Inspired by comments by member Gazedrop on post number 6 of this thread:
Where he managed to bypass the ECU's low-voltage inhibiting feature that prevents the engine from starting when it feels the battery is too low. He did this by removing the casing from the starter relay and operating the contacts by hand.
I set about modifying the starter relay to provide a permanent, easy to operate, emergency start button.
This mod costs absolutely nothing and it's extremely simple to carry out.
The idea is to provide the relay with a manual operating button so there's no need to remove the outer casing in case of difficulty. On these relays the casing is just clipped on and can be removed by levering with a small driver.
This exposes the relay mechanism itself and the idea is to drill a hole opposite the armature (that's the pivoted moving part that gets attracted by the electromagnetic coil), and find a suitable plastic plunger that can stick out of the casing and enables you to force the relay contacts closed.
I used a plastic stopper of the sort that close up the end of integrated circuit shipping tubes, but anything similar will do. Don't make the hole too tight, the plunger must be able to slide easily in it. Not too loose either, it would encourage water penetration. Not too short or it might fall inside the relay casing. The "mushroomed" shape on the inside prevents it from falling out, if you can't find a piece like mine, melting the end of a suitable plastic rod to widen it a bit will do.
Industrial relays often come with manual over-ride buttons on them, so this is an electrically sound mod. Nothing technically wrong with it. Here's a couple fitted with such a button:
This mod will help in the following circumstances, both on carbed and EFI models:
If "start" button or its wiring are defective.
If starter relay operating coil is open circuit or its supply is interrupted somehow (broken wire, bad ground, etc).
On EFI bikes the ECU can inhibit operation of this relay because of low battery voltage. Pressing the relay button will force the engine to start.
If the relay contacts have become stuck or welded together and the starter operates continuously regardless of the operation of the start button. This is a fairly common fault, we've had a few here. Repeatedly pressing the relay button might well "jog" the contacts apart.
The engine can be started even if ANY
of the safety interlocks fail: The clutch, sidestand or the neutral switches
This is of course a great feature, but be warned to ensure gearbox is in neutral before pressing the switch.
The engine will start even if the sidestand is down and there's no need to pull in the clutch either.
Starting and charging wiring diagrams for EFI Bonneville in case anyone wants to study it further: