My fan motor keeps blowing fuses. I can get about 45 minutes or an hour of continuous service from the fuse before it gives up to its task. I notice this last month when running the A/C and again yesterday running the heater. Both times I had the fan switch on high. The first fuse didn't look like it was blown but the test light said different. The second and third time it was plain to see that the fuse was blown. The blown fuses didn’t look like a direct ground caused them to blow...no dark spot on the glass. It acts more like I need to be using a 31 amp fuse instead of the 30 amp fuse. I’m 99.9% sure the wiring/connections are sound. The fan motor is new from Vintage Air and the fan switch is NOS...but the resistor is the factory original. The question is what are the signs of a bad resistor and is this one of them?
The resistor should not be in the circuit when operating on high... It's job is to limit current which also reduces voltage to motor, while making life easier for fuse... On the medium-high setting(through resistor) the motor is probably seeing something like 25A and maybe 11V... That's assuming it's drawing 30A at 14v when operating on high... Your statement about needing a 31A fuse is probably close, apparently the motor is drawing 30A or slightly more which over time will cause the fuse to fail... Possibly the motor is stiff and is drawing more than it should when hot?? Would be a good idea to monitor motor current, a ammeter from a under dash gauge set will do if you don't have anything else...
I had the exact same thing happening on my 1976 station wagon. I never did find the cure, just kept a bunch of extra fuses with me on road trips. I got pretty good at swapping them out. Not much of an answer....but there is company in misery.
Another possibility is if you are using the factory fuse block is it's contacts are heating which contributes to the failure... I'm probably going to do a relay mod on mine when I start using A/C next year, can run a larger supply wire and use a 35-40A fuse if necessary...
x2 on what Krazy Comet said. Bad connection will cause a blown fuse. Have it happen all the time in the a/c world with fused disconnects.
That reminds me what I did to fix it!! It was heating up the fuse panel and melting the plastic behind it. I got an inline fuse/wire. Alligator clipped it to both side of the old fuse location. It was a flat, modern type fuse with about 3" of wire on either side. After I did this no more burned fuses.
Something that was overlooked is the A/C compressor clutch, operates from same fuse as blower... I don't remember measuring one(but will do), would guess it pulls around 5A of course it should not be operating with heater only...
The heater only motor requires a 10 amp fuse, the AC blower motor uses a 30 amp because it also powers the compressor clutch. You're correct about the compressor shouldn't have power when the controls are on "Heat"...but will have power in the defrost setting. What has me puzzled is the fan motor was the only thing pulling power and it blew a 30 amp fuses.
Heater only uses a 14A fuse... I doubt a A/C motor will draw much more than std motor... Defiantly no more than 20A BTW my orig '72 A/C clutch draws 4.7A @ 14v, at 12v draw was just over 4A... I'd have measured my motor but the bench power supply will only handle 5A @ 15V, was struggling to test the clutch...
Haven't had the time to mess with it...been busy removing trees and moving the small 10 x 14 building to the other side of the backyard to make room for a 28 x 30 shop. The problem didn't show up until after replacing the fan switch so I'm going back to the old switch and see what happens.
I wired mine to run through a relay on the high setting to reduce the heat load on the switch. As I remember, before I converted my blower to a late model Taurus type, I had issues with not so great airflow and blown fuses and a burned switch. When I took it out I found that the bushings were worn pretty bad and would squeak after running for a while. I unsuccessfully tried lubing them before I gave up and converted to the Taurus fan motor. Something caught in the squirrel cage or dragging it can cause this too.
