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.

 

Wondering if someone knows how to troubleshoot the relay with a meter. I have no low speed fan, only max and auto seem to work. I'm guessing a relay issue -- but replacements are all expensive and old (so prone to same issue). My strategy is to resurrect the dead when possible.

Worst case, I could remove and open the relay. Inspect for visual defect, then check for continuity in obvious circuits.

But does anyone has instructions for troubleshooting (with a meter) the relay without removing it ? I am referring to the part C46179.

 

The fan control relay is actually 4 separate relays wired together. If you open it, I recall it's pretty easy to isolate each relay and check operation as well as clean the contacts while you're are in there.
I attached a file I came across a while back. Page 2 relates to the fan control relay and page 3 shows how it controls fan speed thru the resistor array.

 

Thanks for that pdf Niles!

 

Thanks for the schematics, that helps a ton. Do you have a sense if problems with the relay lean more towards dirty contacts, or actual physical breaks?

 

I think dirty contacts, however, the relay coil connections inside the box are the typical lacquered very small wire. Cleaning the relays can dislodge the coil connection, but it is easily re soldered. The other thing that comes up, I think more frequently, is an open circuit in the resistor array that is in the blower duct above the relay box. It's a phenolic panel with 4 (I think) terminals on it, attached to the duct with 2 screws.

 

Recap: No low speed fan on the ac/heater
Result: Fixed after finding an unconnected small wire on the "yellow relay" in the schematics that connects to ground wire.

NOTES on the fix
Removed relay from the driver side foot plate behind the foot vent panel. Tested continuity on the prongs that should be connected per the schematics. The yellow relay (the one to ground) was not operating. To disassemble, DO NOT remove the screws on the face of the relay, they actualy hold the 4 different relays to the plate. You just snap the plate away with a little prying with something thin. Traced the corresponding bad relay wire. The wire had come off the metal tab it had been soldered to. I resoldered the wire and then resoldered the other similar connectors since they appeared to be weak links. Used a 12v power source (remarkable helpful thing to have) to test all relays, and the rewarding click had returned to the relay that connects to the ground. Reassembled and everything seems to work well now.