C240 Undervoltage

[CMR]

Twice now my display has showed ‘Undervoltage - Switch off consumables’. Once when I had the wipers and side lights on in heavy rain. Switching these off at the time would have meant no longer being able to drive the car! The second time was with just the cd/radio on (which I suppose I live without but I didn’t think they draw much current). Does this mean I should trickle charge the battery? (ie Disconnect overnight and charge)?

Background info: Car C240 Y Reg. New battery fitted July 2006 just prior to my purchase of it. Only electrical alteration was ‘after market’ alarm disconnected since its was going off every time I unlocked the doors. Usage: Only used weekends. Mostly short journeys (10-40miles), occasional once a month long journey (300 miles inc.)

My real concern is what I have read in previous posts where damage has been done when disconnecting/connecting the battery (see below Title W203 C240 strange warning message) and so I’m pretty concerned what I should do. Any help would be much appreciated!

Quote as previously posted:
"Did you replace the battery yourself? I know of someone who disconnected their battery on a W203 and when reconnecting found the same problems as you, wipers, indicators etc. were not working. They took it to the dealer and it transpired that the SAM module had burnt out when the battery was reconnected. This is a well known thing apparently."

Posted reply
If the battery change can cause damage, I would like to understand what additional measures need to be taken to ensure that SAM damage does not occur.

Unfortunately the reply then says
Its a guy called mobeyone on the 'other' forum (mbclub.co.uk) There's a thread about it. I wont post the link here though.

So could anyone help re: the above and what are the risks to damaging the SAM unit if I have to charge the battery (assuming that is I have to charge battery to stop warning message)?

Many thanks for any help

 

[jberks]

My understanding is that you can only damage the SAM if the key was in and turned on. With everything switched off you should be ok - but wait for someone to confirm this - don't just take my word.

In any case, I wouldn't remove the battery - it's probably fine, just needs a recharge. The 300 mile yourneys will help, but this time of year, if it's stood a lot, the battery will go down over time as the car never entirely turns off. try a trickle charger.

 

[CMR]

Thanks for reply. When you say just needs a recharge does that mean go on a long journey? Re: remove battery - only problem I have is as I live in a flat, the car is nowhere near a power socket to connect a charger.

 

[carabind]

Assuming the message is correct (ie low voltage), it can only be the battery, alternator or possibly a slight continuous drain (like the boot light sticking on) Someone like Kwickfit might do you a free battery test, which will measure the batter at rest, and when the engine is running. Voltage displayed will show if either is at fault. Takes about 2mins, worth trying a couple of places to get a second opinion.
PS Does it show the mesage when/before you start the engine, or does this come on after you've been travelling for a while?
If the former, battery, if the latter then alternator

 

[CMR]

The 2 times its come on has been whlist travelling for a little while, (though on both occaisions - short journeys 10miles or so? One time message stayed for a while then dissappeared so I presumed battery had recharged). Battery supposed to be new (couple of months old?) and the car has low milage so hopefully would not be alternator? I'll see if I can give the battery test a go though.

 

[watsone91]

The best answer here is to buy a cheap voltmeter connect it across the live negative terminals on the battery check the earth connections are clean and good start the car and switch everything on headlights rear screen heater air con everything the voltmeter should read no less than twelve a a half volts if it doesnt then the alternator is goosed .

By the way removing the earth lead while the engine is running will burn out the rectifyer in the alternator hope this helps cheers.....

 

[television]

A simple way to check the charging system is to start the car, then turn the headlights on and rev the car a little and the head lights should brighten up. OK a meter is better but not everyone knows how to use one.

A 10 mile trip is not enough to charge the battery on the car, maybe you could leave the car somewhere where this could be done, but do not use a fast charger, 8 hours at 3- 4 amps should do it, any attempt to use a fast charger will damage the components on the car. Your car is not a 203 and the electrics are very different.

Aternators are not made to charge flat batteries.

Malcolm

 

[robbo]

Your car is not a 203 and the electrics are very different.

Isnt it? I thought the W202 was phased out in 2000, so a 2001 Y reg C240 would be a W203 model?

 

[television]

Isnt it? I thought the W202 was phased out in 2000, so a 2001 Y reg C240 would be a W203 model?

Sorry, thought I read it was a 202 though a Y is early 01 and could be a 202 it always causes confusion with out the correct number.

Malcolm

 

[CMR]

Thanks I'll give those tests a go. So basically not advisable to remove the battery to charge it then?

(Car is an early W203 - Like the ones in Mercedes Enthusiast Nov)

 

[television]

Thanks I'll give those tests a go. So basically not advisable to remove the battery to charge it then?

(Car is an early W203 - Like the ones in Mercedes Enthusiast Nov)

This I why I said that not more than 4 amps.
In order to increase the charge rate (amps) you can only do this by increasing the voltage, when charging in the car the max voltage is 13.8 volt.
By limiting the current to 4 amp max, no harm can be done, do not use an automatic charger, it must be fixed or trickle. When joining up the charger must be on its lowest setting, then turned up

Malcolm