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Discussion Starter #1
Hello fellow Smarties,
I'm after some ideas to a problem I have with the charging system on my ForTwo 2008, if indeed it is a problem. Car does a lot of short runs, the other day it wouldn't start, low battery, had to call breakdown as I was away from home, we got it running with a booster and the guy tested the charge, at idle it was showing 14.5volt but when the revs were increased it dropped to 12.7volt, turn on the lights and it was back up to 14.5volt?
I've since changed the battery but it's still the same, I've had the alternator checked and they said that was fine, I know the charging is handled by the ECU so is this normal, or is it a trip to the dealer for a diagnostic and a bank loan!
:eek:

Thanks in advance.
 

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If the alternator will charge at a minimum of 13.5 volts or more at idle with all the electrical accessories on it is good. If the voltage goes down when the engine speed is raised make sure the alternator belt is not glazed or slipping. Maximum charging voltage at higher engine speed is 14.5 volts. The electrical load is monitored by the engine control module but how much control it has over the charging voltage is unclear.
 

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Discussion Starter #6
Thanks guys I'll have a look at the belt and maybe change it, anyone done this, is it much of a job?
 

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I had a tough time getting the tensioner back in place after removing the belt. the new belt was a bit less stretched out I guess. I ended up grinding down an allen wrench to fit between the tensioner and the frame so I could compress the tensioner enough to put the belt in place. I was working from underneath after taking of the splash guard. Once I made up the tool, it was a piece of cake.

 

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Another option, as you noted it had lots of short trips, which could cause this:

Try getting a plug-in solar panel to trickle charge the car while it's idle. I have a 2008 ICE that has a similar life (lots of short trips during the week, longer trips on weekends). You'll have to wire in a direct (fused) line to plug into, as the default socket is switched with the ignition. I have a direct line in the glove box, with a cover for when it's not needed. But usually it's got the solar panel plugged in 24/7.
 

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Discussion Starter #10
Ok guys, I've had a look at the belt and all seems ok, interestingly I connected a multimeter across the battery terminals and took the car for a run, at a steady speed the meter was showing 12.6v or there abouts, took my foot of the gas and it went up to 14.2v, back on the gas and down to 12.6v, this happened every time, turn on the side lights and it charges at a constant 14.2v under all throttle conditions, I could understand this if the battery was well charged but after the car had just been started surely the battery would require topping up, don't figure! Is this going to be a case of paying through the nose for a diagnostic at Mercedes, mmmmm.
 

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Sounds like either a belt slipping or a bad alternator. I wouldn't just replace the alternator, but put a belt in at the same time.

Mine was running at about 11.5-12.3 volts when I changed the alternator. about a week later, the belt broke. Luckily I ordered a belt when I got the alternator.
 
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