Questions and Answers

BMW 318i Blowing Fuses Q. Dear Mr. Ciulla, I have a 1985 BMW 318i with 300,000 kilometers and have been experiencing a electrical problem. The fuse for the defrost blower, tachometer, temperature and gas gage continues to burn out. Every time I replace it instantly burns out when ignition is started. Please advise what may be the problem and how I may fix it myself. Thank you, Max

A. You have a linear step by step troubleshooting process ahead of you. Instant blowing of a fuse indicates a high amperage draw or short in one of the systems this fuse supports. You will probably need a multimeter capable of measuring ohms to perform this process and a couple of jumper wires with alligator clips.

I would start with the heater blower as it is easiest to get to and also is the most likely candidate based on the amperage required to operate it relative to the other items you mentioned.

Locate the blower motor and unplug it. At this point you can either measure the resistance across the terminals of the motor or try installing a new fuse and turning the key on (you may go through a lot of fuses using this method). If the fuse doesn't blow you then know that the blower motor is the culprit and will need to be replaced.

If the fuse still blows, it could be the wiring to the blower motor. Using your multimeter on the RX1 scale or ohms on a DVOM check the wiring for shorts. You can easily verify the operation of the blower motor by running the jumper wires direct from the battery to the blower motor and momentarily touching them to the terminals, if there's a large spark and no movement of the blower then it is internally shorted and needs replacement.

If the blower checks out okay, then I would move on to the fuel tank sending unit next. You again can isolate it by disconnecting the connection at the tank and trying a new fuse, or you can measure the resistance with the multimeter.

Perform the same deductive checks you performed on the blower motor then move on to the next system. Eventually you will find the cause of the problem.