: How I Rebuilt a 92 Seville Heater/AC Blower Motor Resistor w/Pics



skor
07-08-12, 07:08 PM
If anyone here ever needed to replace a heating/cooling blower motor resistor (controller) on a '92 Seville/Eldorado, or any GM car that uses this same resistor, you know how expensive these things can be. I'm going to show you how I rebuilt one of these resistors using common discrete components for about $15. To perform this hack you will need basic electronic knowledge, such as the ability to read a schematic diagram, and the ability to solder.

I was driving along when I hear a buzz coming from the heating/cooling blower motor. I go a little farther and I hear BANG, and the motor stops. I pull the blower motor and this is what I find.

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The squirrel cage disintegrated. I checked inside the blower housing to see if there was anything loose, but I could find nothing except pieces of the fan. All of this stuff was OE, no one has been in there since the car came from the factory. I'm sure someone at GM thought that saving a nickel on plastic was a good idea.

Well, I bought a new motor/fan assembly and installed it. Wasn't too bad, about $40. When I tried it, it was a no-go. Next thing I do is check the fuse (30 amp) it was fine. Then I checked the blower motor controller. It was blown. Crap! Apparently when the fan blew up, it jammed the motor, which then took out the motor controller. I'm thinking to myself, "Nice engineering. The components on this circuit blow before the fuse does. WTF is the fuse for?"

So I call the Caddy stealer and he tells me $225 for a new motor resistor. Crap! I Check Autozone and they want $180. Crap! So I get my Google-Fu on and I end up at a auto heating/ac repair forum. A dude on the heating/ac forum is an electrical engineer and he was so pissed when the same thing happened to his Caddy that he came up with this circuit that you can build from cheap discrete components.

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The connector block has 6 pins, but only 4 are used. There are only 4 wires on the harness plug.

The FET source is connected to the battery through a 30 amp fuse....it's always hot, even with the key off. The FET drain goes straight to the blower motor. The TIP3055 base is connected to the the control signal coming from the climate control computer. I don't know what this signal looks like since I don't have a scope. I put the VOM on it and when the climate control is switched on it can vary from .5V to 8+V. TIP3055 emitter goes straight to chasis ground.

The cases of the semiconductors I used are TO-247. Normally I would mount stuff like this on mica with heat sink grease, but radio shack only sells mica for TO-220....too small. I ordered sil-pad from Digi-Key. The sil-pad cost $.24 a piece. Resistors used in the circuit are all 1/2 watt, 5%, carbon film.


Now I'm not an electrical engineer, but when I was in high school, I took a basic electronics course. Give me a schematic and a parts list and I can build it, and build it I did.

This is what the motor controller looked like when I pulled it out of the car.

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I cut the original circuit board off the heat sink; it contained some of those "thick film" things which are great for mass production but can't be repaired...it would be like tying to unbake a cake. I bolted the discrete bits to the original sink and soldered it all together. Does it work? You betcha ya. I ran it for a half hour outside the blower housing. The transistor stays cool, the 3 FETs get a bit warm but nothing that could be described as hot. This is what the DIY version looks like.

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Total cost of parts form Digi-Key (including postage) was about $15. Total out of pocket savings: $165. Being able to say, "Eat me!" to GM: Priceless.

I'm happy to share, and I hope this will be of help to someone.

naif
07-08-12, 07:10 PM
Good job.

Try to protect those exposed wiring and solder joints (since bumps and such could make for a bad day) - maybe coat the electronics part with lacquer and then pot the eletronics?

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There is a potential issue there - if for whatever reason your system voltage goes too high, you'll run the risk of blowing out the MOSFET gate since it's just getting switched from battery voltage to ground. The worst that should happen is that the blower gets stuck on, though :noidea:

Submariner409
07-08-12, 07:14 PM
Moved to Cadillac Tech Tips.

Good dope.

skor
07-08-12, 07:29 PM
Good job.

Try to protect those exposed wiring and solder joints (since bumps and such could make for a bad day) - maybe coat the electronics part with lacquer and then pot the eletronics?

-----

There is a potential issue there - if for whatever reason your system voltage goes too high, you'll run the risk of blowing out the MOSFET gate since it's just getting switched from battery voltage to ground. The worst that should happen is that the blower gets stuck on, though :noidea:

I coated all the exposed solder joints with JB Weld. What would redneck engineering be without JB Weld?

skor
07-08-12, 07:37 PM
Good job.

Try to protect those exposed wiring and solder joints (since bumps and such could make for a bad day) - maybe coat the electronics part with lacquer and then pot the eletronics?

-----

There is a potential issue there - if for whatever reason your system voltage goes too high, you'll run the risk of blowing out the MOSFET gate since it's just getting switched from battery voltage to ground. The worst that should happen is that the blower gets stuck on, though :noidea:

Thanks.

I coated all the exposed solder joints with JB Weld. Where would redneck engineering be without JB Weld? :thumbsup:

As regards your concern about a blown out MOSFET gate, the original circuit suffers from that very same failure mode. I've seen the OEM circuits fail where the blower motor runs continuously, even with the key off. In that regard it's no different than the original circuit, except that it will be MUCH cheaper to repair than buying an OEM part.