How can I diagnose gradual electric power loss in a VW Golf Mk5?

Question

I'm experiencing loss of electrical power to my Mk5 Golf. It seems to happen mostly when the car hasn't been run for two or three days.

We changed the battery a few months ago. More recently, there was a loss of power and the car wouldn't start. We'd diagnosed it as the alternator, which was replaced. This worked fine for a week or so, but I'm now at the problem where I'm experiencing loss of electrical power - car sounds uncertain about starting, clock is reset etc. There's still some power, because the radio, lights and clock etc still operate, even if the car doesn't start.

The remote locking feature only half works, too. I can lock remotely, but I can't unlock remotely - I have to use the key. The fob has started to show a red light when the unlock button is pressed.

I'm assuming the electrical power is being lost somewhere else. I'd previously checked for doors not shutting properly or anything else I could think of that might leave a light on, but there was nothing to suggest that was happening.

So how might I start to diagnose how the car is losing electrical power? I'm completely stumped.

Answer 1

The fault you describe appears to be a simple matter of 'parasitic drain' on the battery.

Disconnect the battery earth terminal, connect a ampmeter between the cable and the battery post. Run the ampmeter to outside of the bonnet and close the bonnet without trapping the ampmeter wires. Make sure all doors are closed, they do not have to be locked. After about 10 minutes, check the amp reading.

After around ten minutes all modules will go to 'sleep'. The ampmeter reading should now be at about 0.03 amps to 0.5 amp max. The doors must be closed as opening the doors activates the initial operation of the fuel pump, the bonnet being open activates isolation of the windscreen wipers. Both of these mean the vehicle will not go to sleep.

If the ampmeter shows a high reading, you must now systematically disconnect circuits until the amps drop off. One way is to disconnect one fuse at a time, check the ampmeter. If it has now dropped, that is the circuit that is causing the drain. Rectify as necessary. If the amps do not drop replace the fuse and then disconnect the next fuse. Repeat the procedure to find the drain. If you have none standard accessories, such as amplifiers, the disconnecting routine should start with these.

Finally, even if a battery or a replacement alternator are new there is an outside chance that they may be defective and this should be borne in mind.