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Alternator problem

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HowardRT's picture
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I have searched the archives for information on the alternator problem that I am having with my '86 420SEL but have not figured out what is wrong. The symptoms are that immediately after starting, the voltage at the battery is over 13.5 volts. Turning on the A/C it remains at 13.4 even at idle. Within a few minutes it drops to about 11.5 volts with A/C on and then off. I jumpered the ground from the engine to the battery neg. terminal with no change so the ground strap is OK. The battery light on the dash does not come on while the engine is running even when the voltage is low. It does light when the key is on but goes off when the engine starts. I took the alternator off and had it checked at a local parts place. It was charging but that agrees with what I find - when cold it charges.

I guess the question is if the alternator is bad or the voltage regulator. The other question is why doesn't the indicator light show that battery is discharging? I would appreciate any suggestions and response from anyone who has had similar problems.

Thanks

Howard
'86 420SEL
'83 240D

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Howard

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Jim Hansz's picture
Kalispell, MT
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1969 230
1990 420SEL
Re: Alternator problem

Just guessing here, but if you have some miles on the car then the brushes in the regulator package may be worn and losing full contact once the engine warms up. Could be a diode problem, too. Replacing the regulator may cure the problem. Assuming that you have also checked out the battery condition and it is OK and not the source of the problem. If you have not checked the battery then you should to ensure its OK.

Regulators are available from Rusty at BuyMBparts.com for around $30.

The light shows what is happening with the alternator, not the battery. It senses alternator output and if the alternator output is above the reference voltage then it will stay off.

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brandlj's picture
West Monroe, LA
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1986 300SDL
1985 380SE
Re: Alternator problem

I would look at the regulator. My 300sdl would only put out 11.5 volts, I changed the regulator and all is well now. It was really worn down at age 23 with 115,000 miles. Very easy to replace.

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John

1985 380SE

1986 300SDL

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HowardRT's picture
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Re: Alternator problem

Since I posted the question, I took the regulator off and you are right. Jim, the brushes are worn down and one indicates only partial contact with the armature. There is also a contact that is made when you install the regulator and it seems to be corroded so that it makes poor contact. As to the battery, I had made a trip of 100 miles and the engine died. I jumped to the conclusion that the battery had a shorted cell so I replaced it. I drove home thinking it was fixed but the new battery ran down as well. I am hoping someone can replace the brushes but if not I will look into contacting Rusty to get a new regulator.

Thanks for the advice. I will let you know the result.

Howard

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Howard

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HowardRT's picture
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Re: Alternator problem

I purchased a new regulator and was amazed at the difference in the length of the brushes. After installing it, the voltage at the battery at idle is 13.7 V. I checked it after it had been running for some time Problem solved. That is a considerable savings over what a new (rebuilt) alternator would have cost.

Thanks for the help and advice.

Howard

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Howard

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OfficeLinebacker's picture
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Re: Alternator problem

I'm experiencing a very similar problem which started right after a PS/WP belt broke. Glad I searched. I'll look up a regulator for this car as I'm awaiting test results on the battery.

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