Hopefully someone will have the owners/parts manual for that generator and can look up the part number for you. I suspect it will be a proprietary part number though.
If nobody here can help, you could try one of the Cummins service places in the UK. Go to their service locator page, select UK from the Region options, then enter Wales as the Province on the next page.
Thanks Tom, tried that, only one outlet for Onan in the UK it appears, but will try phoning them Monday - nothing for that series of generators on their website, though. Ideally could do with finding somebody with the same circuit board, I think they're very common, to read the lettering on the diode.
That was my thought too (also what else did the diode damage when it blew)! The answer is I've no idea, could even be a fault somewhere else - not on the control board. Unless there are other suggestions, I could either replace just the diode, or try to replace all the components, then try again. However, there's no marking on RS1 or CR1, and I haven't been able to source any of the 3 relays manufactured by NAIS. These are DS2Y-S-DC12V-H271 and DK1a1b-12V AW3033 (2 of these). If I can't identify/get these components, I really have only one option before getting the board repaired/replaced. If someone is able to have a look at an identical board, that would be a great help!
My Onan manual shows the following service places in the UK, both on the other side of Offa's Dyke:
Cummins Power Generation Ltd. tel. 01843-255000
Dudleys American Motorhomes, tel. 01993-703774
My owners manuals aren't much help in identifying parts, so we'll have to hope that someone with the same generator can look at that diode and read off the part number.
Another option might be a board from a scrapped generator, if you can find one at a scrap yard. But that might be tough over there.
Yet another thought - try emailing one of the part coordinators at one of the U.S. Cummins locations. e.g. my nearest one is Cummins West; Click on the Contacts link to see the email links. They'll need model and serial numbers.
Ned makes a good point about the possibility of something else being the cause of the diode failure, although I've seen folks have similar problems which were not caused by something else. Some years ago a friend built a replacement control board from scratch by literally copying the board in my generator on a prior coach. Popped it in and it worked just fine from then on.
Trying not to buy the complete board (unless used). Just out of interest, here's a picture of the same generator (scroll down). http://cgi.ebay.com/ebaymotors/Onan-RV-Motorhome-4000-GenSet-Gas-Generator-NR_W0QQcmdZViewItemQQcategoryZ50068QQihZ014QQitemZ330008269122QQrdZ1
That's all they list! I can get mine fixed for USD 70, shpping there and back, + cost of parts. Seems a lot to replace a max. of 16 components, could do it myself in 20 mins.I'll post the result of my enquiries Monday (working all day tomorrow) - in the meantime, if anyone has a board they can check (CR2), do let me know!