Battery drain

[Stu at shmx]

Hi all
My 1st post here . I've searched and still carnt put my finger on the problem
2007 a3 2.0 diesel . Got a drain that will flatten battery over 2 days
Right this might take some time
I've a 5 w bulb in series on the neg pole . Lock the car then disconnect the battery and clip bulb on . Car won't go to sleep and the bulb pulses , I've pulled every fuse . And disconnected everything I can . Dash out ,heater control out , every plug under bonnet , .Door looms unplugged, central locking module and the list goes on
So if I lock the car pull the neg lead off and put the light on battery and to ground then unbolt the wire from front of fuse box to centrol locking relay the bulb does go a bit dimmer but then unplug the can-bus module behind the glove box and bulb goes out in about 20sec
I'm out of ideas on this one
Help
Cheers Stu

 

[ScottyP45]

Putting the 5w bulb in series will cause other odd (non permanet) issues with the ECU and other controllers, as your creating a "potential divider" (google that) and the bulb gets a share of the voltage, dropping the value across everything else and things start acting up worse. You need a proper shunt ammeter that reads current but has near zero resistance so it has negligible voltage drop across it. Put that in series with the negative and see what current is drawn. I'd put my money on the rear window demister or the mirror heaters. A relay could be welded at the contacts and keeping something on.

You would then pull non essential fuses first, working upward in importance leaving most controllers till last thing to see if it's a simple peripheral but if it's a controller, that would be hard to trace. Disconnecting any of them could cause a canbus error and set the others chattering over the network creating the impression the culprit is still "on".

The pulsing is probably the ECU rebooting from low volts and pulling a canbus cable may just be triggering a total shutdown.

 

[Stu at shmx]

Ok thanks I'll give that a try .

 

[ScottyP45]

No worries mate, there will be about 3 to 5 amps drawn on reconnecting the neg lead as the controllers all boot up then it settles to about 2 amps constant for a while, then eventually the ECU will go to standby but it's never off completely so there's always a light drain. The immobiliser is always active, as the remote controller for the key fob is always on and it is monitoring contacts and ultrasonics. Id put the meter on it, measure amps and start pulling peripheral fuses. Have you had anything plugged hard into the cig lighter socket or installed anything in the car connected to that circuit or anything else?

 

[Stu at shmx]

Not long had the car so don't know the history. The car don't have any aftermarket extras

Would 3-5 amps cause a big spark on the + side when just touching on the pole . I don't connect + last just tried it today to see if much of a spark

 

[ScottyP45]

Check this thread - post 16:

https://www.audi-sport.net/xf/threa...-not-squirting-sportback.174397/#post-1742406

Yeah will be a bit sparky but only takes a few amps to do that.

 

[Stu at shmx]

I'll post my finding

 

[Chris Parrott]

I had very similar symptoms to you. Turned out to be the alternator.

 

[ScottyP45]

Very good point that occurred to me this morning too Chris, that as you say the alternator may not be charging the battery to full or the battery is dwindling and simply running out on the standby load.

Stu, you will need to measure the battery voltage while running mate, should be around 14.4 to 14.8. Current will start off at up to 60 or 70A and reduce back over time till the battery hits full, usually 35 mins minimum of running. This would need to be checked with a DC clamp meter or actual in-car ammeter which is worth fitting for every day checking. Ebay will have loads for a few quid

 

[Stu at shmx]

well I thought id sorted it , had a amp clamp on it and was reading 0.003 after everything shut down so thought was low enough not to be a problem , but no , car has been sitting for 4 days and battery flat again , stick jump pack on and starts fine , check charging and 14.4
lost on this one and can normally sort most things

 

[abmat]

Is there a chance the battery is knackered and has got to a point where is can’t hold charge?

 

[Stu at shmx]

It's new . Thought that was a duff new one so changed it

 

[Chris Parrott]

I tried a new battery too.
Have you tried disconnecting the + lead from batt overnight? If a diode has go on the alternator it will draw current when battery is connected.

 

[Stu at shmx]

Yep if I take lead off then it's fine did it for few days and no prob . Almost to the point of putting a isolator switch in

 

[Chris Parrott]

Not a bad idea, sounding very much like alternator.