Alternator fault or something causing a short?

[skeffling]

Hello - first post here, so hopefully I'm putting this in the right place!

I have a problem with my 07 plate Mercedes C220...

In short - after being fully charged, the battery goes flat after an hour or so - perhaps less, when the car is left on the drive with the engine off.

I've measured voltages and current as follows:

Voltage across battery, disconnected from car, after being charged = 12.6V
Voltage across battery when engine running = 14V

Current when engine off = above 20A - off the scale for my meter and sparks ensue!
Current when engine running = 8.4A

Current measurement was taken with negative terminal removed from the battery and the meter between the battery's negative terminal and the negative cable.

I've tried a second battery, and the same happens.

So, it seems there is a short somehow, I had thought it was the alternator, but that does seem to be charging the battery when the engine is running...

What could I do next to diagnose this further myself before deciding to spend £unknown with a mechanic ?

Thanks!

[sliks]

That is a considerable drain. You might be able to feel one of the positive wires around the battery bei g hotter than others.

Otherwise remove both terminals from battery, set the meter to resistance/continuity and connect to the negative and positive leads. Wiggle the positive wires as they go away from the battery. The meter should give you an open circuit when you wiggle the shorted wire. So basically find the short to ground.

[Peter-jforf]

As ex Auto sparks, thats an awfull High leak on Battery with all Electrics off !!, i would disconnect the Alternator....ie all cables...do the leak test again...is it all clear now, of course you might get a tiny leak say point 1 / 2 mili amp as your car may have factory Alarm / Imobiliser....But the 20amps leak could well be a Doide gone in the Diode pack in the Alternator...hence a "High leak" as soon as you switch engine off....Yet show a charge when engine running. Peter

[skeffling]

Thanks both, I'll focus on the alternator - will unplug/remove and test etc..

[steveo3002]

alt would most likley be hot to touch too

should be easy to find though...

[Arnie Cunningham]

Yeah, I think I've had a rectifier fail in an alternator before that did this.