In this vehicle, we already have replaced the alternator, but we're still getting the battery warning light on the dashboard; there might be another issue like poor ground connection, blown alternator fuse, and sometimes an ECU problem.
In this video, we verify if the new alternator is working, we test the positive terminal of the alternator for 12 volts, and we checked the feed in the voltage regulator.
What you will need
Follow these steps to test the alternator on a Mercedes Benz.
First, we will check the positive terminal of the alternator, connect the positive test lead of the voltmeter to the alternator's positive terminal, and connect the negative test lead to the ground from here.
We read 12.93 volts which is the battery voltage running to the alternator positive terminal. When we start the engine, the voltage should increase to 13 to 14.5 volts. With the engine off, the voltage should decrease back to 11.5-12.9 volts.
Test Voltage Regulaor
Next, we will provide a signal voltage to the voltage regulator by hooking a wire from the battery directly to the signal terminal of the voltage regulator; we should see the voltage will increase in our voltmeter, the voltage may vary depending on the voltage required to charge the battery, the signal may provide 6 volts or full load, and that is 12 volts.
Disconnect the alternator connector, connect the jumper wire to the voltage regulator's signal terminal, start the engine. While the engine is running, connect the other end of the jumper wire to the positive post; the voltage should increase to 14 volts.
The voltage remains the same, so we have verified that the problem is our new alternator.
- Please do not connect the jumper wire to the battery terminal for too long; remember that the signal terminal varies its voltage. The alternator might overload.
- Consider checking your vehicle's wiring diagram before performing the same procedure because, in some vehicles, the module provides a negative signal.