Starting fault on scrambler

[manwithredbike]

I know a few on here ride scramblers, and i'm guessing the electrics might be similar to monster from the same era?
Bad week for my friend and his 2017 803cc flat track pro. He unintentionally drained the battery last week leaving the ignition on for a few days and then accidentally connected a charger on reversed polarity. Now, with a fully charged battery, ignition on, all fine until we press the start button. The relays (main, fuel pump and starter solenoid) cycle on and off, not a fast chattering on and off, but a steady rythmic on/off. everything else, clock display, lights etc goes dead (off and on) at the same rythm as the relays, and the starter doesn't turn.
When we turn on the ignition and jump the starter solenoid terminals, the bike fires up and runs fine, so the problem seems to be located in the starter circuit somewhere. All circuits lead back to the control unit so it's difficult to fault find without a schematic of the actual control unit.
The question is, could the reverse polarity have damaged the control unit or some other device connected to it. The ignition was off when the reverse voltage was applied though there are a number of items connected to the battery pre-ignition.

Would appreciate any thoughts or suggestions to help sort it and whether you know anyone who would test the control unit if we need to do that.
Would also be good if anyone knew where we could get a look at a schematic of the control unit. We already have the schematic of the bike wiring.

Thanks in advance -- Phil

[Ron1000]

When you say ‘jump starter solenoid’ are you using the same battery?

Simple things first.

1. Borrow a known good battery and see if starts normally.
2. With everything off what’s the dodgy batt voltage?
3. With it started (by any process) what’s the dodgy batt voltage?
4. Checked all the fuses?
5. Possibly damaged the starter solenoid / relay assembly. Check then replace if required. Super easy to whip it out and bench test it.

[manwithredbike]

Many thanks Ron.
The scrambler battery was weak even before the 'event', there's a new one ordered and we wanted to rule it out before we started checking, so we used my monster battery which is fairly new and in good order. It didn't fit the scrambler battery compartment but we lugged up a set of leads and bolted the connections.
Checked all fuses, all ok. Voltages all seem ok both stopped and running but will re-check them.
Will prob pull the starter relay module out and test it but i suspect that the prob is upstream somewhere as the main relay is cycling on and off too, and the starter relay is fed off that.