TL;DR
The expensive mistake with an E36 charging fault is replacing the alternator when the real problem is a dead battery, a bad ground, a slipping belt or worn regulator brushes. Test in order from cheapest to most expensive. Bentley’s figures: a healthy system regulates at 13.5–14.5 V, and more than 14.8 V points at the voltage regulator.
How the system fits together
- The alternator is belt-driven. Its voltage regulator, which carries the brushes, bolts onto the back of the alternator and can be replaced on its own.
- Terminal B+ at the back of the alternator is connected directly to the battery.
- Terminal D+ gets battery voltage through the charge warning bulb when the ignition is on or the engine is running. That small current excites the alternator. If the bulb circuit is broken, the alternator may not start charging.
- On the E36 the battery is in the boot, on the right-hand side.
So the warning light does two jobs: it tells you whether the alternator is producing current, and it is part of the excitation circuit. That is why Bentley’s first check is the bulb.
Diagnostic order
1. Belt, cables and grounds
- Drive belt: check its condition and that the automatic tensioner holds it properly. A slipping belt can make the warning light flicker at idle.
- Battery cables: clean and tight at both ends.
- Grounds: check the cable from the battery negative terminal to the body, and the ground strap from the engine to the body.
2. Battery state of charge
Load the battery with about 15 A for one minute (headlights on, engine off). Then disconnect the negative cable and measure the open-circuit voltage with a digital voltmeter:
| Open-circuit voltage | State of charge |
|---|---|
| 12.6 V or more | Fully charged |
| 12.4 V | 75 % |
| 12.2 V | 50 % |
| 12.0 V | 25 % |
| 11.7 V or less | Discharged |
Below 12.4 V, recharge the battery and test again before blaming the alternator. If the voltage is fine but the battery still won’t start the car, have it load tested. Under a 200 A load for 15 seconds it must stay above 9.6 V at 27 °C (9.3 V at 4 °C, 8.5 V at −18 °C).
Charge at 6 A or less, never above 16.5 V at the battery, and keep sparks and flames away: the gas is explosive.
3. Charging system check
- Ignition on, engine off: the charge warning light must come on. If it doesn’t, fix the bulb or its wiring first; otherwise the alternator may not excite.
- At the back of the alternator: check for battery voltage between B+ and ground. With the ignition on, check for voltage between D+ and ground. If either is missing, the fault is in the wiring.
- Engine running: measure at the battery. 13.5–14.5 V is normal. Above 14.8 V means the regulator is most likely faulty.
- Output under load: ideally with a load tester. Without one, run the engine at about 2,000 rpm and switch on every big load: lights, fans, rear window heater, wipers. Battery voltage should stay above 12.0 V.
Never disconnect the battery with the engine running. It can damage the alternator and the engine electronics.
4. Regulator and brushes
The regulator comes off the back of the alternator with a few screws. Check the brush protrusion: minimum 5 mm. Clean the brush contact surfaces and check the slip rings for wear. BMW doesn’t sell the brushes separately, but aftermarket brushes exist and are soldered in. A complete aftermarket regulator is the simpler fix.
5. The alternator itself
If the wiring, battery, belt and regulator are all good and output is still low, the alternator is the likely culprit. Many parts suppliers can bench-test it, including the rectifier diodes. A replacement should have the same rating as the original; the maker and ampere rating are marked on the housing.
Battery goes flat when parked
That is a parasitic drain, not a charging fault. To measure it:
- Switch everything off and close the doors.
- Connect a digital ammeter between the battery negative post and the negative cable.
- Wait at least one minute for the reading to settle.
Bentley’s figures: 0–100 mA is normal, depending on how many systems need constant power; 400 mA or more points to a problem. To find it, pull fuses one at a time until the current drops; that circuit holds the drain.
Torque values (Bentley)
| Fastener | Torque |
|---|---|
| D+ wire to alternator (M6 nut) | 7 Nm |
| B+ wire to alternator (M8 nut) | 13 Nm |
| Alternator pulley (M16 nut) | 60 Nm |
| Alternator to bracket | 43 Nm |
To remove the six-cylinder alternator, release the belt tensioner by prying off its cover and levering it clockwise. Mark the belt’s direction of rotation if you are going to reuse it.
After an M43 → M50 swap
The M50 uses its own alternator, bracket and belt drive, so fit them as a set from the donor engine. Check that the B+ cable reaches the alternator and that the D+ wire from the warning-light circuit is connected to it. Make sure the engine-to-body ground strap is fitted and tight. Measure the charging voltage on the very first start.
