bmwrepairs: Testing starter solenoid on BMW?

Solenoid or Relay

Usually the starter and solenoid are replaced as an assembly. Finding a replacement solenoid is not as easy as finding a complete new or rebuilt starter. It's very possible the cost of the solenoid could be equal to the cost of a rebuilt starter motor. If the solenoid is worn, chances are that the starter motor has an equivalent amount of wear. Replacing both as an assembly reduces the chance of having problems with the starter motor soon after replacing the solenoid.

To replace the solenoid:

Remove the starter motor.

Disconnect the copper braid lead from the solenoid terminal. Be careful not to twist and pull on the lead as the nut is removed.

Remove the 2 or 3 screws at the nose end of the solenoid. A hand impact and Phillips bit may be necessary.

Remove the solenoid. Use care to not lose the spring in the solenoid.

To install:

Install the solenoid onto the body of the starter. Coat the threads of the screws with a suitable thread-locking compound and tighten.

Connect the copper braid lead to the solenoid terminal. Be careful not to twist or pull on the lead as the nut is tightened.

Install the solenoid and reinstall the starter motor assembly.

Testing

Starter Solenoid
Bench Test

Note the radio security code and the radio presets.

Disconnect the battery negative cable then the positive cable.

Remove the starter from the vehicle.

Disconnect the field coil wire from the field coil terminal.

Check for continuity between the solenoid terminal and field coil terminal with a continuity tester. Continuity (resistance) should be present.

Check for continuity between the solenoid terminal and solenoid housing. Continuity should be detected. If continuity is detected, the solenoid is good.

If continuity is not detected in either test, the solenoid has an open circuit, is defective, and must be replaced.

On Vehicle Test

Before testing, assure the parking brake is set, the transmission is in Park (automatic) or Neutral (manual), and the battery is fully charged and in good condition.

Connect a voltmeter from the small terminal (terminal 50) on the solenoid to ground. Turn the ignition switch to the START position and test for battery voltage. If battery voltage is not found, inspect the ignition switch circuit. If battery voltage is found, proceed to next step.