A few years back, my 2011 GL550 had a red battery light pop up. I scanned it and the message came up with : "Operation fault of component G2 (Generator) P0620". I did some research and it turns out it could either be the entire alternator or only the voltage regulator which is a part of the alternator. The red battery light has never came back after resetting the fault codes, but for good preventative measure I decided to tackle this project and also replace the belt, tensioner and idler pulleys all at the same time. MAKE SURE YOU DISCONNECT YOUR BATTERY BEFORE STARTING! FCP Euro sells a kit for the belt/pulley/tensioner and also the voltage regulator. Part numbers are "Mercedes Drive Belt Kit - INA 272273BELTKT1" and "A0041541906" respectively. There are other DIY involving the belt/pulley/tensioner removal. Just search up M273 belt removal and be sure to have a diagram of the belt beforehand. After removal of the belt, there are only 4 screws, one positive cable, and one electrical connector holding the alternator in place. I did remove the engine belly pans for easier access from the bottom. Disconnect the positive cable and electrical connector first. Then remove the 4 screws. Note which screw has the ground cable attached. Slowly wiggle the alternator out.

Here is where it got confusing for me. If you pay attention to the electrical connector that you disconnected, shine a light onto the connector and you'll discover that the connector is a 2 pin connector but ONLY has 1 pin that actually conducts electricity. However, shine a flashlight onto the male end of the connector that is part of the old voltage regulator on the alternator, and you get a 2 pin connector with 2 spades inside. Then, go and open the new voltage regulator and you'll find that its a 2 pin connector with ONE spade inside. After bringing my alternator to the dealership, parts department, and service department, the service technicians were nice enough to clear up the confusion. One of the 2 spades on the 2 pin/2 spade connector on the old voltage regulator is a DUMMY connector and goes to nothing, so do not panic when you see the old regulator having 2 spades when the new one only has 1 spade. I only took a picture after I installed the new regulator:

New Voltage Regulator with 1 spade


Apparently, Mercedes made 2 types of alternators in the X164 chassis, Bosch and Valeo. Mine was a Valeo alternator and the voltage regulator was also Valeo. According to the service technicians at the dealership, the bosch alternators are more simply and less confusion/less of a headache.