Leak to earth

Please can you give me some advice on a very annoying problem with my 2001 Peugeot 306 cabriolet. It has a leak to earth that flattens a fully charged battery within a week. The alternator is working fine and I have made all the usual checks like seeing if the boot light is staying on, but I suspect it is going to be a lot more technical to rectify. Would I be able to use a multimeter to narrow down the source of the leak? Tony Graham

The easiest way to discover the source of the leak is to disconnect the negative battery terminal and place a small test light in the circuit between the battery terminal and the negative lead. Although this can be done with a multimeter, using a 5W test bulb is a safer option than using the AMP setting on the meter, which may be overloaded inadvertently.

With the test bulb in the circuit, there will be a very faint glow from the element – this is nothing to worry about. A brighter illumination will indicate a drain. Now, with the bulb illuminated, you should systematically

remove and then replace each fuse in the fusebox. At some point the bulb should dim and this will indicate the circuit that is causing the drain.

Although you are happy that the alternator is charging correctly, this may still be the source of the drain and it is worth disconnecting the main lead as part of the test light process. Another possibility is that the alarm system is faulty; the sirens on these vehicles do tend to fill with water, which can cause battery drain problems.