Alternator/Charging issues on 300D - PeachParts Mercedes-Benz Forum

240dddd · #1 12-04-2010, 03:26 PM

I am helping a friend troubleshoot a battery charging issue on a 79 300D. The car has drained two known good batteries. However the charging light on the dash is not coming on, i.e. the light is working but goes out when the car is running.

Here's the troubleshooting steps I have done:

1. I pulled the bosch alternator and had it tested at a few places. All came back with failed result.

2. I bought and installed a new voltage regulator and had it re-tested. It came back failed again, however the person testing it said it failed on the "stator voltage" test (13.9 volts I think). He pulled the wire off of the single prong that is next the three prong connector and re-tested. This time it passed.

3. I put the old voltage regulator back on and had it tested again. The results showed failed again, but only on the stator voltage (14.2 volts this time). After pulling the single wire the alternator passed.

4. I had the shop (Napa) pull a new alternator for a 300d and test it. The new alternator also failed on the stator voltage. They tested a second new alternator with the same results. The stator failed but the rest of the alternator passed.

Not knowing what the 'stator' is referring to (napa didn't know either) it looks like the original alternator was functioning. In fact I don't think there is a second single wire seperate to the three prong wiring harness that attaches to the single prong on the alternator.

Two things I am requesting help on:

1. Can anyone provide any insight on what the stator voltage is and whether it's even relevant to the 300D?

2. If the battery isn't charging but the alternator is functioning, what would be the next troubleshooting step(s)?

thanks

JHZR2 · #2 12-04-2010, 04:10 PM

Sounds like a bad diode. I had this on my BMW. The alternator would put out voltage/current, but the diode (blocking device) was bad, so the battery would discharge back into the circuit.

The oddity here is that youve tried additional alternators with no luck (if Im reading it right). This would signal to me a discharge of some other sort... Not an internal short in the battery (since youve tried multiple), Id guess some sort of soft short in the car wiring that is not carrying enough current to set a fuse.

Id try both with the alternator connected and disconnected, to put an ammeter inline with the negative cable and see what the load is with the car off and sitting.

The alternators used in these cars are weak by today's standards, so may get flagged in a test though they are OK. It could be something silly liek a trunk lightbulb staying on...

Diesel911 · #3 12-04-2010, 07:03 PM

Alternator Diode check check with meter from my notes
With your meter across the battery, on Volts DC, you should read 12.5 - 14VDC

If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.

In post nuber 3 in the below thread I made an Alternator test cord that somewhat duplicates the wiring to the Ignition sitch and Battery Charging light. I keep posting the pics but so far as I know no one else has made one.
'83 W123 battery drain problem
'83 W123 battery drain problem

mach0415 · #4 12-05-2010, 07:01 AM

A stator is the outer windings of an alternator, which is inside the housing. The rotor is the inner piece that spins inside of it, and the pulley is attached to it. On the end of the rotor are 2 integrated slip rings, which your regulator/brushes assembly contacts. The stator is where AC voltage is induced. The more winding turns around the stator and rotor, the more amps produced. If you can see the picture, the stator has three windings that are separate, but bolt to a trio of diodes, each containing 2 diodes (so 3 groups of 2 diodes made into one "diode unit" or "diode trio." Diodes act like a check valve, because and alternator produces AC current, but car accessories operate on DC current. Therefore a diode's purpose is to convert AC to DC voltage. If we lose a diode, we lose one or more windings and we end up below a 13.5-15 volt (15 volts is typical charge rate for very cold weather) charge.

You can check a diode by using a DVOM after disassembly of the alternator. Make sure the diodes are disconnected to test. Good diodes will show continuity in one direction, using ohm setting (some have a "diode" test selection), but out of limits or "OL" in the other direction (switch your test leads on the component). OR You may test it and get either continuity in both directions or "OL" in both directions. If this is true, it will need to be replaced.

I would agree that modern alternator tests may not be adequate for these alternators, as they are computerized and looking at wave forms coming from various parts and matching them with certain programmed parameters. A DVOM is adequate to test an alternator's charging voltage and individual components. But a manual load tester will test amperage and voltage at a selected load to check charging amperage - which is a concern with the stator or rotor. This is where it can show proper charging voltage (and your light stays off) but amperage is inadequate to recuperate current flow to the battery under electrical load.

If I were you, I would find a shop that has a manual charging/starting system tester, like a VAT 40 (pretty popular device - google it) to test the good 'ole manual way. Good luck!

Skippy · #5 12-05-2010, 09:47 AM

I'm in the middle of a similar problem. My charge light didn't come on, but my alternator wasn't charging. After eyeballing schematics for a while, I installed a spare voltage regulator that I had laying around. The charge light came on, but this time the alternator was actually putting out some (not within spec) voltage. I checked the AC output and got over 20 VAC. I concluded I had one or more bad diodes. Unfortunately, I had to put the car in storage at this point, as I had to take a "business trip" overseas. I plan on putting in a new/rebuilt alternator when I get home.