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Anyone Use a PWM for the Blower Motor?

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Notime2d8

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Seems like a cool idea to me. The infinite control would come in handy.
 
I actually built a PWM controller for my car and got it running, but after the $20 resistor kept working for a long time, I gave up on converting my speed knob to a potentiometer. I've never had the resistor pack fail in a square body because they are nice beefy wire, but I've had problems with the blower relay many times, so you could gain some reliability there. I think the keys will be setting the limits, how you mount the pot and the overall installation. You might consider a back-up plan for if the controller fails - either bypassing it or a plug to use one of the resistors.
  • Limits: you don't want the motor to stall, so the minimum duty cycle needs to get it moving under worst case temperatures. I assume your controller has a pot to set this.
  • Pot: IMO, a linear pot mounted in place of the factory speed switch would be slickest. Regardless, it needs to be convenient to access, as you will probably play with it a lot (especially at first). You could also keep the stock switch with 4 resistors (or pots) from the 4 posts for 4 preset speeds. That's not as cool as continuous, but in the right form factor, it could be a plug-in replacement for the resistor pack.
  • Installation: at max speed, the fan draws a lot of current and needs some heavy wire. The easiest install is under hood since the high current source and load are there. Some weatherproofing is in order, but the unit also needs airflow. In cab requires more wire and maybe a firewall hole. The setup is no more reliable than your wiring.
  • EMI: You may need extra filtering on the supply to this controller to keep from interfering with the radio, especially if you listen to AM.
 
I use only two positions on my heater switch---off and high !..
I use high speed and once the truck gets warmed up,the airflow going through the cowl with the blower off will maintain the temperature,even overheat you if your on the interstate..


I have snipped the wire for the lowest speed on the blower on a few trucks,because with the switch in the "off" position,GM rigged them to stay running on low speed ,as part of their "fresh air ventilation system--that will send chills up your spine until the engine warms up some )--the old blower motors would sometimes emit a loud screeching noise on low speed that would make you have a heart attack,and think your engine just spun a bearing !--but runs quietly at high speed..

I even drilled an oil hole in the bushing cap to oil the motor on a few trucks and they still squealed on low speed..oiling them lasted a while,then it would screech again..I didn't feel like replacing the blower unless it was completely dead..
 
I find that on a road trip running A/C I want a speed between 1 and 2. For defrost I'm always wishing there was something between HI and the next speed down. Of course losing the relay gives you only HI and off, which is much worse. Losing the blower altogether is the worst because you can't use A/C at all (evap will just ice) and you may not be able to clear the frost enough to drive. That's why I say a backup plan is nice.
 
Dredging this back up as it's a pretty short thread and the video is fairly pertinent for anyone new to this idea. Plus I've thought about this ever since my fan switch started acting up.

My fan switch finally failed, and as expected/common it was the high side melting the switch/terminals.

Took the switch apart, based on its design and what's available for PWM components, looks like this project should be doable. Found there are sliding potentiometers that will fit the fan switch cutout, and 30A PWM controllers are everywhere for less than $20.

The difficulty however is finding a PWM controller that will work without both the ground and positive fan motor leads being connected directly to it. Apparently most are designed such that the ground is the variable, the voltage to the motor stays constant. The blower motor ground of course isn't (easily?) accessible, and trying to isolate it is IMO starting to get into "just buy a new switch" territory in terms of modifications needed. It's a neat project, but not really worth it if the components aren't off the shelf and cheap.

By any chance is anyone up on PWM controllers, perhaps current automotive applications, that knows where it would be possible to source an affordable PWM controller that will work with just the + motor lead? Or a simple way around that limitation?
 
The problem is that an N-Channel FET is better than a P-channel. The options are to use an N-channel on the low side (chopping the - lead as you noted), use a P-channel on the high side, or use a charge pump or boost converter to get an N-channel on the high side (i.e. it runs off something like 20V). It's probably also measuring the current through a low-side shunt, as is most common. Honestly, it might be better to run the two wires dedicated to the fan motor (as a twisted pair), as opposed to pushing the nasty PWM harmonics through the chassis. In a perfect world, this controller would mount under-hood somewhere between your fuse center and the fan, or on the HVAC box where the stock relay went.

The blower has a ground wire with ring terminal. If that's the only connection, it's pretty easy to break it to return through the controller. If it's also tied to the case, that requires isolated mounting.
 
I think the AC housings were some sort of fiber/non-metal design, but the non-ac/HD heater housings are steel, at least mine is.

Guessing there is no easy way to work through this then. AC units I suppose there would be then.
 
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