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PWM Fan Control

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SeanD

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I have a 2005 Envoy and I am replacing the mechanical (Belt driven) fan with an electric fan. The mechanical fan uses and electro-viscous clutch that has a valve controlled by PWM to control the mechanical fan's RPM. it uses a Hall sensor with a 5v reference to report actual RPM back to the vehicle's computer

If the computer already sends a PWM signal to the relay, am I over looking something, or is it really as simple as just hooking the battery to the source, PWM signal to the gate, putting a reversed bias diode parallel to the fan motor and hook the drain to the positive on the fan motor and then just ground the fan motor?

Also, suppose I wanted to manually put the fan at 50% duty cycle minimum whenever a +12 voltage is applied? I want to tie in to the A/C compressor, so the fan forcibly goes to 50% minimum whenever the A/C compressor turns on, or 51-100% if the trucks computer tells it to.

As for the Hall sensor, I believe the computer is comparing the actual engine RPM to the fan's RPM according to the Hall sensor. Is there an easy way to fake this? The engine's computer will essentially go into fail safe mode and turn on the check engine light if it does not report the expected RPM back for more than 120sec, or if it's disconnected completely. I have the ability to program the computer to just ignore the hall sensor all together with a few keystrokes, so if it will needlessly complicate circuit design, I can just leave this out easily enough. I thought about using a 2 speed Lincoln Mark VIII fan, and just hooking up the PWM output to the "high" input on the fan, and the "low" input on the fan to a relay which would disconnect the high input and manually connect the "low" input to 12v, but this would not allow the fan to run at more than 50% until the A/C compressor turned off, even if it really needed to run on high.

I'm mostly self trained through dabbling with random projects, and when playing around with several thousand dollar vehicles cooling systems, I like to ask questions before I bake things. Besides, stuff like this is fun, and I usually come out knowing more than I started. So Win/Win.

The reason I'm doing this if anyone is wondering, is because at idle, the truck's computer turns the EV-clutch off, and it doesn't pull air through the a/c evaporator, so I get hot air from my a/c vents when stuck in traffic for extended periods. Also, removing an 8lb fan from the front of my engine is going to give me a few free HP and some free MPG's. The alternator on the truck is way more than adequate for my source, it's rated at 160 amps, the fan I'll be using -should- be under 50amps current draw while running.
 
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