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I'm having an odd problem with my 1974 450SL. With the ignition off, the battery light stays on and then goes off when I turn on the ignition (without starting). Could it be the alternator? The alt. charges fine when running...

Thanks in advance,
Andrew
 

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2010 Mercury Milan Hybrid, 1993 BMW 325i convertible
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AndrewR said:
I'm having an odd problem with my 1974 450SL. With the ignition off, the battery light stays on and then goes off when I turn on the ignition (without starting). Could it be the alternator? The alt. charges fine when running...
I don't know about the 450SL, but on my old Chevy this was caused by a bad voltage regulator. It overcharged when the car was running, which damages the battery in the long term. Replaced the regulator and it cleared up.
 

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Thanks, i pop by the workshop tomrw see if they can check it out.
Quite freaky to see it glowing there in the dark!
 

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I would disconnect the battery when the car is not in operation. At the very least, it will prevent battery drain....or perhaps a fire!!!
 

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1976 450 SL
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I wonder if Andrew every figured out what the problem was? My car just did the same thing and the battery died just sittin there.

I put my hard top back on last Sunday, went for a ride ,I did turn on the heater and fan. Came home and parked it, that night I saw the battery light on, I started it up and the light went off, when I turned the key off the light came back on. Before this the car ran fine.

Then next morning the battery was dead. I disconnected the battery cable and charged the battery,, it took a full charge.

Thats where I am now not sure what to do nextt,,?? Drive it to the local alternator shop and have them check it?

Maybe I crushed the dome light wire putting the top back on,,my trunk light quit working also,, I have to wait for some help pulling the top back off.
 

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It Is What It Is, Dude
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I wonder if Andrew every figured out what the problem was? My car just did the same thing and the battery died just sittin there.

I put my hard top back on last Sunday, went for a ride ,I did turn on the heater and fan. Came home and parked it, that night I saw the battery light on, I started it up and the light went off, when I turned the key off the light came back on. Before this the car ran fine.

Then next morning the battery was dead. I disconnected the battery cable and charged the battery,, it took a full charge.

Thats where I am now not sure what to do nextt,,?? Drive it to the local alternator shop and have them check it?

Maybe I crushed the dome light wire putting the top back on,,my trunk light quit working also,, I have to wait for some help pulling the top back off.

Battery light on with the key off indicates a failed diode trio. Replace the alternator.
 

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It Is What It Is, Dude
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.


My personal opinion, not agreed upon by many here, is buy a Bosch Remanufactured unit. Reasonable price, readily available, two year warranty and a 24 hour roadside assistance policy.
 

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1976 450 SL
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dome light?
I just went to check out the dome light,,, dont have one. Nor do I have any visible window defroster wires in the back glass.

Wonder what the wire that comes out of the hard top is for? I have never connected it before.
 

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1975 450SL
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As mentioned above: Frotzed diode(s).

A less expensive remedy is to have your existing alternator rebuilt. I was able to get mine done in about 6 hours locally, but I had someone that knew someone that worked at a place...;) The cost was $90.00, much less time and money than the hassle of ordering a remanufactured unit from a parts house, and hugely less than a new one.

A quality rebuild will change out the bearings, replace the regulator/brush unit, take care of any grooves in the rotor, and replace a defective diode module. Mine even came back repainted to look like new.

Check at a GOOD auto parts supplier (NAPA, Associated, etc..) or a good mechanic for a rebuilder. The desk guy in the parking lot on a cigarette break identified my unit as I walked up to the door, so I was pretty confident they knew what they were doing.


Just as a "way down the list" possibility: How old is the ignition switch? They have been known to fail in odd ways. Euro type ignition systems have a lead from the switch to the alternator. If the switch has failed, and left that lead powered, you would get the same symptoms.

Changing out the switch is more complicated than the alternator, but still a DIY job.

Scott
 

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Hey thanks for the replies! And Happy Thanksgiving to all.

Would the failed diode trio continually drain the battery with the car off? My battery had enough charge to start it before it died overnight.
 

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Thanks Dave,, that hopefully will eliminate a bad ignition switch,, you guys are great!

Go eat some turkey,,,
 

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...Nor do I have any visible window defroster wires in the back glass. Wonder what the wire that comes out of the hard top is for? I have never connected it before.
WIres are so fine as to appear invisible, but they are there. The wire you refer to is the plugin for the defroster... The power is switched thru a timed relay, so the only power up for a while (length of time, I do not know)

I have to get the light just right to even see a hint of them :)
 

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Following up,,, I picked up a rebuilt Bosch with the internal voltage regulator, turns out some SL's have an external voltage regulator such as 73 Sl's (so I read) my 76 450 SL has an internal that possibly,,, originally was external and been modified to internal,according to a note that came with the rebuilt Bosch.
Enough on that from an amatuer.

I paid $114.00 for this Alternator that was "in stock" and an exact match of what I have at the local battery house, we are lucky to have such a well stocked parts store !

Piece of cake to replace,but an air impact wrench made the removal of the pulley nut an essential tool for this step. The car is back up and running.

This link helped me determine mine was an internal voltage regulator, supposedly a 73 SL, but the photo shows exactly what I have Mercedes Voltage Regulator Install

Since I found this thread while searching for a solution to my problem, and this thread is almost 10 years old !!, I hope someone else can benefit from my input when I am dead and gone. :D

Thanks again for the help! If it wasnt for the help I get here to keep this thing running ,,my car would probably be sold to someone that would eventually let it rot! Steve
 
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