Issue on 420SL alternator and battery -Richard

richard420

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I have a 1987 SL420. I've had the battery replaced twice after it failed (car wouldn't start). I've found the voltage on the new battery when engine is running (tick over) is 12.15 volts. When revving it's 12.3 volts.

The alternator is the original Bosch one with a regulator clipped to the outside of it.

So it's not charging enough. Or the alternator isn't working right.

Has anyone else had this problem?

Any idea where I could source an new alternator with the same regulator set up?

Thanks

Richard
 

Richard Moakes

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Sounds like the regulator may be faulty, it's a good place to start. I would also check the earthing of the alternator, it's very easy to lose some volts with a poor earth connection. Easy to test, just put a meter set to dc volts across the body of the alternator and the -ve pole of the battery.
 
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richard420

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hello Richard thank you for your suggestions I am not sure how to remove the regulator, do you suggest removing the battery connections before i start to remove the regulator?, also I will test the earth connection thats a good idea.

Regards Richard Ayres
 

Richard Moakes

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I would definitely recommend disconnecting the battery if you plan to remove the regulator.

The regulator should be held onto the back of the alternator with two screws, undo those screws and gently lift it away from the alternator with a twisting motion to allow the brushes to clear the housing.

It might be worth getting the numbers off the regulator to get a replacement before you start, I would expect it to cost around £25

It might be a more significant fault with the alternator, no easy way to rule that out, and if that is the case I would be looking for an exchange unit from a Bosch dealer.
 
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richard420

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I would definitely recommend disconnecting the battery if you plan to remove the regulator.

The regulator should be held onto the back of the alternator with two screws, undo those screws and gently lift it away from the alternator with a twisting motion to allow the brushes to clear the housing.

It might be worth getting the numbers off the regulator to get a replacement before you start, I would expect it to cost around £25

It might be a more significant fault with the alternator, no easy way to rule that out, and if that is the case I would be looking for an exchange unit from a Bosch dealer.
Hi, richard thank you so much for the info, I have tested the earth connections and it is perfect so onto removing the regulator.
 

Richard Moakes

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One thought, does the battery charging light come on when you turn on the ignition and then go out when you start the engine?
 
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richard420

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One thought, does the battery charging light come on when you turn on the ignition and then go out when you start the engine?
yes it does, there is a small charge into the battery of around 12.233 ticker and 12.144 with head lights on clearly way to low, I never thought to see what charge I have been getting until this latest 8 month old new battery went flat and I started looking into the cause, thank you for your help.
 
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richard420

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yes it does, there is a small charge into the battery of around 12.233 ticker and 12.144 with head lights on clearly way to low, I never thought to see what charge I have been getting until this latest 8 month old new battery went flat and I started looking into the cause, thank you for your help.
 

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