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alverstone wrote:...Turn the key and the starter motor just makes a horrible clicking and grinding sound. A jump start at the battery starts the engine, so it's not the battery....

That sounds to me like it is indeed the battery. It may have an internal short in a cell when the temperature rises.

A jump start does not prove the battery is at fault. From a recent experience with a mate trying to find out why the car would not crank at inconvenient times and after replacing a battery it did not solve the prob- the good oil is take it to a competent auto elec and get the fault diagnosed. The mates car had a short in the starter motor - difference was 12 volt in but not out. The checks will possibly not cost anything. These things do not fix themselves.

OK, so if it won't start (and he describes a very low voltage condition that causes the starter solenoid to rapidly engage and disengage - thus the clicking/grinding sound) and an external jump (with an assumed 12 volt source) makes it start (and assuming that the relay was put in the start circuit), what else could it be but the battery other than completely stuffed ignition circuit wiring or a stuffed ignition switch?

the example I cited is only days old... battery replaced... car would start with power pack or jumper leads.. culprit - starter motor internal short. Proper problem definition sometimes with multiple causes has to be established

I'll take your word for it but, I find it difficult to see how a starter motor with an internal short will not work with about 12 volts but will work with about 14 volts. The OP's mechanic stated that he only had about 4.1 volts at the starter, so that indicates low power at the source (battery) or through the starting system wiring.

It'll be interesting to see the outcome.

So the car starts fine in the morning after being parked for, say, >10hours?

alverstone wrote:A jump start at the battery starts the engine, so it's not the battery.

Not saying it's definitely the battery, although first thing I'd do is to borrow a known good suitable one & swap, but can't agree with your conclusion above that it's not the battery.

There has to be a fault which can be picked- suggest talk to a few car repairers or your local Bursons etc and get a recommendation.Try Tracy Martin's book How to diagnose and repair automotive electrical systems- readily procurable on net or maybe your library.Or maybe the bloke who sold you the relay might look again?

Might be some leads in this for you re place to go in Perth:
viewtopic.php?f=18&t=41294

alverstone wrote:In answer to 1red5's question, the car is fine overnight.

OK, so this almost rules out a dud battery.
What is the relay for? Pardon me if I missed it, the starter?

alverstone wrote:It was the same issue a few times last year after I had a new alarm fitted, but only in March when we get the hot and blustery easterlies.

Wonder if it could also be the alarm itself.

alverstone wrote:Oh, the same guy who fitted the relay instaklled a new battery 8 months ago and tested it during the recent work.

I love it when we get the full story . You have a wiring or starter circuit issue - see a reputable auto electrician.

I have an aftermarket immobiliser on mine that started going feral on hot days. The starter circuit got weaker and weaker until it no longer worked and got bypassed.

I also had erratic starter problems caused by a loose earth strap. The main earth return for the starter motor is from the body near the boot to the PPF (that's the ladder-like beam between the gearbox and diff) just in front of the diff. If the bolt on that works loose then you get a different starter experience every time you try it.