Fri Nov 20, 2020 11:46 pm
There are certain warning lights that are required by regulations to have a system to check that the lamps are in working order. On an E30 these are the brake hydraulics, the handbrake, ABS and brake wear.
I'm going to make a couple of assumptions - your car doesn't have ABS, and the brake wear indicator has been 'dealt with'.
The check system turns these lamps on with the ignition switch, and then senses the voltage on the blue wire from the alternator to turn them off when the engine starts.
The blue wire also controls the battery charge light. 12 volts is applied to one side of the lamp from the ignition switch when it's turned on, and then, when the engine is started, an equal voltage from the alternator, on the blue wire, is applied to the other side, so it goes off.
Your charge warning light is staying on, so the chances are the alternator is not producing the correct voltage on the blue wire. Remove the regulator pack on the back of the alternator (two small cross head screws - remove the air filter assembly, and its a five minute job) and check the state of the carbon brushes, but it's likelt to be a failure of one of the diodes contained on the diode pack in the body of the alternator.
