Battery or alternator

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renault12ts

MB Club Veteran
Joined
Mar 5, 2009
Messages
16,671
Car
2005 W215 CL500.
Last night I was out playing taxi for my eldest daughter. The red "battery/alternator" red screen and audible alarm sounds three times.

The onboard diagnostic shows 10.5 volts at idle and about 13.5v when moving. When I got home I put the Ctek on and only the first thre lights went on. Left it overnight, all lights on and started healthily.

Later in the day it also started healthily. But I've just been out, it started, but not too vigourously, I only drove half a mile to the garage (it's very cold) to get some things...when I came out it only just started. When I got home, engine off and when I tried to start it...dead.

So...battery or alternator? The Ctek is on again...two lights again.
 
I'm going to say alternator.

Start the car and switch on all lights, heated rear screen, and blower motor. Then test the alternator output at the battery with a multi meter.
 
I'll try that in the morning when it's charged. Thanks.
 
I'd go for battery (just to be contentious). If the voltage is low at idle but still goes up to 13.5v when running at higher revs, it seems the alternator is doing something. A dying battery may charge up overnight and give all the signs of health, enough to start a time or two, but then not hold charge properly, thus leaving you stranded. I've had it recently on my C43 and on my motorcycle.
 
I hope it's the battery, it's just turned two years old...but has a three year guarantee.
 
I would offer that alternator output is on the low side. Taking the nominal battery voltage as 6x 2 [ 2.2] volts= 12v [13.2] the alternator is not going to charge the battery at 10.5 volts. Likely the brushes are worn and depending on the alternator model you should be able to replace the voltage regulator/brush pack for a lot less cash than a reconn'd alternator.
 
So...it's the alternator.

It will either be rebuilt or renewed depending on what they find.
 
Last edited:
Alternator rebuilt...£305 lighter in the pocket.
 

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