Alternator failure mode - PeachParts Mercedes-Benz Forum

ah-kay · #1 08-22-2016, 01:09 AM

Just want to check with you guys before I buy a replacement alternator. 300SDL alternator.

1) Alternator is not charging. Battery flat. Charge battery and measure voltage - 12V. When idling, also 12V or 12.1V

2) Battery light goes off on dash as it should,

3) Replace the voltage regulator and same result, not charging. Carbon brush of old regulator is worn a bit but should still give good contact.

I have not opened up the alternator yet, but what could cause not charging. All fuses look good.

Thanks

sixto · #2 08-22-2016, 01:23 AM

Voltage is low for a running engine. Take the alternator to Autozone for a quick test.

Sixto
83 300SD

Diesel911 · #3 08-22-2016, 01:26 AM

The most definative way to go is to remove the Alternator and have it tested. I don't know about all of them but the local PepBoys actually gave me a little print out after they tested it.

If the Alternator tests OK you have some battery or wiring problem (to include the ground strap).

Clearly 12 or 12.1 volts is not charging.

If your Alternator has the Electrical Connector like the W123 Alternators do the 2 large slots shouls have battery voltage all of the time (with the connector unpluged from the alternator).

The single smller slot should have slighly less then battery voltate when the Ignition is in the run position; meaning not in the pre-glow position or the cranking position.

Mxfrank · #4 08-22-2016, 06:51 AM

This sounds like the rotor is open. You can test this by touching your meter probes to the slip rings. There is no easy repair, other than replacing the rotor as part of a complete rebuild. Toast.

jay_bob · #5 08-22-2016, 07:15 AM

I agree on checking the rotor

No excitation = no electricity.

Maxbumpo · #6 08-22-2016, 10:49 AM

I had a similar issue (very low or no charging, but light in dash was not lighting up). The dash light would glow faintly sometimes at night. The problem was tired electrical connectors at the harness plug to the back of the alternator. On my '87 300TD, the spade connector (male) is on the alternator, and the female connector on the wiring harness side. The female connector was not clamping the spade tightly. I diagnosed by attaching my DMM to the battery posts with leads long enough so I could put the meter on the ground and be visible from under the car. With the front of the car safely (!!!) up on ramps, start engine and carefully get under car. Wiggle the alternator harness while watching the DMM. If you can make the DMM reading change (mine would vary between about 12v and about 13.5v as I moved the harness around) then the issue is most likely in your harness.

I cleaned everything up on the harness connector, and tried very carefully to open up that connector in order to squeeze the female connectors so they were tighter and would take a better bite. My harness connector was so old and brittle that it broke apart a little bit, but I was able to patch it back together afterward. The proper repair would be to replace that harness with a new(er) harness.

jay_bob · #7 08-22-2016, 01:14 PM

Another option:
Go to the JY
Find a 124 with a gas engine
Pull the alternator and the wiring harness
Get the alternator tested to be sure it's working
Or buy a rebuilt alternator, just be sure to tell them you have a (whatever kind of car the donor was) instead of the car you actually have
Go ahead put a new VR on it if the salvaged alternator works, cheap insurance
Install it in the car along with the new harness

The gas 124s had bigger alternators than the diesel 124s. The bigger alternator meant that the plug in connection was too weak to handle the increased current, and the connection is bolted instead. Much superior to the plug on connection.

It is a direct swap, you just have to unthread the harness from the grommets along the fender. Dish soap is very helpful here. The gas car harness will bolt right into the terminal block near the battery.

Diesel911 · #8 08-22-2016, 09:44 PM

If the spring inside of the Alternator is corroded like the one in the Pic it can cause issues.

Mxfrank · #9 08-23-2016, 11:56 AM

That's not a spring, it's actually the contact which connects the D+ circuit to the voltage regulator. If that's corroded, the mating contact on the VR is also corroded. When this contact becomes non-conductive, the symptom is that the indicator light never comes on, not while running and not with the key in run position.

Jim Anderson · #10 08-23-2016, 11:39 PM

Alternators have rectifiers (diode bridges that convert AC to DC) that can go bad and checked and replaced seperately.

ah-kay · #11 08-23-2016, 11:51 PM

That is the plan. i have a spare one from an out of service SDL. I will open up the old one to see it can be fixed.

sixto · #12 08-24-2016, 02:27 AM

It might be time to move up to a 120-150 amp alternator from a mid-90s gasser. You'll need larger gauge wire to the battery or run a second OE gauge wire.

Sixto
83 300SD

Do D Do · #13 08-30-2016, 08:23 PM

Is there anyway a worn out ignition tumbler can repeat these same conditions as the original o.p.? I am having the same issue. Thanks.

Mxfrank · #14 08-30-2016, 08:49 PM

If the battery light is coming on before you crank, then the D+ circuit is powered. The ignition switch has done it's job, and the fault is in the alternator.

If the light doesn't come on at all, then the D+ circuit isn't completing. It could be the ignition switch or wiring, but a problem in the alternator is more likely. I went through three rebuilt alternators this month, all with open rotors. If you need an alternator, make sure the vendor takes returns.

ah-kay · #15 09-01-2016, 04:48 PM

I am stumped. Never thought alternator problem is so difficult to resolve.

1) Put in another good alternator, same symptom, no charge.
2) Take the old one to Auto store and it is found to be good as I suspect it would be.

The 'charging' light goes out as it should so what gives? Alternator is spinning as it is on serpentine belt. Car drives well but no charge. Check the 300SDL wiring schematic but nothing comes obvious, could be broken wire? There isn't any inline fuse, is it? Any ideas?