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.

[JoePolo]

Originally Posted by RustyPile

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.

Ahh thanks, so any idea where the jumper wire “Sheepdip” had the issue with is?? Thanks

[Rust_never_sleeps]

Originally Posted by JoePolo

Ahh thanks, so any idea where the jumper wire “Sheepdip” had the issue with is?? Thanks

Compressor clutch is literally between the compressor and the pulley. That clutch magnet is "excited" electronically. Two wires in, and one grounds out on the passenger side of the housing IIRC.
Might look like the attached wiring diagram

[Sheepdip]

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

When I had this fuse blowing problem it took me and a buddy a couple of hours thru process of elimination and an Ohm Meter to finally find it. Mine was the little jumper wire or diode was bad; I proved it by clipping/cutting it. I just replaced the whole plug-in pigtail, which by the way if I remember correctly, they make one with, and one without the diode..glad to pass the info along.

[JoePolo]

Originally Posted by Sheepdip

When I had this fuse blowing problem it took me and a buddy a couple of hours thru process of elimination and an Ohm Meter to finally find it. Mine was the little jumper wire or diode was bad; I proved it by clipping/cutting it. I just replaced the whole plug-in pigtail, which by the way if I remember correctly, they make one with, and one without the diode..glad to pass the info along.

Ahh mine worked with the harness unplugged so I assume that diode is good. Figured it would pop even unplugged.

[Rick Bollinger]

You can ohm the compressor clutch. It will show some resistance but not none. I just checked my old compressor that worked when I pulled it off. It read 4.1 OHM's My new sandan read 2.9 and it has never been ran. I don't know the lowest number but if it's reading 0 then it's toast.

[JoePolo]

Originally Posted by Rick Bollinger

You can ohm the compressor clutch. It will show some resistance but not none. I just checked my old compressor that worked when I pulled it off. It read 4.1 OHM's My new sandan read 2.9 and it has never been ran. I don't know the lowest number but if it's reading 0 then it's toast.

Yup it’s reading 0 I did that last night.

[Sheepdip]

Originally Posted by JoePolo

Ahh mine worked with the harness unplugged so I assume that diode is good. Figured it would pop even unplugged.

I would say the diode is good as it jumps between + to - mine was definitely bad.

[Rust_never_sleeps]

That's a pretty ugly failure scenario, I would never have thought of it grounding there, thanks for that, Sheepdip

I have this bookmarked off the Summit page (for when I get to rebuilding mine)
Pretty good pic of what they look like if your clutch hasn't gone kinetic

[JoePolo]

Agreed I would have gotten there eventually just by unplugging things and trying fuses haha but was great to have the heads up.

[Rust_never_sleeps]

Just FTR, that page(p74 in my copy) from the HEATER AND AIR CONDITIONING section of the manual seems complete.....
BUT the full truck wiring diagram(594 for my type) in the Electrical and Body section only shows one wire to the fan and no relays.
I'll typically go to that full diagram first, so I added a note there for next time

[PbFut]

Clutch replacement is not difficult but you need a decent set of clip ring pliers and puller. These parts are quickly disappearing from the catalogs. I would not put off a purchase thinking better times will come.

[Rust_never_sleeps]

May not be necessary, but most multimeters do have a diode test function

[RustyPile]

A diode is simply an electrical check valve.. It will pass current in one direction and block it in the other. A "diode tester" DVM is not necessary.. Set the resistor testing mode to 1K ohm- 10K ohm.. Like most electronic components, a diode cannot be tested while it's in the circuit, so disconnect one lead. Connect the meter leads to the diode leads. Make note of the reading, then reverse the leads and take another reading.. One reading should be high resistance, and the other reading should be low resistance.. If the diode is shorted, it will test low resistance on both readings.. If it's open or partially open, it will read high resistance on both readings.

[Rust_never_sleeps]

Originally Posted by RustyPile

A diode is simply an electrical check valve.. It will pass current in one direction and block it in the other. A "diode tester" DVM is not necessary.. Set the resistor testing mode to 1K ohm- 10K ohm.. Like most electronic components, a diode cannot be tested while it's in the circuit, so disconnect one lead. Connect the meter leads to the diode leads. Make note of the reading, then reverse the leads and take another reading.. One reading should be high resistance, and the other reading should be low resistance.. If the diode is shorted, it will test low resistance on both readings.. If it's open or partially open, it will read high resistance on both readings.

I've had that feature on every MM I've owned since the 80's, so I just use it instead. :shrug: