E28 Charging System
By Mike W., Wed Dec 23, 2020
Original thread: FAQ E28 charging system
Written for E28s. It also applies directly to E23s and E24s, and mostly to everything from 2002s through E34s, with some detail differences.
70s–80s BMW (Bosch) alternators hardly ever fail. Regulators fail, brushes fail — the alternator itself very rarely. Replacing the alternator because the battery light comes on is like replacing the engine because it will not start. Most of what follows also assumes you have a good battery.
You need a meter. Set a digital meter to 20 V DC. Do not use the 10 A terminal.
Does the battery light in the cluster work?
The light is usually needed for the alternator to excite. (Sometimes there is a jumper inside the cluster — do not assume there is.)
- Confirm the small blue wire is on the alternator D+ terminal (the small insulated post). It started life blue; decades later it may not look blue. The terminal itself is often broken off.
- If the wire is attached, remove it and ground it. The cluster light should come on. If it does not, the bulb is most likely burned out, or there is a wiring problem between the cluster and the alternator (unusual).
- Fix the light first. You need it to diagnose anything else.
Then:
- Light comes on when grounded, but not when attached to the alternator → replace the regulator. The light should work with a bad regulator, but it does not always.
- Light flickers, or comes and goes → suspect the regulator, specifically the brushes.
- Light comes on with the key off → probably a bad diode. Replace the alternator, and disconnect the battery ground when you park if you want the car to start in the morning.
- Light comes on with the key, then goes off when the engine starts → check voltages (below). If it stays on after start, replace the regulator.
The regulator
It is two screws on the back of the alternator. Often rebuildable with new brushes; once in a while they just smoke. Calling it a five-minute job on an E28 overstates it, but it is quick. Brushes are cheap. A spare is a good idea on a long trip.
Voltages
Measure them.
| Condition | Expected |
|---|---|
| Engine off, everything off, charged battery | 12.55–12.75 V |
| Running, light to moderate load | 13.5–14.7 V |
| Running, at the battery, ~13 V | Might start in the morning, might not — and it will not crank long |
| Over ~15 V | Too high. Bright headlights, short bulb and battery life. An older BMW manual says 14.5 V max; a tenth or two above that is usually fine, half a volt is not. |
If running voltage at the battery is higher than sitting voltage, the alternator is working. Low running voltage is more often a regulator or a wiring problem than a dead alternator.
Some E28s have rubber or urethane alternator mounts to cut vibration. Those must have a separate ground strap between the alternator body and the engine or alt bracket. You also need the ground strap on the passenger side, bridging the motor mount from engine to body/subframe.
Voltage drop
If voltage at the battery is low, measure at the alternator: big red B+ wire to the alternator case.
If the alternator is more than a tenth or two above the battery, you have wiring problems. The E28 hot path is alternator → starter terminal → battery. Check each joint.
With A/C on high and high beams on, voltage may drop toward 13 V at idle and pick up with revs. A drop of more than a few tenths between B+ and the battery positive, or between the body and the battery negative, is the fault. Wires rarely fail; ends and terminals do. A tight, clean-looking terminal can still have resistance — you cannot see voltage drop, you have to measure it.
Worry about volts, not amps. Amps follow volts.
A phantom draw (something on that should be off) is a different problem. This page is only the charging system.
Notes from the thread
- Alternators with isolator / rubber bushings and a missing ground strap will not charge, even with a known-good alternator and a working battery light.
- A no-charge / no-light car with a good bulb and a good alternator has been fixed by pulling the cluster and cleaning / tightening lamp-holder contacts, terminals, and sockets.
- All the dash warning lights (ABS, Brake, Park Brake, Charge) staying on after start, then going out on the first throttle blip, has been cured by a new voltage regulator with worn-out brushes.
