748 Electrical Issue

Check the large multipin loom connector, often suffers from internal corrosion.

 

May be obvious but check switch connector (usually around the for area)

 

Double check the ognition relay, they operate but the internals can corrode and cause intermittent failure, my M900 needed the seat removing to start until I replaced the relay?! Your issie sounds identical

 

When this happens does the engine turn over but not fire up or will it not even turn over?

 

No idiot lights so I reckon it's the ignition relay being intermittent

 

Yeah, I wasn't sure about that, are we talking no headlamp, low/main, and no dashboard idiot lights or no just no dashboard lights?

iirc there's a multi pin connector that goes to the LH switchgear which takes a feed from the key switch. Corrosion in there or the LH switch itself would likely cause the issue of no headlights when the key is turned to on, overcome by fiddling with the switches on the LH switch gear. However afaik it's a different multi pin connector that feeds the idiot lights and then a 3rd connector that carries the connections for the tacho and the temp gauge. That last one also carries an injector enabling signal - hence my question about whether the engine turns over but doesn't fire or simply doesn't turn over.

Either way I'm with others above and I'd be pulling ALL multi pin connectors apart and checking for green corrosion inside them, was just trying to work out which one to attack first.

 

I'm no electrical engineer but I'd start with a few basics all over the bike...

1) I would check all earth connections, making sure that there is no corrosion and all are making good solid contact, especially those around the front part of the loom.
2) Check the main loom connectors, especially those on the right hand side of the frame (the multiplug as mention by others).
3) Try changing the ignition barrel, although if it stays running, this would indicate the barrel is okay because otherwise ignition circuit.
4) Star diving into connecters around the handlebars and cleaning once all the above is done.

The final thing, is that it could be a damp ECU. I had all sorts of issues with miss-fires on mine, and weird issues with fuel pump cutting out and re-priming and MotoRapido struggled to find the issue, they swapped the ECU, then once mine had been in the workshop a few weeks, they re-fitted it and it was perfect, they decided that damp had got in around the rubber bung where the chip can be changed, and I can't argue with them as it never broke again.

ECU's are obtainable and given the age of these bikes now, if you can get a good working spare I would have one. You only need 1 dry solder joint to reek havoc

 

If no idiot lights and no fuel pump prime I'd definitely change the ignition relay

 

and with no starter motor working ie the bike is completely dead I'm beginning to suspect two separate issues.

 

An English colloquial phrase

https://en.wikipedia.org/wiki/Tell-tale_(automotive)

 

If the ignition relay doesn't engage the starter motor won't turn either, all the same circuit

 

Yep, but a different circuit from the lights, no?

 

I'm not sure on that, certainly the parking light is a different circuit but I think the main lights go off with ignition

 

They go off with key out but are independent of the bike running. Anyway let’s see what the OP finds.

 

I had a similar issue on a 2001 748, couldn’t get any lights, no fuel pump prime, and no starter, repeatedly switching ignition on and off eventually worked, again once running it was fine, turned out to be the yellow relay mounted to the left of the battery, managed to find that by going round all the relays tapping them until it sprang to life

 

Yup, yellow relay