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  #1  
Old 07-11-2010, 06:22 PM
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Cool Charging Problem: 16.7 Volts at battery - Voltage Regulator or Diodes?

O.K., here's one for you electrical wizards. I hope it's just the voltage regulator (easy & cheap) and not the alternator, but I don't want to start arbitrarily throwing parts at it without a proper diagnosis.

On my '95 E300D (606 engine) I'm getting almost 17 volts at the battery terminals while the engine is running.

I recall something about using the VAC settings on my multimeter to check if the diodes are bad. I'm getting a reading of 35.9-36.1 VAC while running.

Battery seems to be O.K. (holds a charge properly)

Opinions & advice?

Thanks guys!

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  #2  
Old 07-11-2010, 06:27 PM
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Voltage regulator.

If you continue at that voltage, there will be many other problems.
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  #3  
Old 07-11-2010, 10:12 PM
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X2 with Jeff,
Dont run the car like that. You risk destroying your battery. You could even generate enough heat in it to boil it dry or cause it to go BANG !! (been there done that) Probably the reg is shorting the rotor down to earth. Replace the reg & problem should go away If you dont do other damage first.
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1980 300D now parts car 800k miles
1984 300D 500k miles
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  #4  
Old 07-12-2010, 09:07 AM
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What lead you to check the voltage in the first place?
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  #5  
Old 07-12-2010, 11:37 AM
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I had...well still have...basically this same problem in my w123.

Charging Light at Speed & very High VAC readings...

I haven't really fixed it yet. Not driving it seems to greatly reduce the symptoms.

I ordered a regulator this morning. I used a different voltmeter and got 0 VAC. That AC voltage seems insanely high, like mine. I hope it's not the alternator, or I'll head to the pick n' pull.
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  #6  
Old 07-12-2010, 12:07 PM
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babymog & layback40: That's why I want to fix it.

oldwolf: I discovered it because, when used in that car, my radar detector indicates an overvoltage condition.

atomix8: I read through your post - thank you for the link. My multimeter is also a cheapo. I don't know if the AC voltage readouts should/could be trusted, but I'll check it with a different meter.

Thanks for the input guys - I'll check back later.

.
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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.
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  #7  
Old 07-12-2010, 05:31 PM
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Quote:
Originally Posted by layback40 View Post
X2 with Jeff,
Dont run the car like that. You risk destroying your battery...
I'd be more worried about the various control modules in the car. Those are far more expensive and much harder to replace.

-J
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  #8  
Old 07-12-2010, 07:11 PM
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Step number one, (do it tonight) should be taking the voltage regulator out and visually checking for obvious problems.

There's only one way they "normally" fail -- the brushes get too short. You can literally jack up a corner of the car, roll under it, take out two exposed phillips head screws, reach up and grab the regulator, yank it out, and then look at it in a good light and see what it's looking like. If the brushes are worn down to nothing or stuck in their holders, you've found your problem. If not, it was a free diagnostic that took 15 minutes or less, and you can pop it right back in.

They COULD fail in other ways, but this is check #1 for a fast, easy "so that was it!" test.
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  #9  
Old 07-12-2010, 08:37 PM
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Your system is charging so I think you have good regulator brush contacts. More likely if it is the regualtor it is an internal electronic failure or loss of proper regulator case refferal to true ground.. This particular regulator is not the strongest on the market. One thing for sure is the regulator is not reducing or controlling if you wish the alternator output properly.

Nothing wrong with the above mentioned prior post test but keep in mind the fault may not be visable.

Last edited by barry123400; 07-12-2010 at 09:03 PM.
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  #10  
Old 07-12-2010, 09:49 PM
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Quote:
Originally Posted by bustedbenz View Post

There's only one way they "normally" fail -- the brushes get too short.
How could short brushes contribute to an overvoltage condition?

Quote:
Originally Posted by bustedbenz View Post
You can literally jack up a corner of the car, roll under it, take out two exposed phillips head screws, reach up and grab the regulator, yank it out...
Are you sure about that?
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  #11  
Old 07-12-2010, 09:58 PM
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Quote:
Originally Posted by tangofox007 View Post
How could short brushes contribute to an overvoltage condition?



Are you sure about that?

Answer #1: I don't really know, but it seemed like the easiest "first diagnostic" to get out of the way. It was one more piece of data to have in any case.


Answer #2: That's exactly how it came out of both of mine. About what should I be unsure? Jack up car... get under it... take two screws out of regulator where it sticks into the back of the alternator... remove it... that's how mine came out of our 240 and our 300SDL. Did I miss a different design? Sorry if so.
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1987 300SDL
1982 240D
1982 300SD


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  #12  
Old 07-12-2010, 10:12 PM
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Consider that suggestion withdrawn. Just re-read the thread and realized for the first time that it's a 606. Ugh.

My mistake. My apologies. I have no idea HOW the regulator comes out of one of those.
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Past cars:

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  #13  
Old 07-12-2010, 10:43 PM
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Compu,
You will see I mentioned "other damage", Your comment was exactly what I had in mind !! As I am not totally familiar with all the electronic modules in that car, I left it for others to expand, Thanks !!
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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
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  #14  
Old 07-13-2010, 01:11 AM
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606 is probably not new enough to have a computor involved in its function. I am not sure. Certainly newer mercedes models will have a complex alternator circuit but I would expect as much.
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  #15  
Old 07-13-2010, 03:37 AM
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17 volts is about where the regulator failover voltage would be when the regulator has failed. Or there could be a build up of carbon dust inside the case.

If there is enough carbon dust inside the alternator case, you can have shorting to ground from the brush sliprings to the rotor shaft and through the shaft bearings. This would cause excessive current through the rotor field and too much voltage produced. Carbon tracking is what this is called in the dc generator/motor world.

You can remove the alternator (when cold), remove the regulator and disassemble the unit. Clean all the carbon off the rotor and away from the slip rings. Use what ever you want to clean with, just stay PH neutral. I have used hot DI water to flush out the carbon when available. Keep the water out of the bearings though. Let everything dry and reassemble. Renew the regulator brushes if necessary.

BTW the diodes can't produce a high voltage, they only rectify the voltage that is generated. This is all on the regulator/rotor field.

Good luck.

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Last edited by moosejaw; 07-13-2010 at 03:48 AM.
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