Got a 2009 2.0lt diesel S Max that has devolped a strange fault that I can't sort out. It has a new battery and alternator and the charging system is working correctly. It will run perfectly for a days and then suddenly the battery is totally flat in the morning, jump starts straight away and then all is normal until it happens again. The fault seems to correspond with the stereo/sat nav unit turning itself off, the unit will go off and stay off, a few days later battery is flat, after boosting to start, the stereo unit returns to normal until the next time.  The battery and charging system have been checked and are working correctly. Any ideas?  Thanks.

1 hour ago, mark.diver said:

... The battery and charging system have been checked and are working correctly. Any ideas?  Thanks.

Yes, you likely have a fault causing a parasitic battery drain. Unless you are handy with a multi meter, time to consider taking the car to an auto electrician who can use their expertise in tracking down the fault, and then advising on a fix.

46 minutes ago, StephenFord said:

Yes, you likely have a fault causing a parasitic battery drain. Unless you are handy with a multi meter, time to consider taking the car to an auto electrician who can use their expertise in tracking down the fault, and then advising on a fix.

I've used a multi meter on it and there is no drain whenever I've tested it, seems like an intermittent drain which makes it even harder to locate, was just hoping that someone had some experience of where to start looking.

First suspect would be the stereo.  Sounds like it could be shorting out somewhere which would then drain the battery.

It is possible that it's a red herring though, and that it's shutting off because of the low battery voltage, instead of being the cause of it.

39 minutes ago, TomsFocus said:

First suspect would be the stereo.  Sounds like it could be shorting out somewhere which would then drain the battery.

It is possible that it's a red herring though, and that it's shutting off because of the low battery voltage, instead of being the cause of it.

That is a possibility, I was starting to think along those lines, but can't see how that would explain that the stereo coming back to life after jump starting the car, and sometimes the stereo fails and it is weeks before the battery goes flat, maybe not connected.

 

4 minutes ago, mark.diver said:

That is a possibility, I was starting to think along those lines, but can't see how that would explain that the stereo coming back to life after jump starting the car, and sometimes the stereo fails and it is weeks before the battery goes flat, maybe not connected.

You'll have to do some testing.  Next time it fails, check the battery voltage straight away, and see if there's a parasitic drain an hour after parking.

Alternatively, you could try simply replacing the radio with a good used one.  If it doesn't make a difference, you can sell it on to recoup the costs.