Q: My car is a 93 bmw 740i and there is a fuse that keeps blowing when i turn the a/c on. If i have the a/c on when first starting the car, the fuse will blow and the car wont start. it is fuse # 17 and it is responsible for the on board computer and the electric windows and the lights for the dashboard and the theifproofing system. if the a/c is off the car will start normally but when i turn the a/c on it will blow, the battery light will come on and the LED display will start nonstop irratic flashing and it will say "FASTEN SEAT BELTS" then when i turn the car off i will have to replace the fuse again before the car will start. i also pushed the check control button and it said "BMW NR and then 7 numbers that i cant remember. if anyone knows where i can look up the codes please let me know. Thank You
A: You have a complex problem here. Fuse 17 is not even directly concerned with Automatic Heating/AC Control (for AC/Heating is Fuse 20). Fuse 17 provides power to a lot of circuits/modules some of which are engine computer (includes starting), transmission, mirror memory,ABS/ASC, central body electronics, etc. etc. Fuse 20 concerns central locking, rear lights, antitheft, AC/Heating Both 17 & 20 affects power windows, sunroof, instrument cluster/check control, electric throttle, brake lining light, and onboard computer. Why turning on the AC blows fuse 17 and not 20 is the problem. Perhaps, the joint controls they have are adversely affecting fuse 17 and not 20. I think that since the ignition key (fuse 17) provides power to the AC switch (not the AC system/module), which switches the final stage (aka blower resistor), and switching voltage to the blower relay, a short in either the blower relay, final stage, or blower motor will blow fuse 17. The logical repair would be to first replace the blower relay, if fuse 17 still blows then replace the final stage, if the fuse still blows, then replace the blower motor. Another item worth looking into would be the AC Pressure Switch on top of the compressor. This signal goes to the engine computer. And since the engine computer also provides switching power to the compressor relay, a bad relay or compressor could also blow fuse 17(in addition to the pressure switch). You see, it is not a simple matter. Diagnosis like this cost 2.5 to 4 hrs. of labor.