Strange problems started on my way home from Penn. tonite, and I'm lucky to have gotten home. The first was when I put on the fogs and the ABS light came on.I shut them off and the light went off. Ok I thought, just a coincidence. About an hour later I switched on the High beams and the radio goes off. Switch off high beams, radio slowly comes back on. Try it again, same thing. So I go to change lanes and use the blinkers, and the SRS light comes on, and won't go off. Now it's like -oh no- the car's going to shut down. I notice the instrument lights are barely on. I stop at a light and the idle drops to 500 and the headlights are dim. I just get it home with hardly any light, so I turn the lights off and the idle comes back to normal. Lights on, almost dies. I've never had any electrical problems with this car before, so I don't know where to start. Does this sound like a voltage regulator problem, alternator, or??? Anyone experience something like this before? Any help would greatly appreciated.

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

Sounds like things aren't getting enough power. As you guessed... the first thing I would check is that the voltage is at least 13 with the engine at idle.

The engine doesn't need any electricity to keep going, but it does need juice to run the ABS and SRS, as well as the electronic idle control.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Sounds like your alternator died and the battery gradually got lower and lower. The regulator in the alternator is the first thing to check. Monitor the voltage at the battery and start the engine, if it will still start (put a charger on the battery yet? if not, do so and leave overnight). Make sure the battery connections are clean and tight.

If the battery voltage sits at around 12 or less and doesn't go up to 13-point-something or more when you rev the engine then it is likely that the regulator is bad. A lesser possibility is the alternator itself.

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"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

This sounds like a voltage problem, and may or may not be the voltage regulator. It could also just be an old, tired battery. Or it could be lousy connections (corrosion at the clamps) to the battery, or a poor set of ground connections. The fact that the problem arose while you were driving on the highway points in the direction of the voltage regulator or alternator. Under those conditions the battery is typically on there for the ride. However, the most frequent voltage regulator issue is worn brushes and they usually show by lighting the battery warning light very dimly at first - nothing you would notice in the daylight, but something you may notice in the dark. You did not mention that so it is feasible the voltage regulator is not the problem and it could be something else.

When you start trouble shooting, I would try to figure out if it is a bad connection first. Remove and clean down to shiny metal, the battery connection clamps - do the post and the clamp inside diameter. Check the negative terminal to ground connection (follow the negative terminal to the point where it bolts to the car's frame, usually about 8 inches or so away from the negative terminal on the battery. Unscrew it and sand down the connector, the nut or bolt surface that contacts the electrical eye/tab and then the surface the bolt and electrical connector eye/tab contact on the frame. I usually coat these things in a non-conductive grease to keep the corrosion down. Nothing is perfect though. This is good whether the problem lies in the connections or battery or the voltage regulator just because it is good to keep these connections clean and tight.

Next, check the voltage on the battery terminals without turning the engine on, with the doors shut, no radio, no lights, no other loads. It should read over 12.5 Volts. If not, you don't know if it is a bad battery or a bum voltage regulator or alternator that has failed to maintain the charge on the battery. Also if it is lower than 12.5 Volts, the car may not start - the lower the voltage the lower the probability it will start. If you have over 12.5 Volts, the car should start. If the car starts, check the voltage at the battery terminals with the car running. If someone is available, have them give it enough throttle to allow you to check it at a high idle, like 1500 rpm. Depending on the state of charge, at idle the battery should read over 13 Volts and at the high idle closer to 14.5 Volts.

If running you don't get the anticipated higher voltage the problem is looking a lot like the voltage regulator. The best way to check this is to pull the voltage regulator - it is the thing the electrical connector plugs into on the back of the alternator. Usually held in by two screws on the ears of the flange. Do a search, there are lots of voltage regulator change out stories and helpful hints in the archives.

Once it is out, look at the length of the brushes. If they are both very short, and even, you need a new voltage regulator. If they are worn unevenly, it is likely the alternator is in need of a rebuild, including new diodes. If the wear surfaces are smooth, chances are the surface they rub on, on the shaft, is ok. If they are worn at an angle or have deep grooves in them, the alternator needs to be rebuilt or your new brushes will just be ground down quickly and you will be doing this again.

If the check on voltage shows no improvement and the voltage regulator brushes are fine, the battery may be the culprit. A bad cell will drag the voltage down, and put a heavy draw on the alternator. However, since you were on the highway when the problem arose, and normally that means the battery is next to useless, I would expect the voltage regulator to be the problem - as it failed to provide the necessary power to run the electrical load, the load was met by the battery. As the battery got weaker, you got weird responses due to lower voltages than the system was designed to operate with - fix the regulator and the problem should go away.

Good luck,

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Thanks for the info. The battery is about 1-1/2 years old. This all happened last nite so I haven't had a chance to look under the hood yet. I'll post my findings when I get a chance to look at it. Thanks again.

__________________
1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

Low voltage = bad alternator or regulator.

__________________

Gone to the dark side

- Jeff