Fuse blows when fan speed on high

[87SQRBDY]

I saw a thread similar to this, but no answers that helped. My blower motor, relay, and resistor are all new, but when fan speed is put on high the fuse blows. I don't know why or where to check to run this issue down. Any help/advice would be greatly appreciated. It's for a 1987 R10 Silverado. Thanks in advance

 

[75gmck25]

On my ‘75 the high fan speed uses a separate 30 amp fuse. It is in a wire running across the firewall from the power junction near the brake booster over to the blower relay on the A/C plenum. This is the stock setup. Maybe a newer truck is wired differently.

The original barrel-style fuse used to get so hot that it would melt the fuse holder and finally lose contact. I replaced a section of wire and the fuse holder with a new 30 amp blade fuse and holder, and it now works fine. Something in that wire and fuse holder was creating too much resistance. I assume the blower motor must draw very close to the max 30 amps for that fuse.

 

[AZ squarebody]

I have the same problem with my 79 K15. The blower works fine on low and medium, but if i put it on high, it blows the fuse inside the truck on the firewall. Then nothing works. I don't have an inline fuse on the red wire coming from the power junction by the brake booster. Any thoughts on that??

 

[Doppleganger]

Think I'd look into wiring up a relay.

 

[Ricko1966]

@75is leading you down the right path resistance causes amperage draw check the condition of the wires and the cleanliness of all the connections on that circuit.

 

[AZ squarebody]

Thanks for the info! I will check that wire. The wire that went from the relay to the blower motor was clearly bad and had been very at the connection for the noise suppressor in the past. I replaced the connection and eliminated the noise suppressor. Maybe I need to replace more of that wire as well as recheck the connections at the junction block. Thanks

 

[AZ squarebody]

I cleaned all the contacts on the relay, resistor, switches in the dash. Checked the red wire across the fire wall and it looks good. It still blows the fuse and shuts everything down when I move it to high speed. This is frustrating.

 

[ronniebomb]

I am also chasing the same exact problem on my 78 GMC K25. I have just spent the entire day replacing the blower motor, relay, and resistor. I cleaned all connections for the main power wire across the firewall. Still blows the fuse...........
I guess I'm going to get quite familiar with my multimeter.............yeeesh!!!

 

[ronniebomb]

Did some research, and found this on another truck forum:



"unplug the wire to the motor. Then replace the fuse, and try all settings on the switch.

If the fuse doesn't pop, the fan motor is bad. If the fuse pops, its the relay or wiring

If the fuse pops with the motor unhooked, leave the motor unhooked, and unplug the relay. then replace the fuse and try again

if the fuse pops at this point with the relay and the motor unhooked, there's a wiring problem. probably a short where a wire chafed and shorted out to ground. Since the fuse doesnt pop on every switch setting, the problem should be between the switch itself, and the wiring out to the relay and motor."



I applied this logic, and found out the replacement relay I purchased from NAPA was blowing the fuse! I put the original one back in, and what do ya know............everything is working as it should!

Thought this might help someone in the future......

 

[WFO]

Did some research, and found this on another truck forum:



"unplug the wire to the motor. Then replace the fuse, and try all settings on the switch.

If the fuse doesn't pop, the fan motor is bad. If the fuse pops, its the relay or wiring

If the fuse pops with the motor unhooked, leave the motor unhooked, and unplug the relay. then replace the fuse and try again

if the fuse pops at this point with the relay and the motor unhooked, there's a wiring problem. probably a short where a wire chafed and shorted out to ground. Since the fuse doesnt pop on every switch setting, the problem should be between the switch itself, and the wiring out to the relay and motor."



I applied this logic, and found out the replacement relay I purchased from NAPA was blowing the fuse! I put the original one back in, and what do ya know............everything is working as it should!

Thought this might help someone in the future......

I'm just curious about what was wrong with it for you to have changed out the original relay.

 

[Finkaire]

The fan speeds go through the resister except for the high speed which is direct I’m assuming. Not sure how the relay comes into play. I’m missing speeds and the resister is good

 

[ronniebomb]

I'm just curious about what was wrong with it for you to have changed out the original relay.

I was letting the truck warm up, and noticed no heat whatsoever. Prior to this, occasionally, the high speed wouldn't work. Or sometimes, nothing worked..........but if I messed with the blower speed switch flipping it up and down....it would start to work. The truck is intermittently driven, and has an original 58,000 miles. So, when I had nothing working, I started throwing new parts at it that I had previously purchased. I had the wrong hi-speed blower motor switch (early version). So, I went to NAPA and purchased a new blower motor and the correct HSBMR. I installed the blower motor resistor, blower motor, and the new HSBMR. When I tried the heat, nothing worked still. That's when I checked for a fuse...........and there is one. DUH!!!
I put a new fuse in, and everything worked, until I hit the high-speed circuit, then the fuse popped. I researched a bit and found some folks solved the problem by cleaning the harness connections along the firewall. There's a junction block, and a 30-amp fuse that have wire eyelets. So, I spent the time cleaning them and putting back together with dielectric grease. Nothing, still popping the fuse. So, last ditch effort, I swapped out the new HSBMR with the original.......and what do ya know.....everything works as it should! I again tried the new HSBMR, and it blew the fuse again. So, I have another crap (said as Trump would articulate) CHYNA part that is a giant FAIL!!
Thought this might help some.....and make others laugh a bit!!

Have a Merry CHRISTmas y'all!!!

 

[Snoots]

The HIGH speed position only closes the relay. No voltage goes directly from the switch to the fan when it is on HIGH.