Help - Battery totally losing charge on cold nights

[Gawdi]

Scratching my head over a problem with my 92 300D.

On nights when temperature goes under zero, the battery totally loses its charge - not even enough charge to work the central locking...

I thought it was a problem with a dicky battery cell, so replaced the battery and have a brand spanking new one (original mercedes) but problem persistes - any ideas?

 

[W210 Fan]

are you sure its a dead battery as the central locking us done by vacuum so maybe you have a knackered hose or resovoir?

if you have a charger or battery test meter I would check the battery is good, if you plug the charger on and the ammeter shoots up to max then its pulling a lot of current as has no charge, if it only pulls a small amount or none then its charged,

 

[grober]

You have something drawing current somewhere, or a leakage to earth which is draining the battery overnight. Remove the positive terminal of the battery and place an ammeter in series with the battery. You should see a current drain which will be higher than usual ( remember there will be a few things which take current all the time such as the clock so this current will never be zero). Then go to the fuse box and try removing each fuse in turn till you see a drop off in current. That should tell you where to look for the fault. This can be trivial e.g. the boot light switch jammed so the boot light never goes off even when the lid is shut.

 

[kusanku]

Could it be your alternator? If it is on the way out, it might be putting out some charge, but not enough. Might be fine during the day, but if driving with lights on, stereo etc, and sitting in traffic, the drain on the battery might outweigh the amount by which it is charged. So battery does not get charged properly, and after being parked on cold night, is flat by the morning.

This happened to me when I had my 123 series. Alternator was working, but not putting out as much charge as it was supposed to. AA man diagnosed it easily with a meter. Any autoelectrics place should be able to check out the alternator easily enough. Had a lot of trouble getting a replacement though, with it being an older car, and had to get the original reconditioned, which only lasted for about a year.

 

[grober]

It could well be your alternator. A faulty rectifier pack/control will flatten your battery quite quickly. The fuse test i suggested wont work but you could remove the connector plug to the alternator at the rear instead and check the current drain with an ammeter as detailed above.

 

[Ade B]

Go through the fuses first as Grober says - my money is on the boot light..

I had an identical problem, the battery would last 1-2 days max.




Ade

 

[jonm]

It could well be your alternator. A faulty rectifier pack/control will flatten your battery quite quickly. The fuse test i suggested wont work but you could remove the connector plug to the alternator at the rear instead and check the current drain with an ammeter as detailed above.

Had this problem with a car a while back, it was intermittent and in fact when the alternator was drawing current it was audibly 'humming'.

 

[NewMB]

Just had this problem as well. Was traced back to a faulty "Audio Gateway" that were eating power overnight. Since (costly) change, no probs.

 

[Stan the Man]

I suggest you check the alternator output . A voltmeter across the battery terminals should read between 13.8 and 14.2 volts with the engine idling. If the alternator output is correct and you have fitted a new battery then follow Grober's suggestion and go through the fuses. Likely offenders include the boot light, Comand (if fitted) and electric seats (if fitted).

Do not disconnect the alternator (from the battery) with the engine running, you will kill it immediately !

 

[grober]

Do not disconnect the alternator (from the battery) with the engine running, you will kill it immediately !

GOOD POINT when I said disconnect the alternator plug I meant with the ignition off of course but perhaps didn't make that clear.

 

[Gawdi]

Thanks everyone... I've ordered an ammeter so will check the alternatr output when it arrives (i did know not to disconnect it from battery.. lol) but I think I've isolated the problem to the boot light...........

It looks like theres a wiring problem, as the boot light goes on an off properly when manipulating the switch on the boot lid, however when you actually shut the boot, the light doesnt go off and infact goes from its normal dull glow into a bright light........

I think as short term solution I'll just remove the bulb...

 

[Ade B]

Thanks everyone... I've ordered an ammeter so will check the alternatr output when it arrives (i did know not to disconnect it from battery.. lol) but I think I've isolated the problem to the boot light...........

It looks like theres a wiring problem, as the boot light goes on an off properly when manipulating the switch on the boot lid, however when you actually shut the boot, the light doesnt go off and infact goes from its normal dull glow into a bright light........

I think as short term solution I'll just remove the bulb...

Bingo

Glad you've found that issue.

I unplugged the bulb and keep a battery fluorescent lamp in the boot as I've still yet to fix mine.. The switch works but somehow misses the strike plate on the boot cill..

Would be worth checking the other bits once the multimeter arrives to make sure.


Ade