Gents, I'm super excited to be able to report back finally some success!
So in troubleshooting last night, I followed the light of
@Major Ledfoot and chased the red wire back to the battery. Despite continuity and no apparent resistance on the multimeter all the way back, this showed me that it was indeed the wire that passes through the 25A fuse holder in the engine bay, and then straight to the battery +ve terminal clamp in my case. This wire was reading 12V before the fuse holder (and all they way back when unloaded) but only 2.5V after the fuse holder when the circuit was loaded. A good indication that continuity testing is basically useless in this instance, as there are no open circuits or dodgy earths!
So long story short,
@Slowbrick was also right - that fuse/holder was corroded and not letting enough juice through. But stick with me, the story gets better... I bridged the pins of the fuse holder, and suddenly have 12V under load at the fuel pump relay, but still no relay action on turning ignition to position 2.
I tested the relay off the battery using Dave Barton's helpful pinouts and circuit diagram: jump the two left-most pins (labelled 30 and 85) to battery positive, and then earth separately pins 86/1 and 86/2 to test each side of the relay. It was clear that both sides of the relay were working fine.
So back to the fuse panel in the cabin, and the earths. Taking all of the fuses out, cleaning all the ends and the terminals with the Dremel, and doing the same with the earths, felt very satisfying. And it worked! The relay would now happily click away when ignition hit position 2.
This morning I went for a more permanent fix on the pre-pump blade fuse holder, and just cut it out and replaced the whole thing and the wire to the battery with a new 40A cable and waterproof fuse holder. Now she purring like a kitten.
I'm going to go ahead and guess that the hot/humid weather we had in Melbourne while I was away, combined with the car being sat for a while, brought some corrosion issues to the fore.
So basically everyone was right! I should have started by chasing that red wire back as
@Major Ledfoot suggested, as it would have allowed me to isolate the error so much more quickly.
@tbro,
@VolvoDeger and
@Slowbrick were right about the fuses/fuse blocks being at issue. My next step is to take Dave Barton's advice and use a regular cheapy 4-pin relay to take most of the load off the fuel pump relay, as it still runs a bit darn warm with all that current. This just seems like a no-brainer.
Thank you again, so much, for all of the kind and generous advice. I'd be very honoured to buy you all several beers!
I hope if nothing else, this thread can serve as a useful tool for those troubleshooting similar electrical gremlins - there are some gems within from the wise.
I never want to look at this PoS again:
Replaced with one of these, which neatly tucks behind the original bracket!
