This is an odd one, and it seems to have been fixed by whatever powers exist working beyond my understanding.
The other day the AC in my 1986 series III stopped working. I checked the fuses and sure enough the 50 amp fuse that feeds the fan was blown. Easy fix, unless it blows again. So I get a fuse, install it with the ignition off and mode selector also off. As soon as the fuse touches the clips the fan goes on. An odd occurrence. A quick check with the S57 electrical shows that the power to the fan relay is N (brown) always live. So it must be the main fan relay (actually 4 relays in one box) has stuck contacts or something. I extracted the relay, opened it up expecting to find all sorts of problems but everything is in order. All the coils pull the contacts in and no shorts or stuck contacts to be found. I bypassed the relays to check the speed resistor array, all in order there as well. So I cleaned the relay contacts, closed it up and reinstalled the relay.
So then it must be a servo micro-switch screwing up the operation of the relay. I happen to have one of the factory AC test sets and this morning I plugged it in to check servo and amplifier operation. Again everything in order. And the extraordinarily complex mechanical system is working perfectly.
So I cleaned a bit, fixed nothing I can identify and am at a complete loss to discern the reason for the blown fuse and ignition off blower operation.
What do think?

 

This is a real brain tease that requires some mulling. But....from quick glance at the schematic.......

Is the brown/yellow wire at the relay pack 12v at all times? It shouldn't be. If it is, try disconnecting the water temp switch on the heater core inlet pipe. Any change? If not, I suspect a problem in your mode switch.

Cheers
DD

 

I may never get tot he bottom of it; the system now works perfectly. NY power would also operate the fans thru the Y line thru maximum resistance to produce the lowest blower speed. When I put the fuse in it was more like the 2nd or 3rd blower setting.
The thing is all the 12V control circuits get power from 35A fuse 6, powered by the ignition switch. With ignition off the only power to the system is the N conductor to the relay and it can normally only get to the fans if control voltage is present.
I've given it more thought and can advance this scenario:
A momentary start surge when I turned the ignition on with the AC system in AUTO in the fan circuit caused by dirty contacts on the operable relay. The fuse blew, but prior to that the contact heated to the point there was a minor adhesion of the contacts, a stuck contact but not really fully welded. Like a bad spot weld. So I was on the right track with the relay diagnosis, but in ham-fisted attempts to open the relay box, the adhesion broke loose and was not apparent when I got in. So perhaps I fixed it without knowing it. Cleaned and treated contacts should be fine for many years.
Now if the surge was something else (it could be a temporary dead short in the wiring somewhere) it could reoccur, then I'm in a world of hurt. Potentially very difficult to find.
But for now back to motoring.