TLDR: I think my heater blower motor seized due to extreme cold (or snow/ice in the housing) and blew a fuse -- then worked after time in the garage to warm up.

Brief history of the basics I've already tackled.

• Blower motor died about two years ago. Replaced, ran fine.
• Present day... blower motor started squealing and only ran at top speed.
• Due to squealing and top speed issue, replaced both the motor (with blades/cage) and heater resistor assembly. (Rock Auto)
• Note: replacement motor had positive and negative connections reversed, so I reversed the wires on the motor wire harness.
• Everything worked fine.
• One month later, working fine on my son's way to work, no fan at all after his shift.
• HVAC fuse under hood was blown.
• Replaced the above fuse and it blew immediately.
• Checked blower motor by connecting direct to battery, doesn't work.
• Hooked multimeter to motor wiring harness and ran dash switch from low to high -- same full voltage to motor for every setting.
• Checked continuity on heater resistor seems fine.
• Used multimeter to check the wiring harness going in to the resistor and got the following.

Heater fan dial on 1 (low) -- full voltage to 2
Heater fan dial on 2 -- full voltage to 2 and 4
Heater fan dial on 3 -- full voltage to 1 and 2
Heater fan dial on 4 (high) -- full voltage to 2

• Seems wrong to me that dial settings 1 and 4 should both have power to the same place on the harness.
• I did a quick continuity check on the actual fan dial, and each setting puts out power to a separate pin, so I believe it's not at fault.

Is there another component between the dial on dashboard and the resistor under the hood?

Anything else I might be missing.

New motor is being sent, free of charge, from Rock Auto, but I don't really want to install it until I can get variable voltages at the motor wiring connection (to show things are working).

Any help is appreciated!

Note: Live in Manitoba, Canada and it's been very cold here recently, as well as two blizzards in a row. Mentioning in case it's relevant.

EDIT: Traced some wiring and noticed the resistor harness is also connected to a Fan Control Relay (GM part # 12177233). Relevant? Relay clicks when heater dial is set to high, and continuity check shows current being switched, so relay appears to be fine.

Actually, now re: above, the dial settings on 1 and 4 both giving power to 2 (on the diagram) is fine, since 4 on the dial kicks in the Fan Control Relay, defaulting to high power (I think) -- which bypasses the resistor.

EDIT: Now I'm completely befuddled. On a whim, I hooked all the parts back up -- including the motor that wouldn't run -- and the fan motor now runs and I have all four speeds. However, when I hook my multimeter to the motor power harness, it reads full voltage at all four speed settings... hook the harness up to the fan motor and it gives me four speeds. I must be missing some basic electrical principle (perhaps it needs the load from the motor to register the voltage?).

Mind you, this is with a jumper wire still replacing the original fuse that blew. My battery is low from all the testing so I'm charging it overnight. Will try the fuse with a fully-charged system tomorrow.

Is it possible for a fan blower motor to freeze, not allowing it to spin, and causing the fuse to blow? Perhaps sitting in the heated garage while working on things thawed the motor? Are Blazers/S10's known for snow/rain/moisture in the blower housing?

Will add to this once I re-test in the morning. Thoughts/advice still appreciated.

EDIT: Now runs fine without blowing a fuse. Going to assume the fuse blew because the fan motor seized, and assume the seize was cold-related since it's a new motor. Will also be open to it being a bad bearing or something, depending on future performance.

 

Each switch position for motor speed outputs the same 12V from all speed settings from the HVAC controller. You are not changing voltage in the HVAC controller, you are routing the same 12V to different points in the resistor divider network (there are 3 resistors inside). You will get that same result at the other end of the cable that runs from the HVAC module to the resistor divider network, they are plain wires. If you hook everything back up, switch through the different speeds then you will measure some different voltages at the backprobed unused terminals because you are measuring different phantom/unused points on a resistor divider network but the selected HVAC module output will still always be 12V. This is interesting but not useful. Now the voltage at point C does change with different switch positions as a different resistor combination performs a resistor divider with the motor windings. With some resistance measurements I can tell you what voltages to expect there.

Be careful, if you are blowing fuses then the motor could draw enough current to damage wiring or start a fire. The highest setting works sometimes as a motor is dying because its enough torque to overcome the seizing bearings.





George

 

Thanks for the extra info.

Started it this morning and everything is "fine".

Fan motor must have seized for some reason. I'm going to go with cold-related for now, since it's a new motor and the circumstances above, but could also be a bad bearing.

 

This motor is likely failing. You will get the most life out of it operating it on high. Also, look for a small hole at bearing ends of the motor for a couple of drops of light oil.


George