2005 Holden Astra Pricing and Specs

Going back to first principles, anywhere that a power wire that should be insulated is allowed to touch an earthed part of the car is a potential source of a short circuit. As such, there are about a million places where a wire or cable carrying 12 volts from the battery could be shorting out by touching an earthed part of the car.

Typical sites of a short-circuit include anywhere where a wire runs through a metal bulkhead (say, from the car’s interior to its engine bay) anywhere a wire is continually flexing (the wiring inside a door jamb where it passes through the body) a switch of any sort or anywhere else where friction could have worn away the wire’s insulation. Sometimes the insulation just gets old and falls off the wire (Mercedes-Benz cars from the mid-90s were notorious for this).

The clue will be in what components have suddenly stopped working. Checking the fuse box to see what fuses have blown in response to the short circuit is also a good indicator or where to start looking. If you suspect the short-circuit is at the battery itself, there will be no need for forensic investigation; a short at the battery will produce a violent spark and lots of smoke. Unless, of course, the car is fitted with a great safety feature called a circuit breaker on or near the positive battery terminal. If you’ve suddenly lost all power to the car, that’s where to start looking.

Finally, don’t assume that a loss of power to the car or some of its components is due to a short circuit. Sometimes a fuse can simply fail for no good reason and call a halt to play. If you’re unsure, an auto electrician can work miracles that the rest of us cannot generally fully comprehend.

You could have a fault with the car’s wiring. The starter-motor takes its power from the fat, red and black leads that attach to the battery, but it also needs a low-voltage signal from the ignition key that tells it to do its thing. If there’s any sort of loose or missing wire, you might not be making the necessary electrical connections. You could also find that there’s an on-board security system that is blocking the starter-motor from turning. You could even find that the ignition key has a flat battery and is not unlocking the car’s on-board computer.

You say that the battery has voltage, but I wonder how you checked this. You can often put a multimeter across the battery terminals and see 12 volts, but this can be what’s called a surface charge that’s strong enough to light up the dashboard, but then disappears when you put the battery under load (by trying to start the car). Attach the multimeter and check the voltage while actually trying to start the engine. You may find you have a dead battery after all.

You’d expect any noise from under a car’s bonnet to be caused by the engine, but if this noise occurs before you’ve started the engine, you’re on to a real mystery. Perhaps the noise can be heard as you crank the engine but before it actually fires? If that’s the case, the clunk or knock could be coming from the starter motor. Or perhaps the action of the starter motor is causing the engine to rock back and forth on worn engine mounts and allowing the engine to hit on the cross-member or some other component under the bonnet. Perhaps it’s a loose exhaust system that’s banging on something as you crank the engine. Once the engine fires and is running, the frequency of its vibrations changes and the clunk goes away.

But I’m afraid that without more information, this one remains a mystery to us.