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Discussion Starter #1
I've been having trouble for some time but cant quite decide on a course of action.



Changed the battery recently for a good spare...holds a tad over 13v after charging. It'll start the car and it'll run with a charging voltage of just over 14v. But if I turn the engine off it struggles to start it again to the point where the starter is clicking away. So...Battery or starter motor?
 

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Discussion Starter #2
Forget it guys...it's the battery. Having just recharged it again and showing 13.8volts I hooked it back up. It didn't have the power to start it..it turned over, briefly and then the clicking started again. Checked the voltage and it was down to 12.8. I think it's FUBAR
 

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Registered 2002 R170 SLK V8 5.0
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2,593 Posts
I agree with you, don't buy a cheapie either ..................

A decent Battery is everything on any Car full of Electronics :wink:

HTH, Cheers Dave
 

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Super Moderator CA 2012 SLK55 AMG w/P30
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13,148 Posts
Forget it guys...it's the battery. Having just recharged it again and showing 13.8volts I hooked it back up. It didn't have the power to start it..it turned over, briefly and then the clicking started again. Checked the voltage and it was down to 12.8. I think it's FUBAR
Gee, I have to think its something else happening here. I just checked Schwarzie and after 12 hours, sitting at 12.3 (no load) and she starts like a champ.

My magnum is at a week standing now and granted there might be fewer things going on (but it does have an active factory alarm and a lot of electric stuff) and she's at 11.7 and no problem starting either.

Like I said to start (pun intended :wink: ), there has to be something else causing that no start. IMO.
 

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Premium Member 2001 SLK320/2000 SLK230
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2,719 Posts
Something is wrong. Anything over 11.6v in a good battery should be plenty to start (with a good battery most cars are designed to start with 9v if it does not go below.)

A battery that comes up to voltage too fast is probably bad and needs a good load tester to know.

Any more I buy only AGM batteries.
 

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Registered 2006 SLK200k
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25 Posts
You have a voltmeter, so my advice is
Your (nominal) 12 volts are being used, while cranking (or clicking) between the battery positive terminal and the negative.
To test the battery you need to measure the voltage while cranking, at the terminals you should still have 9 or 10 volts.
If within a few cranks the voltage drops further it is the battery capacity, or the charging system. You say you have 14 volts charging at the terminals (not the leads) so I guess its OK.
If you then move your negative probe to the engine side of the earth strap you should have an almost identical cranking reading, if you lose more than half a volt clean the terminals/strap.
Move the positive probe to the starter motor side of the solenoid and test again, more than a volt suggests poor connections on the solenoid (or duff solenoid).
You want the maximum voltage drop to show across the motor.
If the motor is getting 9 volts and not spinning fast enough then it is suspect (or again its connections).
 

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Registered 2002 R170 SLK V8 5.0
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Hi,

Glad you've got it sorted.

A few often overlooked pointers here ..............

@Sandy&Ben is on the right track here :wink:

A Battery always needs testing under load, and not with Multi-meters either ........... (Heavy Discharge or "Drop" Test).
If OP's Battery with it's 12.8 Volts had been Drop Tested it would have likely read 7 - 10 Volts Max ....................

AND

On any Car with Electronics that is simply not enough .

On a more advanced Car like say a W220 S Class, W210 E Class etc, the Electronics will throw low Voltage Codes if the Battery drops much below 11.2 V and it will not even attempt to Crank !!
A V6 and any Post Facelift R170 will do the same.

On the "older skool" 4 Cylinder pre facelift R170 it still has more Electronics than most realise and will still need a good strong Battery ..............

Just because a Battery starts the car doesn't mean it is not at the end of it's useful life, R170 has Modules that will not be happy with low Voltage, the TCU is a common example of one to go into limp mode over a low Cranking Voltage ;)

HTH, Cheers Dave
 
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