Alternator, Please Help
 

Ok, I started to drive my 79 300SD with 75K miles and after two days it died. (drove two nights with the lights on 35 Min drive) I went to start the car on the third day and my new 1000cca batt was Dead as a doornail. I charged it (13.5VDC measured), started the car and the voltage output would not get above 12.7 VDC. With the lights on voltage slowly decreased. Also I pulled the connector off the back of the alternator and it had some corrosion. I slipped the connector on and off to try and clean it up I also jiggled it while monitoring the voltage (engine running of course), no change.
I need to straighten it out this weekend. I am very well educated as I have had many American alternators apart and rebuilt them. I have read some previous threads about the brush assembly coming out with the regulator, does this apply to the 79 300 SD? This sounds like a dream come true after dealing with GM regulators. So let me get to the point, should I replace the brushes, regulator, or consider a rebuilt alternator, or am I missing something altogether.

Help!
PeterG

Peter,

I am not absolutely sure about the alternator in your vehicle, but the engine was the same as the later w123 300D Turbo, so I will assume the alternator was the same as the alternator that came with that car.

To answer your question, yes, the brushes are part of the voltage regulator assembly. The plug you disconnected on the back of the alternator is plugged into the voltage regulator assembly. It should have had a little wire spring clip holding the plug in place.

Anyway, the voltage regulator comes out by pushing the spring clip off the plug, unplugging the plug, and then unscrewing two screws on the "ears" of the flange on the voltage regulator. The unit then kind of rolls down and to rear, out of its openning, in the back of the alternator.

If the brushes are really small nibs, like a quarter inch long or so, they are shot. They are about an inch or more long when new.

If they are evenly worn, you only need brushes.

If they are unevenly worn you have either an electrical or mechanical imbalance in the machine and the alternator needs to be rebuilt. I would get one from FastLane that is rebuilt by Bosch. If the brushes are evenly worn, but the wear surfaces look rough and gouged, there is a problem on the copper contacts on the rotor, like a burr or other form of raised metal that is causing the gouges. Replacing the brushes in these instances will likely work for a few days, weeks or months, but once you have a mechanism that causes increased wear, changing the soft part of the wear couple will not solve the problem.

Hope this helps, Jim

It can't be said better than what Jim indicated. Let me assure you that a new voltage regulator might not do the trick for the long haul if one of the diode's is messed up. The tracks that the brushes rub on must be in good condition. One of mine was messed up and the new voltage regulator wore out in one month's time.

You can shine a flashlight up into the back of the alternator once the VR is removed. This should reveal the general condition of the two tracks on which the brushes rub.

I think the number you want is 13.8 volts and this is easily achieved when the two components are functioning properly.

Don

Thanks Jim and Don for your input.,
Do you think with the low milage, could I get away with just the Reg, and Brushes if the slip ring looks good, or would the bearings be questionable. I dont know how long the bearings last in the MB alternators, Vs american, and how reliable are the diode packs on these? Can you replace just the diodes?



79 300SD 75K miles
89 Jeep 180K
69 Shelby GT-350 Convt, soon to be sold.
96 Chrysler Concorde, (Maybe they might learn something from Daimler about front ends) 120K and has had problems with steering since purchased.

Hopefully yes. I suspect that it'd be fine just to pop the VR in. Might last forever, who knows. Mine would have if the ring hadn't been trashed.

You might do a search because some guys have been successful in just replacing the brushes in the current VR ($3.00 only). I took my VR to the place that rebuilt the alt and they put new brushes in free cuz they were doing the alt anyway.

Don

Peter G,

Once you get the brushes out you should look at them. If they are evenly worn, a new voltage regulator (used to cost under $20, but now it is anywhere from $40 to $55) which comes with new brushes, should be all you need. I find changing the whole thing easiest, but there are more frugal subscribers to this site that actually go out and find new brushes for a few dollars and replace only the brushes. That requires some skill with a soldering iron, I believe, as the brushes are spring loaded and you have to make a mechanical connection (retain the brushes so they don't fall out with the springs) and an electrical connection.

If the brushes show any other signs of damage, you are better off with a rebuilt unit, in my opinion. By rebuilt I mean new bearings, new or refurbished diode ring, new or refurbished rotor, and a new or refurbished stator winding. Most local rebuild shops do one to three of the above, and nearly none will give you a rewound stator. As the windings age and are exposed to the elements, as well as the rigors of being asked to charge a Diesel automobile's battery on a daily basis, the winding insulation can get tired. It will only show its weakness at high operating temperatures, which is when you need it most, and many rebuild shops do not do an insulation system check with the stator and rotor at full rated temperature. So they don't see a need to rewind. Bosch on the other hand should be doing this routinely.

Hope this helps. Jim