alternator question - PeachParts Mercedes-Benz Forum

#1 09-22-2003, 02:45 PM

Hello to all you oilburners out there:

Yesterday, I experienced a flat battery in "the Black Bomber", my 1985 300d. For perspective, for the past month and 1/2, the battery lamp, the brake sensor lamp, and the oil pressure lamp were on SLIGHTLY until revving the engine. Then they would go out..............continued this way for about 1.5 months....I figgured that one of the fuzes was weak, needing to be replaced.
I ran MANY MILES with MANY STARTS off the battery. The charging system seemed to be holding its own. I thought that the regulator and brushes were on the way out.
Anyway the auto would not start. After a jump, she fired right up and I drove home the 35 miles with the headlights getting VERY DIM!!!

I made it home

....thanx to the oilburner.
I got a replacement regulator at pep boys of all places! Borg Warner.....$39.....This was installed after I made sure that the battery terminals were very clean. They were clean to begin with, as were the body grounds.
With the replacement regulator installed, and the car running for 5 minutes or so, the digital meter read that I was LOOSING voltage........If I really revved up the engine I might get an increase of 1/10th of a volt....not good.
I removed the regulator and CLEANED it off with alcohol and returned it to pep boys for a full refund.......excellent!
My father in law said that the brushes need to be "burned in for aprox 10 minutes or so" before voltage would flow properly.
The charge lamp on the dash did NOT light, which is a common symptom of the faulty brush pack regulator.
My question............if I replace the alternator with a genuine REBUILT BOSCH unit.........ARE THE 55amp and the 65amp units INTERCHANGABLE?????I mean the electrical connection as well as the mounting brackets.
Should I try to get another regulator and let it "BURN IN" longer.
Also, is the alternator in the car stamped as to weather it is the lower amperage or the higher amperage...........or only with the part number.

Thanx much and lets keep those diesels humming,
Spo out.

diesel don · #2 09-22-2003, 02:59 PM

thots:

1. the diodes on the alt could be bad thereby negating the help of the new regulator. this happened to me as i burned through two regulators in a week.

2. i simply had my alt rebuilt and i'm 40k into it now and works well.

dp

lrg · #3 09-22-2003, 03:15 PM

Besides Diesel Don's possibility it may be that the armature where the brushes make contact is worn. A good auto electrical shop can turn the core (as long as it isn't too far gone) so the brushes make a better contact and it should only cost a few dollars. Either way I thing you want to remove it and have a decent alt shop check it out on the bench.

gsxr · #4 09-22-2003, 04:32 PM

No offense to your FIL, but I believe he's incorrect. New regulators should work almost immediately. I believe you'll need to replace the alternator, or get it rebuilt. If those options are too expensive (which I doubt), a decent used one may be cheaper from a major MB salvage yard. And yes, AFAIK, the 55 and 65 amp versions should be interchangeable.

#5 09-22-2003, 05:45 PM

Gentlemen.....and I use the term loosely;
Thank you all for the quick input. On my way home from work today, I stopped at a local independant MB shop and consulted with Edmund, the shop owner. After he inspected my old regulator, it was his opinion that the brushes still had a little meat on them. Also, the brushes were NOT scored and burned.
He asked if the battery light came on when I turned the key fully. (not while running after the jump start)....I could NOT RECALL.
Edmund stated that the brushes allow continuity of the lamp circut on the dash. He thought that I probably need a replacement alternator..........Nice Guy and very competant mechanic......He also stated that he believed that all 123 series alternators have the same mounting bracket and the same electrical connection. They should be interchangable....................
...........What would you folks out in the forum replace it with (if NOT REBUILT)............This is NOT a question of what BRAND of rebuilt......ie; only a BOSCH FACTORY REBUILT>..........but, a question of weather or not to go with the 65amp unit...........it DOES get COLD here in the winter, and I WILL be driving the Black Bomber fo\r at least another 10 YEARS

.

Thanx again,
Spo out.

gsxr · #6 09-22-2003, 06:32 PM

In this case, bigger is better. I upgraded my W124's 70A unit to a late 90's 143A unit. It took some electrical modifications but works great. You won't see much difference between 55 and 65, but if you have to replace it anyway, I'd go with 65 assuming price isn't a big factor. Do some price shopping as well, a 65A alternator should NOT be very expensive.