Parasitic drain on P38....

Hi guys

The truck,4ltr petrol, is killing batteries. I've checked that nothing is still lit up at night, changed the RF receiver and at my wits end. Brand new battery reading 11.98v after one day stood and reads 13.98 when running. Checked all the fuses and none are blown. No warning messages on the dash indicating any issue with the alternator.

Any suggestions would be greatly welcome as will need to start relying on the truck as the weather gets bad, the Golf just doesn't cut it in the snow! but at the moment I can't trust the truck for more than a couple of days to start....

HELP PLEASE!!!!

Thanks Bruce

Pop the hood then lock the car and leave it for 10 mins. Get yourself a metre and set to read DC Current (DCA). Begin by pulling the first fuse and put the meter across the terminals where the fuse was. Record the current. Repeat until you find one that is pulling more than it should.
Worth remembering that if you are saying that the battery lasts 2 days (48 Hours) and you have a 75 AHr battery then you will be looking for a current reading of just over 1Amp.

Might be a failed diode in the alternator if it's dropping that quickly.

Cheers guys, have a multimeter but not sure what setting it should be on to test the fuses?
Is there any way of checking the diode in the alternator? Please bear in mind I'm a novice!!! Laymans terms are best for me!!!

set to read DC Current

I had a similar rapid drain problem last year which turned out to be a dodgy heated windscreen connection so you could try checking the two relevant yellow relays to see if that helps. Took my electrician a while to work that out and members on here were brilliant with suggestions so i'm sure you'll work it out.

P38 battery drain is often rightly blamed on the RF receiver and the BECM not going to sleep.BUT around 30% of the ones I have fixed have been for other reasons.
Have a look around for old phone kits or tracker systems - these often have nicad batteries in them which have their own charging units built in.Now the nicads are old they are being charged constantly,which will rob the cars battery.
Another one is the memory in the factory CD changer,the BECM will stay asleep,but the CD changer will keep dragging current.

Cheers guys, noticed the rear heated windscreen doesnt work so will check the relay for that; in the fuse box under the bonnet I think? Don't have a factory fitted CD changer.....were they standard?

With the RF receiver changed already can't be that?..

Just been out to check and the alarm LED is still flashing so still working but is a brand new battery....tomorrow is another day!

Another reason why the battery may be draining is the becm could be waking up due to mobile phones, faulty alarm systems (households), any signals from transmitters can wake the becm up for ten minutes at a time reseting the suspension etc. I found disconnecting the blue lead which connects the window antenna to the receiver worked for me but this means getting a little closer to the car to activate the locking system.

BruceVeedub said:

Cheers guys, noticed the rear heated windscreen doesnt work so will check the relay for that; in the fuse box under the bonnet I think? Don't have a factory fitted CD changer.....were they standard?

With the RF receiver changed already can't be that?..

Just been out to check and the alarm LED is still flashing so still working but is a brand new battery....tomorrow is another day!

Click to expand...

I assume that the replacement RF receiver is the latest spec and not an Ebay special?
Charge voltage is on the low side so I would go with Wammers and suggest perhaps an alternator dode failure.

RF receiver from dealer :-(

If you disconnect the heavy wire from the back of the alternator & leave overnite it will eliminate it or not

Unplugged it on the old RF unit before I replaced it but it didn't make any difference :-(

I mean the wire on the ALTERNATOR, if the diodes are faulty as mentioned earlier then they allow leakage, however when you disconnect the wires, if the battery stays up then it confirms that the diodes are shot. If you still get drain then it is not an alternator fauklt. It is not related to the RF unit

Sorry misunderstood! Will give it a try at the weekend

Hi
is your ignition barrell where the key goes in coming out all the way when the key is taken out, mine sticks in so i have to tap it so it comes out otherwise the battery will go flat,

BruceVeedub said:

RF receiver from dealer :-(

Click to expand...

Are you sure they sold you the latest version RF receiver? You could always unplug it and see if that cures the drain.

mgmayo said:

I mean the wire on the ALTERNATOR, if the diodes are faulty as mentioned earlier then they allow leakage, however when you disconnect the wires, if the battery stays up then it confirms that the diodes are shot. If you still get drain then it is not an alternator fauklt. It is not related to the RF unit

Click to expand...

Maybe a daft question but I'm not mechanically minded or adept; would a failed diode on the alternator cause a drain if the truck isn't being used?

yes, that's the problem, the diode 's purpose is to allow current flow in on direction, when it fails it allows it to go to earth and drain the battery

Cool, or not! Thanks for that mate