Alternator Wiring - PeachParts Mercedes-Benz Forum

pleiades · #1 03-12-2010, 03:41 PM

('82 300CD)

History: My alternator was giving me some trouble last May, so I pulled the voltage regulator, which had worn brushes, and replaced it.

Charged well after that; the dash light would illuminate intermittently but it seemed to be working fine; drove it 500 miles without a hitch.

Until the other day, when it seemed to have a little trouble starting, and later in the drive the headlights ran down and the electrical system cut out (thank God for diesels!)

Trying to root out the problem (with the voltage regulator eliminated) I pulled the alternator wiring harness and found that there's a short between all three connections, as well as between the three connections in the small electrical box just forward of the battery itself. I tried this with and without loading the system (turning "on" the lights).

This can't be right.

Diesel911 · #2 03-12-2010, 05:40 PM

Don't forget to follow up and fill us in on what you found. Have not read of this happening before.

pleiades · #3 03-13-2010, 11:49 AM

Holy loose belts, batman: the alternator belts worked loose enough that I could turn the alternator fan by hand. The belts are now tightened properly.

Looks like these apparent shorts were just due to "ghosting" from the various nodes and semiconductors in the charging/electrical system circuit.

I pulled the dash charging/battery light; it had an intact filament and continuity across the contacts. To verify function, I reassembled the dash and then checked the "small blue wire" connector on the alternator wiring harness with the key out and with the key in accessory/glow. No voltage with the key out, battery voltage with the key in/car glowing, which confirms continuity through the dash/bulb portion of the circuit.

Now, with the alternator wiring harness reattached, I fired up the car and noticed two things:

1) Battery light at dash does not light with key in/car glowing when all other idiot lights come on, indicating no continuity through this part of the circuit.

2) Alternator does not appear to be charging at all. Voltage drop across battery terminals is a steady 12.5 volts with or without engine running.

From what I understand about how the charging system is wired, a fault inside the alternator or voltage regulator circuit is preventing battery current from flowing to the field windings, preventing the alternator from charging. Correct me if I'm mistaken.

Having just changed out the voltage regulator in May of last year, I'm certain it isn't the brushes (I'll pull the regulator next to check). Either a diode in the rectifier network has blown (another the symptom should be no/poor charging with an AC ripple across the battery terminals; my vehicle exhibits a low (order of 50 mV) AC ripple which I understand to be normal for this alternator) or some of the solid state wiring in the voltage regulator has gone bad.

My hypothesis is that poor charging (which can be attributed to the loose belts) caused an undervoltage in the battery (500 mV when I pulled it for trickle charge) which led to failure of the voltage regulator.

Anyone have any thoughts?

Diesel911 · #4 03-13-2010, 03:39 PM

Some what over my head.
At the Connector as you said you should get no Voltage from the Blue Wire measured at the connector; until you turn the key on.
The other tow Fat Red wires at the Connector should have Battery Voltage all of the time.

I cannot find the pic of it but I made a test Cord for my Alternator. I used an Alternator Connector from the junk yard and wired a singel wire to the 2 large Femal Spade Terminals on the Connector and where the Blue Wire comes out of the connector I put in series a 3 watt light bulb (to take the place of the Dash warning light) and soldered it to the single wire. I put a small Battery clip on one end.

When I plug the Test Cord Connector into the Alternator and clip the other endi to the + Terminal on the Battery iit duplicates the circuit and the Alternator charges.
So If I think there is a problem with the wiring I can use that Test Cord to check that.

Any chance that it could be a problem in the Ignition Switch?

snookwhaler · #5 03-13-2010, 04:22 PM

Here is a thread that I put up weeks ago about "no warning lights".

Warning lights on at start?

In my case it was a voltage regulator problem (worn out). Did you install the regulator with the alternator in the car? If so, it is possible that you could have installed the regulator so that the spring is not in the "slot" on the regulator. It is also very easy to break off a brush if you install the regulator with it in the car. The above will cause the problems you speak of along with the "no charging" syndrome. It is also possible that one of the brushes on the regulator is "hung up" in the bore and not contacting one of the slip rings.

If the belts were loose, I would DEFINITELY pull the alternator and inspect the bolts. The lower bolt has to be really tight or it is subject to vibration and premature wear. While it is out of the car you can pull the regulator again and have a good look at the alternator, take it in and have it tested, etc...

pleiades · #6 03-15-2010, 06:17 PM

I think the belts were loose because the pair I had wasn't matched well. One was much tighter than the other. The two on there now are both pretty evenly tight.

I pulled the regulator to check the brushes, which were fine; reinstalled just for kicks and the problems persist. A new regulator is on the way from Phil, should be here tomorrow.

I'm still a little confused by the fact that the dash light circuit seems to be working when not attached to the alternator- no volts at the blue wire with key out, battery volts at the blue wire with key in and energizing circuits- but the dash light won't light when the alternator is plugged in. Is there some sort of alternator/regulator grounding connection I could be missing?

[Edit]Additional Data:

There is an obvious grounding strap from the regulator the alt. case, under one of the regulator screws. Tested positive for continuity with alternator case (which has continuity with battery negative/motor/chassis ground).

Terminal on alternator which connects to "blue" wire (field/regulator terminal) does not have continuity with ground. From what I understand of the schematic, it should. I'll report back when I have the new regulator installed; any comments or suggestions are appreciated.

pleiades · #7 03-18-2010, 06:30 PM

Installed a new regulator, no effect. Pulled the alternator and found the problem: no continuity between the slip rings. Dead alternator.

So in conclusion, if your battery light won't illuminate and your voltage regulator is known good, you have a dead alternator.

Diesel911 · #8 03-18-2010, 11:33 PM

Thanks for the Follow up.