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.)

1. 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.
2. 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).
3. 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.

The battery light is how the alternator first gets field current: a 2 W cluster bulb is only about 0.17 A. Contrast that with 3–4 A once it is running. That is why a worn regulator or a lazy excite can sit there after start until you blip the throttle — residual magnetism and a bit more rpm are enough to start generating. Wiring a ~12 Ω resistor in parallel with the bulb would let it excite at cranking speed, but then the indicator would not work.

Some later clusters put a diode between the alternator and the light, and a resistor so a burned-out bulb no longer means no charging. Early cars with a simple bulb in the D+ line will not charge if that bulb is dead.

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 big-blue manual says 14.5 V max; a tenth or two above that is usually fine, half a volt is not.

Modern flooded batteries are happier around 13.7–14.2 V. 15 V will cook electrolyte out of most of them. 1980s batteries were a different chemistry and lived with a slightly lower setpoint; some later cars never quite reach what a new battery wants. Aftermarket adjustable regulators exist, but they have been seen to spike — if you try one, watch it on a meter. One long-term report of a Regitar VRB195 (marked 15 V) ran a steady 13.8–14.0 V.

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.

Ground straps

Alternators with isolator / rubber bushings and a missing ground strap will not charge, even with a known-good alternator and a working battery light. Measure running voltage from B+ to the alternator case itself, not to the battery negative or the body. An M5 with isolator bushings and no strap was "not charging the entire time I owned it" until the strap went back on.

Cluster contacts

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. The light has to actually light to excite the field.

Several warning lights at once

ABS, Brake, Park Brake, and Charge all coming on together after start, then going out on the first throttle blip, is a common complaint. On a stock cluster the charge lamp going to ground (D+ not generating) is the only "official" reason for Charge. The others look like independent circuits on the ETM.

In practice:

• Many of those lamps also go through the SI-board processor, so a low-voltage or SI-board fault can light more than one.
• ABS (and possibly brake-lining) modules are voltage-sensitive — they will light the lamp below about 13 V even if the switch itself is fine. Park brake is a dumb switch.
• When it happens, measure at the alternator right then. More than once the alt simply was not charging, and the other lamps were along for the ride.
• Worn regulator brushes have cured the "all lights until I blip the throttle" version.

From the discussion

Additions above are from the FAQ thread: Mike W., RossDinan6, austin8753, [[1]], Chicagotrader92, and tig.