I first noticed the battery light come on when I had the headlights on and the music on, the light would start to flicker at times of maximum current draw. The light got slowly worse (brighter) for a few months, and the battery was pretty old so I replaced it. That didn't affect the light at all, it was on most of the time after awhile.

A few days ago the battery light stopped being lit all the time. Around the same time the radio started turning off when the volume was up. Then the car began to start really slowly, and finally not at all without a jump.

The voltage on the battery when the car is running is 11.2. When I floor the accelerator it goes up to 12.4, but it usually hangs out around 11.4.

These reading seem too low to me.

The other day I got under the car while it was running and unplugged the alternator and measured the voltage. There were two large terminals and one smaller one. The voltage from ground to the large terminals was 30 to 40 volts, I am not sure why. From ground to the smaller terminal it was 14.2 volts.

What is the next stop to diagnose this? Should I cut the three wires that go to the alternator and measure the current on them?



Here is a pic of the back of my alternator.

Remove the two flat head screws, remove the voltage regulator which will likely reveal worn brushes, obtain and install a replacement.

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1987%40%401987&makeid=MB%40%40MB%40%40X&modelid=300%2DDT%2D002%40%40300D++Turbo&keyword=voltage&catid=All@@voltage&subcatid=F4010@@Voltage+Regulator&mode=PA

Plan on replacing the head gasket soon because my guess is the grime on that alternator came from the a breach in the head gasket and is spewing from below the #1 exhaust runner.

Sixto
87 300D

1.Never unplug or disconnect Alternator leads with the engine running...
(Dead Diodes are generally the result)[Rectifier Pack]
2.Have you VERIFIED all grounds (And power leads) are GOOD?
(Including the Engine to Chassis main Ground strap)
3.If all your leads/connections are good your problem is most likely the
Voltage Regulator [Attached by the two slotted screws in my remake of
your picture]
4.I'm Lazy (Or Efficient).If It ain't putting out minimum 13.5 V at Idle...Off to
Advance/Autozone/Whoever and test the Puppy.An Ammeter is certainly a
wondrous Tool.(Expensive though)

Attached Thumbnails

 

__________________
'84 300SD sold
124.128

I installed a new voltage regulator the other day. The old one had very warn brushes. The idle voltage is 12.15 and when I rev the engine it goes to 12.95.

Its still not 13 volts but I haven't been having starting troubles like I did last week so I am going to call it a success.

However the day after I installed the regulator, a lot of strange things started happening with the car. I don't know if its a coincidence or not, but all these things must be related. They all started happening at exactly the same time while I was driving on the highway at night. Luckily none of the things affect the drivability, but I have never seen so many things go out at once.

* The oil pressure gauge does not work
* The RPM meter is at 0
* The fuel gauge is at empty
* There is a buzzing noise when I drive. It comes from the left of the steering wheel, near the fuse box. It goes away when I use either turn signal and returns when I am no longer signaling. It sounds a little like the mercedes buzzer except that its a lot quieter.
* The mercedes buzzer goes on when I open the drivers side door and have the key in. It never did that before.
* The damper that opens when I turn on the heat is always open so I get a cold draft through the vents when driving


I have never seen so many things go wrong at once, there must be a common theme here. No fuses were blown. What could be wrong?

(It's next to the EDS box behind the Battery)

(See Picture)

Check the 10 Amp fuse in the top

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__________________
'84 300SD sold
124.128

It sounds like your ignition switch is not making good contact or a wire has been knocked loose. I don't believe that the OVP affects the fuel and oil gauges.

__________________

Gone to the dark side

- Jeff

The gauges (including the tach) all work off of fuse #5 in your car. Replace fuse #5 whether it looks good or not. The buzzing and the passenger door may be caused by a partial failure of the N2/4 "Warning Module," which is located behind the instrument cluster.

The worn brushes may also have worn the alternator's slip rings so that they do not make good contact with the new brushes. That could explain the charging voltage, which is not bad but not good and therefore suspicious.

Why these things should have started at once I cannot explain.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

That turned out to be sage wisdom, fuse # 5 looked fine but once I took it out of there it fell in half and replacing it fixed the problem.

Fuse # 6 (turn signals) did the same thing today, I wonder if that is a coincidence.

The car is now idling at 12.2 and goes to 12.95 when I rev it. Its not enough - I have to keep charging the battery at least once a week, or else it will end up needing a jump.

Replacing the voltage regulator on the back of the alternator did not make any difference at all in the measured voltages at the battery. I am glad I got new brushes in the alternator but that wasn't the problem.

What should I take a look at next?

Maybe the alternator didn't enjoy the oil soaking.

Sixto
87 300D

Are the contacts/ connections clean. I had a similar problem with my car recently and cleaning the connections made all the diference.

__________________
81 300D Turbo 190K sold
83 300sd 319K best $500 I ever spent-daily driverw/ 2 tank WVO set up

If you have checked, cleaned, repaired, etc., all of the connections, including the vital engine-to-body ground strap (near the starter motor) and the problem continues, I would next suspect the alternator. By removing the new regulator/brushes, you may be able to look at the slip rings (the brushes ride on them and feed electricity to the rotor). If the slip rings have been badly worn by the old brushes, they will not make good contact with the new brushes. The oil, as Sixto mentioned, won't help.

If the brushes are indeed badly (and unevenly) worn, the alternator will have to be replaced. This you can do yourself. Remove the serpentine belt and the alternator wires (disconnect the battery first!) then unbolt the alternator.

Hint: If you need to turn in the old alternator as a core, be sure to remove and keep the regulator, since it is new and will make a nice spare to keep for "next time."

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I think I have the connections right, but its hard to be sure. Can I test those all at once by connecting a solid ground wire from the alternator case to the battery ground and see if that causes the voltage at the battery to start going up?

Can I start the car with the alternator disconnected and test it, maybe hook up a headlight or volt meter to it?

Does the battery usually charge well at idle or do you usually need to rev it / be driving to charge it much?

Might be more practical to take the alternator to Autozone or some place that will test it for free.

Sixto
87 300D

Are these cars like the W123's and the alternator won't charge if the dash battery light is burned out? I know in his original post he stated that the battery light finally quit coming on. Could have been a bad voltage reg, then the battery light burned out. The voltage reg. replaced but still no charge if battery light burned out.
***this of course assumes that the battery light is necessary for the alt to charge like the W123. If this model is different I hope somebody will chime in***

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

I spoke with a mercedes mechanic today and he said that this car does not use the voltage from the light to energize the alternator, so the light is not a possible cause of the problem.

However when I turn the key almost all the way, the following lights go on: Brake, antilock, glowplug. I thought they were all supposed to go on for test? Its been a month since the battery light has lit up for any reason, I think its burnt out.

I used a clamp-on inductive ammeter on the wires coming from the alternator, it was only putting out half an amp.

I disconnected the alternator and started the car and measured the voltages at the connector that plugs into the alternator. The two large spade connecters read 11.97v (battery voltage) and the smaller one read 11.65v (to ground).

At this point I think its a bad alternator so I have decided to remove it. I read some of the threads on the subject but I am still not sure how to loosen the serpentine belt.

The mechanic I spoke with said its not required to remove the fan. I have a 13mm deep socket ratchet and I am trying to figure out exactly what to do with it.

Here are some pictures of the belt tensioner, how do I proceed?