In the world of alternators the one used on most all E30’s is quite simple and straight forward. The Bosch alternators are a a two post design, with D+ and B+ terminals.

B+ (also called 30) is the larger of the two and routes through the starter, then the battery junction box and finally to the battery. For all intents, its a direct shot to the battery, no fuses, or switches.

D+ is the exciter wire that is wired through the ignition switch and cluster. The exciter wire is critical, if it does not provide initial power to the alternator, the alternator will not charge.

With the key in accessory mode, and the car off, there should be a battery voltage reading at D+. If power is confirmed, the next step is to see if you have power at B+ while the car is running. If power is confirmed, you will have to confirm continuity in the the cable to the starter, then the battery junction, and to battery. If no power is confirmed, you have isolated the problem to the alternator itself; its either the voltage regulator, or an internal alternator fault.

There is an important distinction to remember between early (84-85) and later E30’s.

Later E30’s have a resistor built into the cluster, where even if the bulb burns out, it will still provide power to D+. In early cars, there is no such resistor; if the bulb burns out, there will be no power to D+ and the alternator will not excite and therefore will not charge.

This entry was posted on November 14, 2021 at 8:27 pm and is filed under Do it Yourself. You can follow any responses to this entry through the RSS 2.0 feed.