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Alternator or bigger problem?

Old 07-08-2016, 03:51 PM
Junior Member
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Join Date: May 2016
Posts: 1
Default Alternator or bigger problem?

Hello, I have a 1984 mercedes 300sd with 220,000 miles on her. She is very well cared for and has been her whole life and in amazing shape. My issue I am having after acquiring this car a charging issue so here goes. My first sign of low voltage was the battery , brake lining , and parking brake light on in the car that happened for a while and would come and go and car still started and ran fine. Well one night it just didnt and wouldnt charge anymore so after doing some research I determined it must be a voltage regulator. I ordered a new one from mercedes source and a nice tool to make the job easier. Well bingo back in business I drove the car around all night with lights on in mixed traffic it started fine at a few locations I went to. No dimming of the lights all was fine and seemed back to normal minus the lights on the dash still on brake battery etc.. well yesterday evening I tried starting and it was hesitant to start but turned over. this was not normal so I said i better shut it off and see if it will restart so i dont get stranded some where. Well it would not restart. I have not tested the alternator and the battery is brand new. What could it be, I dont get why this seems so random. and even after the car started the first time I replaced the voltage regulator the dome lights instanlty got brighter like it was charging.

thanks again

Old 09-07-2016, 10:57 PM
Junior Member
Join Date: Sep 2016
Location: California
Posts: 2
Default Alternator

Sure you have figured this out by now. Pull the alternator and have it bench tested at your local auto parts store. Try to get the Bosch replacement. Also if like my 82 they come with a 55 amp standard . The upgrade gives you 65 amps worth the money.
Old 09-08-2016, 09:42 PM
Join Date: Jul 2016
Location: Prince George B.C.
Posts: 96

Hello, My advice is this. #1 Put a battery charger on the battery, make certain the fluid level is equal in all of the cells and all are at least 1/4 inch above the plates , and fully recharge it. Next check that your battery posts and the clamps are clean , lightly greased with dielectric grease and are tight.
#2 using an accurate voltage meter test your recharged voltage. It should be 13 1/2 volts. Then have the battery load tested in place. Bad cells kill new alternators as well as old ones.
#3 after the battery load test leave the voltage tester connected. Start the engine. a good alternator will immediately raise the voltage to around 14 volts or slightly higher as the engine is running and recharging the battery. Turn on all the lights , your air heater motor and your wipers and the voltage should not drop. If it does, either your alternator is faulty or as another member mentioned is under capacity and requires upgrading. Sadly a lot of parts stores are simply parts re sellers and testing both your battery and alternator yourself is much easier in place, takes about 10 minutes and is cheap to do. Good luck Allan
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