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  #1  
Old 02-27-2008, 12:53 AM
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w126 alternator

i believe my alternator is shot.
i say believe, because no professionals have checked it, but my more-car experienced friends have told me that's what my symptoms indicate.

when i got the car, i was told the battery was bad, because it needed to b jumped if it hadn't been driven for about 3 days.
ok. changed the battery.
she works.

for about a week.

then, today, i go to start her up. nothing. no sounds at all.
my friend tries to jump it, she doesn't start, but i can hear her try. and the radio turns on, but she's not quite there.

so, i have two questions.

does this sound like an alternator to you folks? anything i can check myself to be sure?

and

how do i go about changing the alternator, if this is the question?

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  #2  
Old 02-27-2008, 01:20 AM
compress ignite's Avatar
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Auto-Zone or Advance

Drive it to either...they'll put a tester on it with the engine running.
You want between 13.5 volts and 14.3 volts at idle with no load.

You are testing for the Alternator.
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  #3  
Old 02-27-2008, 01:42 AM
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problem with that plan:
it won't quite turn over, even when being jumped.

I gather from other posts, though, that you can pull it out and have them test it, is this as accurate/thorough? do they charge for this?
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  #4  
Old 02-27-2008, 11:06 AM
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I had a battery problem on my '94 SL 500. I checked all over the place for current leaks, i even removed all the fuses and the battery still went dead in 3 days. New battery, old battery did not matter, they all went dead after three days. Took it to an electrical shop, they said there was a service bulletin about the ignition switch being defective,wanted to do a major dash teardown to access the ignition switch, i declined the service. researched for days on all the Mercedes forums, finally went to a Chrysler forum, everyone universally agreed fused diodes in the alternator. Haven't a clue what a diode is or what it does, or where located in the alternator. Replaced the alternator the next day, have not had a problem since. In my case yes it was a defective alternator. You can buy one of those battery minder things that when the voltage drops below a certain voltage it disconnects the battery around $70.00. Repeated deep discharge of the battery reduces battery life. Unless you have a current leak somewhere else it's most likely the alternator. I agree with your automotive experienced friends.



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  #5  
Old 02-27-2008, 11:58 AM
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Did you make sure that all your cables are clean and tight? Did you check the ground wire from the engine to the chassis? Thsi could cause all sorts of headaches.

You can pull the alternator and have the guys at Autozone test it. They have a machine that spins it up and checks the voltage.

To start the car I would have your friend run his car at a speed above idle for about 10 minutes while it is connected to your battery. I connect the cars battery to battery and just make sure that I disconnect the donor car's negative jumper first when I am through. Starting a diesel takes a lot of juice especially to operate the glowplugs and you are probably not getting it if you just hook up the donor car and try to start yours.

If your alternator's voltage is low, you can replace the voltage regulator very easily. The dealer should have them in stock so you can do it today. If the alternator is shot, do a search on AL129X, which is a direct fit Bosch alternator that is 115amps instead of the puny 65 you have now. It is an easy upgrade and well worth it,

Good luck!
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  #6  
Old 02-28-2008, 09:24 PM
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any tips on the swap? i've decided i'm just gonna replace it.
my local pull-apart that specializes in mercedes parts will sell me the alternator with a 1 year warranty for $65
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  #7  
Old 02-29-2008, 02:50 AM
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Quote:
Originally Posted by bengerman View Post
any tips on the swap? i've decided i'm just gonna replace it.
my local pull-apart that specializes in mercedes parts will sell me the alternator with a 1 year warranty for $65
Kinda high price, but if you do this, I'd spend $20 more and replace the regulator/brushes (very easy with the alternator out). You would probably void your "warranty" but OTOH you wouldn't need it.

I think that diode failures are very rare (lucky you) in comparison to regulator/brush failures.
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  #8  
Old 02-29-2008, 08:24 AM
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Do a search on the forum for AL129X. The alternator comes from the mid 1980's Saab 9000. It is a 115 amp alternator that will bolt into the brackets with no issues. you might have to swap the pulley if it is set up for the wrong type of belt and you will need to run new wires from the alternator to the juction box on the lower right fender. The big bolt on the alternator goes to the screw where the two wires from your original are connected and the small bolt goes to where the small blue wire is connected in the juction box. wiring takes five minutes and once it is installed, you will wonder how you lived with the small stock Alt. Once I installed it I was finally able to run my rear defroster, front def, wipers, headlights and radio without getting a low voltage condition!

I would never replace a stock alternator with another stock one...
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  #9  
Old 02-29-2008, 09:33 AM
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Disconnect the positive battery terminal prior to alternator removal, remove electrical connections to alternator, remove belt adjustmnet bolt then anchoring bolt, review belt condition prior to installation of new alternator, replace if needed, reverse instructions for installation. Only word of caution, when buying a used part, with a year guarantee or lifetime guarantee, if the replacement alternator is defective or has the same fused diodes, you're only going to have to replace it again. Salvage yard does't care how many times you have to replace a defective alternator, they've got a whole shelf full of them. Unless money is really tight, just go out and buy a rebuilt/remanufactured unit. No guarantees there either, but you're chances of having to do the job once are much better. Not that replacing an alternator on a 560SEL is particularly difficult, it's not, it's right there on top. But why do a job twice if you don't have to.





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