Computer Modules

[richardf]

Hello again. And thanks for all of the help that I have received from the forum. I recently took my 1990 Cadillac Sedan Deville to a shop to check to see why my heater fan was not blowing. The blower must be fine because it worked up until my display center lights quit illuminating. The diagnosis was that there are three computer modules on this model year cadillac. Two of them are bad and need replacing. The only one that is not bad happens to be the one that controls the car starting and stopping (ignition). One module they dont make any more so they want to take that one off and send it to a company that makes that module and send it back for the repair job. Car will be out of operartion for several days they say. Said my car must have been struck by lightening. Does this sound right to you. Thanks for your comments.

[Cadillac Jim]

Your car has eight to twelve modules, depending on the model and options. The three they are talking about must be the ones involving the engine, instrument panel, and driver information displays. These are the only ones that an under-hood mechanic would be concerned with, and it's possible that a non-GM under-hood mechanic would talk about these as if there weren't others or even not know about the rest of them.

If your car had been struck by lightning, you would likely be seeing a lot more than a couple of popped modules. Also, you would see the entrance point as a large burn on the body or radio antenna, and the exit point as a large burn on the rim of one or more wheels and tires.

You can pop modules by using the wrong test instruments on them, or using test instruments incorrectly (this is called back-probing, and it puts voltage from an ohmmeter onto the interface chip of a module). This is rare and can be avoided by the use of instruments specifically designed for testing sensitive semiconductors and proper grounding procedures, but the FSM warns against it in test procedures for some connectors.

You can pop modules by using a battery jumper with reversed polarity. If the voltage of the positive terminal of the battery goes negative even by a volt or two this can pop electronics all over the car, including the eight to ten modules that they didn't mention. If this had happened, they would be telling you a different story.

You can pop modules by putting a fast-charge on the battery while it is still hooked up on your car. A fast charger can put 18 V to 24 V on the battery, and this is very bad luck on any car that has always-connected electronic modules, which is just about any car less than 20 years old, or that has a Check Engine light.

Most of the time when a mechanic decides that a module is bad, he does it by elimination; he can't find a problem that the module is reporting bad and he decides that the problem is the module itself. Most of the time when this happens, the module is fine. I can't tell without hearing a lot more, or looking at the car myself, but this sounds very much to me like what is happening with your car.