I may be wrong but this does hint of an over-Voltage relay problem. The
relay may be faulty or simply dirty, with the contacts oxidised so that
current does not flow properly.
Remove the relay and check the contacts. It would be best to simply try a
couple of bursts of switch cleaner between the contacts first to remove the
oxidation. If this doesn't work then some very FINE abrasive paper between
the contacts will work (for a while, anyway!) followed by bursts of cleaner
to remove the abrasive residue.
These relays have been known to be subject to dry or cracked joints as well.
Recheck all the solder connections, and reflow any that look poor or suspect
(grey solder, round solder bubbles or evidence of cracking). You should also
spray a little switch cleaner in the relay socket.
Note I use De-Oxit, the best switch cleaner with lubricant for all my switch
cleaning work, and I would highly recommend it. It's made in the US.The
lubricant does not normally cause a problem with relay contacts.
If this doesn't work, buy a new one!
Al Bolton, G4VSQ
1990 Mercedes 500SE 128,000 miles
Hope this helps
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