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Old 10-06-2002, 01:14 AM
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Join Date: Jun 2002
Location: Monrovia, CA
Posts: 490
Question about Charging Voltage

As I mentioned in a previous post, My 86 300E stranded me and my family about 70 miles from home last night with a dead battery. I bought a new battery, an alternator and a voltage regulator/brush pack today and headed out to bring the car home. The old brushes were fairly worn down; the inboard one protruded about 5.5 mm, while the outboard on was only about 2.5 mm. Both the running faces appeared relatively clean and smooth. I installed the new regulator/brush set, cleaned the alt. connector and put in the new battery. The resting voltage measured about 12.08v (I don't know how long the battery may have sitting on the shelf and I didn't have time to throw it on the charger). With the engine running at idle, the voltage was 12.24v and at about 2500 rpm the highest charge rate I saw was about 12.44v, and it took probably 30-40 sec. to get there. With the headlights on the voltage at idle was 11.97v and maxed out at 12.15v. All measurements were taken across the battery terminals. This seems really low. Can anyone tell my how to test the alternator in the car with a VOM? Does this sound like a bad alternator as well, or could I have gotten a bum regulator? I've also noticed that the battery light does not come on when I turn on the key before starting the engine. Should it?



Last edited by Phalcon51; 10-06-2002 at 10:48 AM.
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Old 10-06-2002, 11:40 AM
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Join Date: Jun 2002
Location: Monrovia, CA
Posts: 490
Well, I just went out to check the car this morning and the battery is completely dead. Looks like I have a low amperage current drain somewhere. The clock stopped at about 5 am. I put the new battery in yesterday at about 3 in the afternoon.

Could shorted windings in the alt. do this, or a bad component in the voltge regulator, when the ignition is off? Can anyone provide me with the ohm meter values for the various parts of the alt/reg/rectifier and which connections to test at?

Thanks for any help,

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Old 10-08-2002, 01:12 AM
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Join Date: Apr 2000
Posts: 412
It looks like the alternator is not charging at all. The most simple test is the battery voltage should be around 14 volts with the engine running. A typical 12 volt battery in good condition at 70 degree farenheit should read about 12.5 volts with the engine off. The battery charge light does not come on on Merecedes when the brushes are burned in the regulator assembly. My guess is the alternator has an open winding somewhere if a new brush regulator doesn't cure it.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T
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Old 10-08-2002, 05:30 AM
Posts: n/a
Alternator problems

Many alternators require some current from the battery to enable generation to start, then start self-feeding excitation current. Initially this current comes through the warning light and a resistor - if either the warning light or the resistor have gone open circuit, you wont get anything out of the alternator without revving it hard. Check to see if this applies to MB electrics, check to see if a big rev (>3k) will start it producing.
As above, you should see ~ 14V when running, almost irrespective of electrical load if alternator is good.
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Old 10-08-2002, 06:58 AM
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I think you have two problems. Your alternator is not currently charging as it should, but I doubt that it has anything to with the drain. To ensure that this is the case, disconnect the plug at the alternator, then put an ammeter in series with a battery cable. You should get very little, if any, current draw. Certainly no more than 100 milliAmps. If you get no current draw, plug in the alternator connector and see if it increases, if it does, then the alternator is indeed the source of the draw.

I expect the current draw is somewhere else. With your ammeter in place, begin removing fuses and disconnecting things until it goes away. Whatever you disconnect that makes the current draw significantly decrease, is the circuit with the problem.
Good luck,
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Old 10-08-2002, 08:13 AM
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Hey Larry,

The one thing I have the most trouble with is electrics, your last post was the easiest to understand beginers diagnostics advice I've seen regarding how to find a draw.

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Old 10-08-2002, 08:33 AM
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Thanks. Electrical Engineering is my education and most of my career before I got off into the warped software side of the world. I've been heavy into electronics for 34 years. Most people try to make it a mystery instead of trying to simplify it.

I hope it helps,
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Old 10-08-2002, 08:35 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
jp got this one.

You must have an excitation current through the indicator lamps! That circuit must be diagnosed or bypassed to turn on the alternator.

You may also have a current drain but your voltages indicate a low battery and no output from the alternator.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 10-09-2002, 05:48 PM
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Join Date: May 2002
Location: Mass
Posts: 1,127
Those numbers indicate no charge at all. If I remember right, you should be getting about 13.52v @2500rpm with minimal load.

Hopefully you have the problem Steve describes, because it is cheaper to fix...

...though your alternator could still be toast.
When my alternator failed, the battery light did not come on either, but replacing the Alt fixed the problem, so warning light circuit was fine.

Best of luck.
1986 300E 5-Speed 240k mi.
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