voltage problem
 

i have a 1994 325i runs great until you turn blower motor on high or rear defrost on then it will start to bog down and red engine light comes on i checked to see if there was 5 volt going to sensor i got it check alter getting 13.2 does anybody know what this is it is also blowed ac relay and fuse but compressor is good please help driving me crazy:

Check grounds in the system, 13.2 seems a bit low but not out of range for normal, do you have an amp clamp to check the alternator amp output? (Voltage may be ok but not enough amps)

+1 Sounds like the alternator output is at rock bottom.