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  #1  
Old 07-19-2004, 08:27 PM
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alternator question

The battery went dead on my 1983 380SL, so I replaced the seven year old battery and checked the charging sysytem, as best I could with tools available (digital voltmeter). I was distressed to see only battery voltage at idle, with no change when I revved the engine to 3000RPM. I checked to see if the battery light in the dash worked in the #2 position, which it did, so I performed the only crude test I could think of, taking off the alt belt to see if the batt light would come on when I started it, which it did. This shows the warning system is working. On GM vehicles, I routinely see 14-14.5 volts at the battery at idle, even on a charged battery. Any ideas?

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa
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  #2  
Old 07-19-2004, 11:54 PM
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Most cars I've seen will show that higher 14V+ at idle as well, then drop to 13 something when the motor is off. Maybe your multitester is bad? I tossed one last week because it was reading 17V on all my batteries.
A good alternator test would be to turn on all lights and A/C on high to see how she handles the load.
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  #3  
Old 07-20-2004, 10:50 AM
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Thanks for the reply, Chevota. I did check my other vehicle with the same meter, and it read 14.4v. I tried turning on all the accesories I could think of, checked voltage at the battery, at C105, which is where the pigtail from the alternator hooks to the batt/starter circuit (on the coolant expansion tank bracket), and at the big batt cable connection in the engine compartment, and I read about 3 tenths lower, about 12.3v. I also checked continuity on the alt pigtail lead, checks good. I'll try to get a meter to measure amps from the alt to see if indeed I have a problem. My wife thinks I'm creating a problem where there isn't any. Some women just don't understand a good old need to fix something
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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa
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Old 07-20-2004, 10:51 AM
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Check the voltage regulator on your alternator. If the brushes are worn down, the alternator is not recharging the battery. Cheap part to replace.
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  #5  
Old 07-20-2004, 02:09 PM
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It may be on its way out like jriewer said. If it's the original alt then I'd say you got your money's worth. You sound like a do-it-yourself kinda guy, you might consider rebuilding it yourself. I have a local hole in the wall starter/alt repair place that sells me any part I need for cheap.
Also, most auto parts stores will test your alt for free. They'll give you an amp and volt reading.
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Old 07-20-2004, 04:48 PM
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Thanks, guys. It's a very easy one to remove, so I'll do that and have it checked. Thanks again for the help.

Paul

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1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa
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