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JimSmith · #14 01-10-2002, 01:41 PM

Mr.W123,

I have changed the brushes and Voltage regulator on W123 Diesels a number of times, and I had the same indications you describe. In fact, if you are driving at night and the typical W123 dash lighting syndrome leaves you with a dimly lit instrument panel, you can see the battery light glowing a deep, dark red before it actually fully lights up. Turning off the head lights makes it go out, but leaves you driving in the dark. Anyway, I have always seen the dash light give me the warning that the alternator was not producing the necessary Voltage to charge the battery, just not always burning brightly, especially when the car is using some of the electrical power in the battery and there is relatively little Voltage left to light the instruments, and warning lamps.

There is a little spring clip over the back of the connector to the Voltage regulator that you push off and then unplug the wires going to the battery/electrical system. Then there are two screws holding the Voltage regulator to the alternator, that also hold the assembly in a position to compress the springs that keep the brushes (little quarter inch or so square cross section graphite rods about two inches or so long when they are new, and about an eigth of an inch long when they are shot) in contact with the rotor of the alternator. As I recall the contact was not with a typical smooth slip ring, but with copper bars embedded in the shaft which can get burrs and things on them that grind the brushes away quickly.

Anyway, you unscrew the two screws and the assembly is loose. You tip is slightly and the assembly comes out of the back of the alternator in your hand. If the brushes are little short things, they are shot. You should look at them to see if they have gouges or other signs of bearing on a rough surface, as the new ones will wear out shortly if that is the case. If one brush is significantly shorter than the other, this is an indication of potential mechanical and electrical unbalance in the machine. If there is any indication of roughess on the rotor contacts, or unbalanced mechanical or electrical performance, you should change out the alternator, or you will be chasing this issue over and over.

A new Voltage regulator assembly typically used to cost under $20, and the alternator rebuild was $65 to $120, depending on where you got it or had it done. So the temptation to just address the Voltage regulator and brushes (a half hour job at most) should be resisted if there is reason to believe the machine has other problems. On the other hand, I have had the Voltage regulator change work for 3 or 4 years, and 75k miles when all that was wrong was the two brushes just wore down and no longer made good, continuous contact.

Getting the alternator out is not difficult, but you have to make sure you are turning the correct screws. I have not done this in quite a while, and would suggest you check out a manual. I know I have bent up the adjustment rod and bracket doing this job before, and made things unnecessarily difficult by being thick headed and proceeding without a manual.

Good luck, Jim