An R107 with a weak alternator kills batteries, dims the lights at idle and can make fuel injection electronics misbehave. Testing takes five minutes with a meter.
Normal charging voltage
With the engine running at fast idle and lights off, you should see roughly 13.8–14.4 V at the battery. Below about 13.2 V suggests undercharging; above about 14.8 V suggests a failed regulator and risk to the battery.
Battery light behavior
The red battery light should come on with the ignition on and go out once the engine starts. If it stays on, glows dimly, or flickers at idle, check the belt, then the regulator and brushes. If the light never comes on with the key on, the bulb may be out, and on many alternators that bulb circuit is needed to excite charging.
Brushes and regulator
Bosch alternators on these cars have a voltage regulator and brush holder on the back, held by two screws. Worn brushes cause intermittent charging. A new regulator and brush assembly is cheap and often cures it.
Belt
A loose or glazed belt makes the alternator slip, especially with lights and blower on. See belts.
Test steps
- Measure battery voltage engine off.
- Start the engine; measure again at 1,500–2,000 rpm.
- Turn on headlights, blower and rear defroster: voltage should stay above about 13 V.
- Check the alternator’s output cable and the ground straps.
- If voltage is low, remove and check the regulator and brushes.
Replacement and upgrades
Remove the battery cable first. The alternator sits high on the engine on most R107 V8s, so it is reachable from above. Rebuilt units are widely available. A higher-output unit is useful if you have added electrical loads, but not necessary for a stock car.
Quick answers
- Can a bad alternator cause a battery drain? Yes, a failed diode can drain the battery overnight; see battery drain.
- Do I need to disconnect the battery? Always, before removing the alternator cable.
- Can I drive with the battery light on? Only briefly; the car will run on the battery until it is flat.
Related reading
- Electrical troubleshooting
- Why is the battery light glowing faintly at idle? Often worn brushes, a weak regulator, or a slipping belt. Check charging voltage at idle and at 2,000 rpm.
- Does a rebuilt alternator last? A quality rebuild with new bearings, brushes, regulator and diodes is generally reliable. Check the warranty and return the old unit as a core.
