A while ago I thought my passenger door lock was flattening the battery as when charger was connected, the motor seemed to run.

Anyway, I’m getting a very odd symptom (odd to me anyway)

So. If I don’t use the car for 4 or more days, battery is so flat doors won’t unlock or any lights show. Will start easily with a jump start.

But, if I lock and unlock the car several times a day over the same period, and NOT start the car, it holds its charge just fine.

I have ruled out battery by replacing it.

I’ve had the antenna Wi-Fi thing fix since owning the car. (5 years+) This is a new issue over last few months.

Any ideas where to check first?

Thanks

If I leave the Ascot for 3-4 weeks, the battery is fine (and the EAS stays up) but if it is left for about 6 weeks, it will be flat when I go to it. As soon as I connect the charger, the rear right door lock will try to lock (even though it already is) once every couple of seconds. It carries on doing this for maybe 15 minutes or so until the battery voltage has risen and then it stops. Whether it starts to do it when the battery voltage is falling and reaches a certain point, I have no idea but I suppose it could do which would accelerate the battery going flat.

The only way to check why yours is going flat is to check the current draw with either a clamp on ammeter or one in series with the battery. Once 2 minutes has elapsed and the BeCM has gone to sleep, it should drop to something like 25-40mA. Any more than that, or the BeCM waking for some reason, there is a problem somewhere.

I’ll borrow a clamp and test it out, thanks.

I’ve noticed my dash cam is staying active. It’s wired in to the becm output, that normally turns off after a little while of ignition off. It’s clearly staying on now. It 100% used to turn off so this is a new issue. What could keep the becm awake?

BeCM SID is here https://drive.google.com/file/d/1bYljTTLbcRvp6wLJ-3qxhWPEhr4agC-G/view?usp=share_link, page 9 onwards gives a list of everything that will wake the BeCM and gives a step by step diagnosis to battery drain.

Huh.. so if you're charging the battery with the bonnet open, the BECM will stay alive. Interesting.

Oh really. I often park mine with the hood open (with a battery tender). With the hood open it seems to be less inviting for squirrels and rats to make themselves at home.

I understood that to mean it will only wake the BECM when the switch state changes since it says edge triggered.

Thanks, that’s a brill document!

Gilbertd wrote:

BeCM SID is here https://drive.google.com/file/d/1bYljTTLbcRvp6wLJ-3qxhWPEhr4agC-G/view?usp=share_link, page 9 onwards gives a list of everything that will wake the BeCM and gives a step by step diagnosis to battery drain.

Some versions of BECM will not lock the car with bonnet open . . others do seem to ? Both my P38 now have the bonnet switch disconnected, which means I can test the locking, sleeping, etc with it open !!

Mine locks with the bonnet open, it just gives me a little beep to let me know. I don’t think I’d disconnect the switch, it seems to be a nice safety feature.

@Harv……unless you happen to leave your keys inside when you pop out to open the bonnet to refill the washer fluid while parked at pump #3 at a Shell service station in the pishing rain only to find yourself locked out and red faced with an understandably impatient boll@x behind in a 22 reg Mercedes, then I would imagine your view of the safety feature may change to one of “Why in the name of all F&%KS would someone design such a thing?”
Not that this ever happened to me….

Oh wow, I didn’t think it was actually possible to lock your keys inside of a P38, though I’m pretty careful to leave a window open if I close the doors with the key inside. I did lock myself out of a Dakota I used to have (with it running at the ski hill 60 km from the nearest petrol station).

The only time it will lock you out is if you have suffered dancing locks for ages and ignored it. To do it, it has to lock itself without you doing anything, so that is the only time it will happen.

Good to know. Thanks Richard