Learn me blower motor resistor theory and practice

Ages ago, I removed the AC system from my V8 S10, and installed a non-AC heater box and blower motor resistor.  At the time, i still had the original HVAC controls inside, and they were in pretty bad shape.  To get the system to work, I enclosed the AC blower motor resistor in a plastic tub under the hood of the car to keep it from touching any metal (it is not the same physical size as the non-AC resistor, so doesn't fit the heater box), and ran the system that way.

During the latest swap, I installed some non-AC controls.  Herein lies the issue.  The wiring is still all AC, and the AC system had 4 fan speeds, versus 3 for the non-AC system.  I'd rather not do a bunch of rewiring - my preference is to run some leads from the current wiring connector (4 posts) to the blower motor resistor (3 posts).  So here's where I make some assumptions, and I may need some help:

1. Does full power bypass the resistor assembly?  Since a fan only running at full is the failure mode for a bad GM resistor, I am guessing yes.
2. Based on that - the greater the resistance measures, the slower the fan should blow?

If those are correct, I'm going to hook the multimeter to the wiring harness, and cycle through the settings, looking for 12 V on the pinout for low and medium.  I'll run leads from low to the highest resistance, and medium to the lowest resistance.  And, make sure the same connector that all the resistors for the AC resistor assembly run through will get connected to the post that both of the resistors for the non-AC resistor assembly run through.   And, if all goes well, I'll have all three speeds working fine.

Clear as mud?

Yes, on high the resistor is bypassed.  Yes the greater resistance, the slower the fan speed.

You should mount the resistor inside the blower case, they are designed to have the air from the blower motor cooling them.

Run_Away said:

Yes, on high the resistor is bypassed.  Yes the greater resistance, the slower the fan speed.

You should mount the resistor inside the blower case, they are designed to have the air from the blower motor cooling them

Thank you.  Yup, the correct resistor will be mounted inside the heater box where it belongs.  Four or so years ago when I rigged up the "temporary" solution, I didn't want to pull the heater box to cut the opening for the larger resistor, because I knew eventually I'd install the new controls.  It just took a lot longer time than planned.

No need to clutter this thread up with specifics, but it appears early-90s GM had very different wiring for AC and non-AC configurations.  Even though the wiring connector is physically the same for the controls, I suspect the power feed is at a different pin, as I could not get power to the blower motor resistor connector.  A little research shows it is possible GM also used a relay for full power to the fan on AC cars, and just directly wired to the fan for non-AC cars.