Electrical System Diagnostics - Finding an Electrical Draw
Disconnect the negative battery cable (-). Connect a 12 volt test light between the negative terminal of your battery and the negative battery cable which you just removed.
1. If the test light is lit-up or shows a faint glow you’ve got some kind of drain on the battery.
Make sure you don’t have any accessories on in your car (don’t forget the dome light if the door is open). You can now begin to track down the circuit that is causing the draw on the battery. Leave the test light set up as before and now go to the fuse box, and one-by-one remove each fuse until the test light goes out.
Once the test light goes out, you have found the circuit that has the drain. Now reinstall the fuse. One-by-one disconnect the wires fed by that fuse until the light goes out. Check the color of that wire. Now reference your service manual wiring diagrams (or fuse panel cover on newer Volvos) to identify the wire by color, and the accessories that it supplies power to.
Now you can find the culprit that is mercilessly draining your battery and begin troubleshooting the component and related wiring.
2. If the test light doesn’t light up, you’ve probably got some other problem such as a bad battery or bad alternator.