W123 alternator denominator - PeachParts Mercedes-Benz Forum

zsmith29 · #1 03-22-2008, 09:21 PM

My 300D has not been starting for me like she used to(a quick jump and she starts right up). I went to Autozone and got the battery/alternator tested.
I got an average reading of around 12-13 volts and 35 amps. The amperage reading was at 2000 rpms.
Is this so low that I need a new alternator.
If I do need a new one I would like to get one that puts out more than the stock 65 amps.

dawsonj3 · #2 03-22-2008, 09:24 PM

Have you checked to ensure that the voltage regulator is still servicable? These alternators usually fail only 1 of 2 ways: the voltage regulator is worn out (can be replaced) or the alternator bearings are failing (time for a rebuilt alternator). See www.dieselgiant.com for a procedure to inspect and/or replace the regulator. How old is the battery, and have the techs tested it to determine that it is still serviceable? How often do you drive the vehicle? Several weeks of sitting idle can cause a battery to drain somewhat.

zsmith29 · #3 03-22-2008, 09:41 PM

I drive the car everyday and have been for several months now. I replaced the voltage regulator w/ a salvaged one. The brushes on the new--er one had about twice the length as the one that I took off my alternator. I will check out dieselgiant's page to see what he's got.

The battery checked good at Autozone.

I don't mind changing out the alternator w/ a reman I just want to be sure that I need to do it b/4 I do it.

zsmith29 · #4 03-22-2008, 10:01 PM

Is 35 Amps low at 2000 rpms?

vstech · #5 03-22-2008, 10:19 PM

Quote:

Originally Posted by zsmith29

Is 35 Amps low at 2000 rpms?

it's the 12-13 average volts at 2000 that is the killer.
35 is low too. it should be upwards of 60, but autozone only tests what it's drawing, not what it will put out. unless they pulled the alt and took it to the bench... NO WAY a free alt test went that far on a 123...

anyway, do a search for saab 115 amp alternator, they are almost a drop in...
or get a gasser MB 80 amp alt.

running-snail · #6 03-23-2008, 07:49 AM

Gasser Saab 80 amp units will work from 80's; early 90's. Also BMW - a 90amp unit from a 325is - looked like it would work just fine if clocked.

These upgraded alternators will require a change in wiring = a good thing given the size and age of the original wires running from your stock alternator.

zsmith29 · #7 03-23-2008, 08:38 PM

Ok that is what I was wanting to know about. The Saab 115 amp alternator is what I am thinking about getting. Does anyone know what years and model will bolt into my '84 300D.

J-Bear300TD · #8 03-24-2008, 12:03 AM

Check your Main Engine-to-Chassis ground. If that's loose, the alternator won't be properly grounded because it get's it's grounding via the bracket it's attached to the engine with. It's a crafty little demon. Checking this will save you a lot of time, money and frustration. It's located on the Driver's side just behind the left front wheel. You should be able to look down and see this braided cable. You'll have to lift the car to tighten it.

zsmith29 · #9 03-25-2008, 02:25 AM

It checked out ok, a little greasey but I took it off and cleaned it.
Anybody know anything about using an alternator w/ higher amperage output?

daw_two · #10 03-25-2008, 06:40 AM

Sound just like what my w123 went thru last fall......The initial test of the battery was good; but 3 days later, the battery test failed. Sounds like a failing cell in your battery. Charge the battery up and then go to AutoZone and have them test your battery.

Of course, I replaced the alternator first......so maybe it was the alternator AND the battery. Or maybe the failed alternator caused my battery to die.

barry123400 · #11 03-25-2008, 10:40 PM

Could be the reverse. A battery that will not charge up properly keeps the alternator working too hard. It is just trying to do it's job but the battery will not cooperate. .