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Known issue on 90/91 models, related to loss of proper resistance in the resistor chip. On this thread posted below, look for the first link Stu posted which covers the 90-91 repair technique. The rest of that thread relates to a different problem in my 96 than what you are experiencing, but some of the tear-down photos etc may be of benefit to you.
Do a search, lots and lots of info here along with the suggested above link. Upper right, hit "Search" In this box, click "Advanced Search" This feature will let you only search C4 specifics by selecting C4 General or Tech which is where you need to look. Scroll down til you see the Category section on the right side and you will see it.
Welcome to the Forum, there is a wealth of info here if you look.