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Fixing a thousand dollar Bosch alternator
for twenty-five dollars in parts

Is your multifunction dashboard display lit up with the "battery/alternator visit workshop" message of impending doom? Are you worried because you've read on the Internet that an alternator costs over $1000? Well, it's true - the alternator in my CLK has a dealer list price of $1050. Rebuilt ones at parts houses list for $650.

Don't be shocked by the prices. You bought the car because it's a well built, expensive German vehicle. These cars have a lot of features, and maintaining it may cost a little money. For some with budgetary concerns, the solution is to take it to their local independent service shop. They may prefer the independent because they feel the dealer charges too much.

Once upon a time, that wasn't a bad idea. Most experienced technicians knew almost everything there was to know about automotive charging systems. Five minutes with a test light and a screwdriver would typically reveal the root of the problem. The basics haven't changed all that much.

Unfortunately, the details have. First, many people don't diagnose anymore. They plug the car in to a diagnostics machine, and that machine tells them what needs to be replaced. Unfortunately, the diagnostic machine for a Mercedes Benz is a Star Diagnosis System (SDS). These are expensive and the overwheming majority of independent shops don't have them. A common alternative is to continually replace parts on a car until the idiot light goes out. That's not an economical solution when Bosch electronics are involved. In this case, a dealer, or someone with experience, would be a much better choice.

The service writers at most dealerships know the alternator itself usually doesn't fail. The voltage regulator is what usually fails. The voltage regulator fails because it's a normal wear part. Being a normal wear part, the voltage regulator is inexpensive. After the voltage regulator has been removed, comparing the old part to a new one will quickly show the spring loaded carbon brushes on the old unit are a lot shorter than the new. Once those brushes stop making contact, the dashboard warning will appear. A Bosch voltage regulator is about $25.

Those who are experienced at replacing Bosch voltage regulators follow a procedure that's something like this:

1. Put the car on ramps or jack stands and let it cool down.
2. Disconnect the negative battery cable.
3. Remove the bottom engine compartment panel.
4. Loosen the accessory drive belt, and slide it off the alternator pulley.
5. Disconnect the plug in the rear of the alternator.
6. Remove the cover and the nut, then remove the cable from the alternator.
7. Remove the two bolts that hold the alternator in place.

At this point, friction is the only thing holding the alternator in place. A big flathead screwdriver/pry bar may be needed to remove it from the bracket. Fortunately, it slides back in a lot easier than it slides out. Some may be wondering, "Slide where?" It's a tight fit on a V8 powered CLK, I believe it's easier with a V6. From the front of the car, the alternator will slide to the left to clear the bracket, then toward the rear of the car, then down, and out.

Now the voltage regulator can removed from the alternator. That's accomplished by

1. Removing the screws numbered 1 in the drawing.
2. Removing the cover.
3. Removing the screw numbered 4 in the drawing.
4. Removing the voltage regulator.

Some people can do this with the alternator in place. I think it's easier with the alternator out of the car. Reinstalling it is the reverse the removal procedure.

© 2009 Marcus Blair Fitzhugh
Signal to Noise