Looks like I've got the Geordie version of the manual. That's okay, probably the best version :-)
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I've developed a theory as to what's gone wrong.
Over the past few weeks I've developed a remote start / stop panel for the genny. Basically a 100M long 3 core cable that goes to a box with 2 spring return toggle switches in my caravan. The switches operate 3 relays inside the genny's electrics box, which interface with the ignition key to operate the starter, fuel solenoid etc. so that I can turn it on and off from the caravan.
Whilst figuring out the electrics, I noticed the genny is kept running by a cascade of 3 relays which ultimately keep the fuel solenoid open so long as 3 conditions are met - fuel sufficient, engine temp ok, and oil pressure sufficient.
My remote design didn't work for ages and I struggled to figure out why until I realised the genny's oil pressure sensing relay was broken. On looking further into that & trying to replace the relay with a new one which still didn't seem to work, I realised that at some time in the past someone had bodged the wiring to bypass the broken oil relay.
When I first bought the genny I stupidly just took the word of the seller that it was all fine and ready to go - just started using it. At the back of my mind I did think 'must check the oil etc', but this didn't happen for ages. I probably ran the genny for 20 or 30 hours total before actually getting round to checking the oil.
When I finally did check the oil a couple of weeks ago it was bone dry in the sump. Yikes. Anyway, topped it up to full and it ran okay.. for probably 4 or 5 hours total before failure.
I'm surmising, the last owner bodged the oil relay to keep it running, but then the oil ran low. Because he'd bodged the oil relay, he had no indication that the oil was all gone, but the engine didn't know either, so it just kept going and the interlock didn't function.
Maybe, he's finally realised he's been running it for donkeys ages with no oil hence it ending up cheap on ebay.
I'm wondering if it was hanging on by a thread, and my sudden addition of a sump full of good clean oil has been the straw that broke the camel's back..