the key.
The radio and clock are supposed to stay on after the key is turned off. It's called Retained Accessory Power, a feature built into all Cadillacs that allows use of powered items (radio, windows, trunk release, etc) for 10 minutes after the key is shut off, or until a door is opened.
The bang into reverse is not related to the PRNDL problem. Could be a motor or trans mount, or maybe something expensive. Check your codes first.
I also have a 90 Deville. My car doesn't have automatic locks. I didn't know they did. The 90 is an old car... You can even shift gears without putting your foot on the brake pedal
The radio stays on after you turn the car off because of the way the Cadillac is designed. You can also roll the windows up/down to verify this (start car, shut off, take keys out without opening the car door. you should still have power).
The radio should automatically turn off when you open the door to exit the car.
As for the clunk you probably have a bad motor mount (all of mine are shot). You can test to see if the motor mount is bad by opening the hood all the way, starting the car, put your foot on the brake, shift into drive and give it a tiny bit of throttle (not too much though). If the engine lifts up quite a bit the mounts are probably shot. If you rapidly shift through the gears you should also see the motor flop around.
The radio doesn't work( thats another problem). The clock stays on even after the door has been opened. The clock only turns off after i have pressed the button for the door. I don't think the motor mounts are bad but i'll double check. If the mounts were bad wouldn't it clunk in both drive and reverse? I checked for codes and all i got when i pressed off and warmer is .7.0 nothing else. What does this mean?