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Old 03-31-2002, 12:06 PM
Registered User
Join Date: Jan 2002
Location: Monroeville, NJ
Posts: 93
Unhappy Alternator Problems

Has anyone found that there are problems with the Bosch alternator on the 560 series engines. I recently bought a 1986 560 SEC and soon found that it was only charging at 12.7 volts with no load. Problem was a burned out diode. My 1987 560 SL appears to have the same problem, and I'm taking it off to have it repaired. Anyone with similar experiences?
M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.
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Old 04-01-2002, 08:27 AM
Registered User
Join Date: Nov 2001
Location: North Central Kentucky
Posts: 1,061
I was having trouble with my battery going dead on my 560SEL. Meter showed low/no charging. Turned out to be bad connection. Look at the terminal block on shock tower by battery. Three terminals under a plastic cap. Two heavy duty wire sets and one small dia. one. Remove the nuts and then the wires. Take a wire brush to everything. Reassemble with some sort of anti-oxidant compound. I couldn't believe that was the problem until I started checking with DVM. 15 years of black dirt on the connectors was enough to stop the current. Was really wierd to see full voltage coming out of alt. , but not showing up at battery.
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Old 04-05-2002, 08:27 PM
Registered User
Join Date: Mar 2001
Location: Memphis, TN
Posts: 129
Funny, I ran into the same problem with my alternator failing this evening. Autozone said that the alternator was the problem, but I never trust them. So I have 3 questions.

first: I have a digital multimeter, how do I test the alternator without pulling it out of the car?

second: Fastlane lists 3 different alternators, whats the difference between them

third: If I fully charge the battery using an external charger overnight, could I use the car for short trips while waiting for the alternator to arrive?

'57 190SL
'86 560SEC AMG
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Old 04-05-2002, 09:12 PM
G-Benz's Avatar
Razorback Soccer Dad
Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
If I fully charge the battery using an external charger overnight, could I use the car for short trips while waiting for the alternator to arrive?
Maybe. I had done that with my VW many years ago...the stock alternator went out (yipee, I get to order the chrome one), so I trickle charged the battery overnight for work the next day.

Did that for two weeks...hard on the battery though.

The VW was a lot simpler electrically, and I avoided as much night driving as possible, as well as leaving the accessories off.

In YOUR case, the engine management systems are electrical, so there is probably more battery draw to be concerned about. Also, as you lose voltage, different components start throwing all sorts of codes to the computer, and your car will probably go into "limp mode".

Unless you have a massive battery like an Optima, I would relegate the car to trips of ten miles or less...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car
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Old 04-06-2002, 02:06 PM
Registered User
Join Date: Jul 2000
Location: Krum, TX
Posts: 30
Generic Test with Voltmeter

Before starting the care measure the voltage across the battery. This is your baseline. Should be at least 12.0 volts.

Start the car. Turn on the headlights with brights. Measure across the battery. Should be .8 to 1.5 volts higher than the base voltage. Might have to rev the engine to get the highest reading.

Sometimes on these Bosch alternators you can replace the voltage regulator and fix the problem. This can be done with some cars with the alternator on the engine.

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Old 04-06-2002, 06:32 PM
Posts: n/a
Another test...

Start the car.Measure voltage at idle,it should
be 12v+w/out any access.Let somebody rev.the engine,at about 2,500rpm. You should have at least 14v.If it stay at 12v. you are not charging.Alternator is shot.
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