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Old 03-14-2017, 12:23 AM
corbitt's Avatar
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Join Date: Jun 2016
Location: Seattle
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Battery Drain: 1981 380SL

Greetings. I purchased a cherry '81 380SL last year. The vehicle has a trickle charger installed. Something drains the battery if not driven or charged. The battery can be depleted in a week. Yes, of course I have replaced the battery! Any tips on diagnosing this are welcomed and appreciated. Is this common? Thank you!!
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Old 03-14-2017, 05:15 AM
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Be sure to post the solution to the problem.

With the engine and key off, place a screw driver / piece of steel on the pulley end / other end of the alternator. If you feel it being pulled to the alternator, a diode is likely conducting in both directions ( shorted ). Just be sure the screwdriver isn't magnetized as it will affect the test.

A diode is a one way check valve for electricity, when it conducts in both directions the alternator windings remain energized draining the battery.

Another test is to use an amp clamp to probe wires looking for a drain. An Amp Clamp is a generic term for a mater that clips around a wire and measures current flow. If you get one, just be sure it will read DC ( battery for example ) and the lower cost ones only read AC ( house power for example ).

If you are tool limited, disconnect the battery then disconnect, insulate the alternator wires, then reconnect the battery and monitor battery voltage over time to see if there is still a draw.

There are other methods of testing but these are quick when limited on tools.
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Old 03-14-2017, 05:37 AM
optimusprime's Avatar
Trevor Hadlington
 
Join Date: May 2014
Location: Worcestershire in England
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You will need a new regulator fitting in to the alternator . . Charge the battery up, and remove the plug to the alternator .You will see then if its the problem, next time you conect the battery up.
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Old 03-14-2017, 02:59 PM
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you can also do this

How to Find a Parasitic Battery Drain: 9 Steps (with Pictures)
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Old 03-14-2017, 03:10 PM
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Quote:
Originally Posted by corbitt View Post
Greetings. I purchased a cherry '81 380SL last year. The vehicle has a trickle charger installed. Something drains the battery if not driven or charged. The battery can be depleted in a week. Yes, of course I have replaced the battery! Any tips on diagnosing this are welcomed and appreciated. Is this common? Thank you!!
Check out the link below to troubleshoot the alternator and its components. If you need more specific info, post a Comment just below the article and someone will get back to you with a more detailed response. Hope this helps and best of luck!

Mercedes-Benz R107 Alternator Replacement and Inspection | 1972-1986 - 450SL, 380SL, 500SL, 560SL | Pelican Parts DIY Maintenance Article


-Dmitry
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