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Old 11-01-2004, 03:33 PM
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Join Date: Nov 2004
Location: Stow, MA
Posts: 1
300D charging problem, alternator?

Hey guys –

I’m having an alternator / battery / charging problem with my 123 diesel (‘84 300D turbo, 423K miles) that’s driving me crazy. Any help or suggestions would be super appreciated!

The problem started a month ago. The battery light (and the surrounding dash lights) started coming on very faintly, but would usually disappear if I revved the engine up. Every time I started the car the battery got weaker until I had to jump it to start it.

I recharged the battery that night and was able to drive it around for a couple days before I had to jump it again. At this point I recharged the battery again and tested the voltage across the battery terminals. With the key off it was about 12.3, at idle with no load 12.5, and with lights, blower motor is was 11.8. I pulled out the alternator (65amp) and had it tested at CAP…they said it was not putting out enough voltage (do they test it across a battery?) and needed to be replaced.

I replaced it with a used 55amp alternator from a ’77 280CE parts car. The charging system on that car was functioning before it went to the junkyard. I put it into my Benz, and she was running great for about a week. No batt light, and the voltage at idle was about 13.5 and under load it was 13.1 volts. I thought I was home free!

Last night, I was driving home and the radio kept going on and off. I immediately suspected that the problem in my charging system had returned. Sure enough, I parked the car, shut her off, and then tried to restart it…and the battery was too weak to start it. I tested the battery voltage and it was 11.4 volts.

I charged the battery overnight and tested the voltage this morning. Battery voltage with the car off is 12.8 volts, 12.5 volts at idle, and 11.8 volts under load. I should also note that the battery terminals are clean and I cleaned the tabs on the electrical plug going into the alternator before installing it.

Is is it possible that this alternator has gone bad on me as well? Do I need a 65amp alt.? Could this be a ground or some sort of drain on the charging system?


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Old 11-01-2004, 05:01 PM
Registered User
Join Date: Aug 2004
Location: NYC
Posts: 49
relevant thread

I don't pretend to know much about your problem but when I had a similar alternator problem I used this link. It might be relevant to you espically reagading the question on different alternators.

Alternator for ' 86 300 E
1986 300E 240K
1999 Mitsubishi Spyder GS 60K
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Old 02-09-2005, 04:20 PM
dkveuro's Avatar
Sword of Damocles
Join Date: Jun 2003
Location: Here an' there.
Posts: 2,548

The cheapest way is to replace the Voltage Regulator in back of the alternator.
There are a few 'copies' of OE out there, so get a known brand like Bosch.

It is real easy to swap out, two Phillips or slotted head screws and roll out the Reg' wires to mess with.
New one is harder to install due to longer brushes.
Roll it in and forward and locate it with both screws before you tighten them up.

You might pull the ground off the battery to be real sure you don't weld the screwdriver to the Alt Batt+ post.

The old Reg will usually have very short worn out brushes. They don't sweep the charge off the rings and you get your classic fuzzy light symptoms.

'corse....this could be all wrong, but hey !
Waddaya want fer free !

Hells bells....this piggin' post is 4 months old !!!!

[ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."
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