Expert: Rob Painter - 2/5/2009

Question
HI  GOOD  MORNING       I'M  WORKING  ON  A  1990  SEDAN DEVILLE    I  CHECKED  FOR  A  BATTERY  DRAIN .   ITS    IN  THE     PASS KEY DECODER  MODULE.       PULLED  THE  FUSE   AND   THE  DRAIN  STOPPED .  IF  YOU  CAN  HELP   I  WOULD   CERTAINLY   APPRECIATE   IT  VERY  MUCH  THANK YOU      FRANK

Answer
Hi Frank,

I never heard of such a problem, however the question would be; what else is on the same circuit as the Pass Key Decoder module?

You will need to acquire a 1990 Cadillac electrical manual with schematics to determine if other circuits are also involved in order to isolate the exact problem. Just pulling the fuse will isolate the problem only partially. What else is tied in? The cigarette lighter, dome light, courtesy light etc?

You can try to acquire a manual from Ebay or possibly find one at your local library.

Not all the circuits are listed on the fuse box.

If you find that only the Pass Key Decoder module is on that circuit, you will have to replace that module.

Here is the problem if you go used: Unless you get the ignition lock and key with the used module, the engine is not going to start because you don't know what resistance chip is required for the module. Then, in that case to read the resistance required from the junk yard module, you will need a tool called an interrogator (glorified ohm meter). Once the resistance is determined, then you can go to a locksmith and get the correct resistor key blank and have a key cut to work your ignition lock.

If you go new, there is no code until it sees the first key (in your case, it would be yours) the code would then be burned into the decoder module.

The other problem you will have is that the car is 20 years old and the module for your 90 Caddy has been discontinued for 15 years.

Corvette used this antiquated, problematic piece of junk that was called an "anti-theft system" until about 2005. If all the terminals are the same as the unit in your car, you can buy one of these new. I think they are less than $200.

You can hope that the problem is not the module and something else in the circuit, but the only way you will know for sure is with a manual.

Good Luck!

Rob