BMW E34 starter not turning or turning poorly
E34 starter does not turn or only clicks - what to check first?
Measure the battery voltage and condition under load. Then check the grounds: the battery ground and the ground lead between the engine and the body can be oxidised or loose. The starter draws a lot of current, so a weak contact is enough to prevent starting.
What if the starter does not respond at all even though the battery is fine?
The relay behind the ignition switch may have come out of its socket, so the control voltage does not reach the starter solenoid. Other causes are a worn ignition switch base and a broken main fuse near the battery. A fault in the ignition switch base can work intermittently.
How do I test the starter myself?
Turn the ignition on and put the gear in neutral. Connect the starter's trigger terminal to +12 V. If the starter turns, it is fine and the fault is in the wiring, the ignition switch or the relay. If it does not turn, the fault is in the starter or its power supply.
When does the starter need to be repaired or replaced?
If the starter turns slowly or engages poorly, the cause can be worn brushes or dirty contacts. The starter can be taken apart, cleaned and fitted with new brushes. The starter can be replaced from underneath. Cheap aftermarket starters have lasted poorly.