Fan cuts out when on for a while

Why would it do that on all speeds then?
If you think about it, you are adding the resistors to the circuit to slow the motor. More resistance equals more heat. While the motor might not be drawing as much current, you still have to account for the added resistance.
Good luck Ross. Electrical gremlins are frustrating to say the least. With the symptoms you are having, after checking all the connections it could even be a broken wire midstream or as you mentioned an issue with the motor itself. I know you will figure it out though.
If you think about it, you are adding the resistors to the circuit to slow the motor. More resistance equals more heat. While the motor might not be drawing as much current, you still have to account for the added resistance.

EDIT - I typically run it on high, with the least resistance... big car to cool... so the switch shouldn't be loading up because of the resistor... as much....

Are saying that if the blower motor is at fault, if it fails at the least resistance (high setting feed, full available amperage to draw upon), lesser amperages shouldn't really make a difference to what the blower is dealing with internally that makes it fail. OK, I think I understand that.

Also, is it true that lower amperage availability to feed the blower motor could make it work harder/hotter?

My thoughts for ruling out the switch and resistor side of things were more centered around the switch and its internal contacts (little spring loaded brass things I expect), and why, if one setting gets hot and fails, why would current flow through the other (and presumably cooler) brass contacts?
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That's right - and I am not going to pull the switch at this point, as I have too much going on. I can deal with it after or during the winter.

So as I kind of understand, with no other evidence of overheat anywhere else, if the blower motor is pulling too much current in its attempt to keep going, the fan switch may have tired contacts inside it which are getting hot and opening up under load? Why would it do that on all speeds then?

I just don't see that - it's something where ALL the power going to the blower fan (after the switch and resistor) is getting interrupted, and from what I see that can only be the blower motor itself...


I'm going to check the bulkhead connections for the feed to the blower motor again just to be sure.
You might try running a test light to blower motor connection. If the test light goes out when the blower stops running, the problem is someplace else.

Would this duplicate the symptoms?

Battery charger hooked up, engine OFF
Key on ACCY
Heater ON
Fan on HIGH

Sit down, watch a ball game in the garage, wait........
Lol @Gerald Morris - you've obviously never dealt with pulling the switch on a 65 or 66 Chrysler - a totally different animal than a Dodge or Plymouth, or a 67 or 68 Chrysler for that matter... there's little to no room up in there. To even LOOK at the switch (which involves lying on your back with your head on the gas pedal and your feet over the seatback), the ash trays have to come out, AC and defrost tubes have to come out, steering column lower cover with fresh air vent cables and controls has to come out (which you then have to put under your head, which is uncomfortable to say the least), the AC vacuum switch, and its push buttons and tubes all have to come out ...and if one wants to actually pull the entire HVAC control unit, the radio should to come out, the speedo should come out for best access, it's a pain, believe me!

Regardless, I went through every single connection point in the chain, from fuse box to the blower motor.

Not a single piece of evidence of bad connections, overheating wiring, melting, nothing, nada. There were a couple of "loose" connections which I tightened up, but they were still OK by comparison to other connections. Blower motor resistor assembly was fine (I have an NOS one on standby, but it was OK), and yes, I can bypass all that to send a full unresisted 12V to the blower motor if I really need to...

Any connection I took apart I cleaned with emery cloth to ensure "new" metal to metal connection, and tightened up the female connectors.

I am now suspecting the blower motor - but why would it quit and come back, quit and come back? If it was going bad, wouldn't it just get worse and worse (slower) and just seize up and die?

Of course when I put it back together everything worked fine, and when I started the car (it's on jack stands to deal with the fuel leak issue) the temps dropped to meatlocker range in about 5 minutes of idling. The proof will be in the next time I drive it with the AC on for any length of time... so I don't know if I'll get to that before I head off to WPC next week, but I can send 12V directly to the blower if I need to.

I will report over the next couple of days. I am waiting on a fuel tank sending unit gasket.

Admittedly, I don't have a factory AC '66, so Mathilda isn't a good example. I had a dealer AC in her when we got her, and immediately removed that. I wholly beleive you when you describe the agony of removing that switch.

Can you just disconnect the blower motor then? Motors fail in funny ways, DC and AC alike. Your blower has brushes and permanent magnets of course, and a wonky commutator, or a bit of debris getting in it might short several of the poles without totally shutting the motor down. DO direct connect your blower, run it for an hour if you don't mind charging the battery to keep it blowing good, and note how it behaves....

you may as well replace that motor if it misbehaves at all. You can always repair the good OLD motor, and run a new aftermarket one for the present.

It looks like you just MAY have had a loose connection in the dash. I've repaired electrical problems aplenty which arose from dirty, aged connections in Mathilda and Gertrude too. Cleaned the headlight switch in Gerti a couple months back, and things have done swell since.

Will pray to St. Tesla 4 U in this matter....
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Yes, please pray to St Tesla for me! Thanks!

So - I currently have a battery direct 12V power wire for the fan hooked up, and in place in case I need it. If.when the fan quits, all I need to do is unplug the feed wire and plug in the new one. I will be installing an inline fuse for that too. I can also use this to see what amperage the fan is drawing. I expect the fan will draw normal load.

I still haven't done the road test to see if my ministrations have solved the issue with the original wiring, but I sort of suspect that they have not, and that it will quit again... if that is the case, then I will do the direct power connection, and see if the fan powers up - if it does, then I know it's not the fan, and I will have to gut the dash to get that switch out, and really inspect the wiring (although I'll suspect the switch in that case). If it doesn't power up with direct 12V, then I know that it IS the fan.

That's where I'm going to leave it for now.
OK - so it looks like the issue is still within the dash, either with the wiring from the fuse box to the blower motor switch, or the blower motor switch itself...

Here's what I went through today:

I finished up installing the "backup" 12V direct feed wire, and instead of running it directly from the battery, I decided to run it off a fuse in the fuse box so it will be protected. I will also be intalling an inline barrel fuse for extra protection, but I wanted the backup 12V feed wire to be getting it's power from a key on 12V circuit. So I pulled the wire through the speedo cable grommet, dropped the fuse box, attached a proper Packard female connector the new wire, and plugged it onto the accessory fuse spade. I then plugged the wire into the fan feed on the firewall, and then connected the battery, turned the key and the blower came on full, as expected. Works perfectly with the key. I figured that if the fan stopping issue persists, I may install a toggle switch on the dash so I don't have to pull over and open the hood and connect wiring... it will be safer, more convenient and I can deal with the problem later. I hoped I'd solved the issue with my work over the past few days...

So on I went with my install (since this setup is supposed to be a backup power feed to the blower motor if the blower stops working again), with the battery disconnected, I unplugged the new feed at the blower motor, and reconnected the original feed wire, and left the backup feed dangling safely. I reconnected the battery, and went to test the blower motor and dash switch... and nothing. Dead. Compressor was happily engaging and disengaging as per the pushbuttons, but no blower at all.

So the issue may well be from the fuse box to the switch... or the switch. Either way, it's not working right now, so it will be back to the feed wire. I gotta find a suitable toggle switch - I'll look in my Wiring parts box.

Looks like a winter project - gut the dash, go through the wiring on the bench, clean up the switch, reinstall. Radio needs a tad of service too, and the speedometer lens could do with a polishing to remove scratches. Might be time to install the LED bulbs in everything too.