Blower Motor Circuit Question

[JoePolo]

Reviving a 72 C10 that’s been dead since the 90s. It’s a factory AC truck.

Blower motor fuse/AC fuse (25A) is popping whenever the switch is turned on from the off position. I had a switch that worked from our Blazer so I pulled the connector and hooked it up out of the panel and blew the fuse again.
With the switch in the off position I have power to the input and I accidentally jumped it with the test light to the terminal just above and it actually kicked the blower motor on. So I know the motor isn’t stuck.

Resistor? God I hope not, I know they are deep in the box.
Relay? On the side of the box. I might have one of those I could test too.

My seat is out right now so trying to get anything that might be under dash figured out now.

Thanks guy.

[Rust_never_sleeps]

First guess when the circuit blows immediately is that there's a short to ground, particularly since your motor isn't bound up.
Should be able to confirm with a multimeter(continuity to ground from the fuse terminal with that switch on)
Relay is at least closing the circuit, so could be OK

[JoePolo]

Originally Posted by Rust_never_sleeps

First guess when the circuit blows immediately is that there's a short to ground, particularly since your motor isn't bound up.
Should be able to confirm with a multimeter(continuity to ground from the fuse terminal with that switch on)
Relay is at least closing the circuit, so could be OK

Thanks I’ll run that test. I am assuming I need to use the fuse terminal that always is powered even after the fuse pops? Sorry I’m pretty handy but electrical isn’t my strong suit.

It pretty close to instant pop, motor definitely doesn’t click on. I actually left the test light on the terminal at the switch while clicking the switch on and the test light dims.

[Rust_never_sleeps]

Originally Posted by JoePolo

Thanks I’ll run that test. I am assuming I need to use the fuse terminal that always is powered even after the fuse pops? Sorry I’m pretty handy but electrical isn’t my strong suit.

It pretty close to instant pop, motor definitely doesn’t click on. I actually left the test light on the terminal at the switch while clicking the switch on and the test light dims.

1. For continuity, you'd want the other terminal, since that runs to the motor

2. Ground short is usually instant, might be indicating a different problem, could be just due to inherent delay.

[JoePolo]

Originally Posted by Rust_never_sleeps

1. For continuity, you'd want the other terminal, since that runs to the motor

2. Ground short is usually instant, might be indicating a different problem, could be just due to inherent delay.

Got it, I'll test it tonight. I can also resistance test the switch itself I would assume.

The reason I have this switch to test with is, our Blazer was doing a weird thing, and still is (I never solved that problem either). Where the blower motor on high will basically intermittently cycle to slower speeds (low and medium seem to work as they should) and sometimes be stuck as slower speeds for the most of the time when in high. Changed everything but the resistor. I always put off getting to the damn resistor. Why GM buried it so far in the box is beyond me.

[RustyPile]

When tracking down shorts and other issues that would blow a fuse, I use to tool in this link. It's very easy to use, connects to the fuse box terminals of the blown fuse.

https://www.jbtools.com/sg-tool-aid-...ntpage-nosto-1

[JoePolo]

Got a minute to do the test. Definitely the circuit has a short. It instantly reads 0 ohms. I unplugged the blower motor with the same result.

Guess now I need to start tracing out wires after I buy that tool.

[Richard]

Disconnect the relay and try the switch. IMO you should replace the 50+ year old resistor and the relay regardless, They will fail if not already. A few screws on the s-duct and two to remove the s-duct adapter on heater core box. Resistor is right there. With the seat out it is easier.

[Sheepdip]

Another scenario...Unplug the A/C compressor and try it. I had this problem, and it turned out to be that little jumper wire that grounds the clutch coil was bad/grounded out. That will pop your fuse instantly

[JoePolo]

Originally Posted by Sheepdip

Another scenario...Unplug the A/C compressor and try it. I had this problem, and it turned out to be that little jumper wire that grounds the clutch coil was bad/grounded out. That will pop your fuse instantly

Which wire is that? Mine has a small little wire with what looks like a resistor in it right at the connector. I will be trying that next running it with the AC compressor unplugged. Thanks.

[RustyPile]

Originally Posted by JoePolo

Which wire is that? Mine has a small little wire with what looks like a resistor in it right at the connector. I will be trying that next running it with the AC compressor unplugged. Thanks.

That's not a resistor, it's a diode. Its purpose is to "cushion" the voltage spikes generated when the clutch coil's magnetic field collapses.