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Automotive electric fan controller for 2-speed fan

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ccernst

New Member
I've gotten this bug to replace the mechanical fan in my truck with an electrical fan. When my mechanical fan engages, it really sucks the power out of my engine and it becomes a real dog. Many who've done this to their Rangers have noticed more power available for moving the truck.

There are many designs out there covering the mechanical/physical aspect, but the controller idea is a bit lacking for wiring up a 2-speed fan (commonly found in the Taurus and Lincoln Mark-VIII). I think I know enough about circuits to come up with the general idea...but sizing the parts out, I'm lost. Since this is a critical piece of my truck, this needs to be robust.

Here is the design I came up with, followed by a description of sources and operation...and then my questions. I apologize for the bad quality of my schematic...
7913-e-fan_controller_v3.jpg


Starting with my battery (12v constant source), this goes into my first relay. First relay I want controlled by either a temp on/off switch or a signal tapped off of my air conditioner compressor that tells it to turn on/off. My temp switch looks like a sending unit, but normally has a high resistance up until 200 degrees, then resistance is lowered until it is cooled down to 185, which then resistance goes back up. This is one way the first relay's coil will be energized/de-energized. Alternatively, when I have my Air Conditioning (AC Request) on, I want the fan to come on in the high-speed configuration. If AC is off, then the coil is driven from an ignition switched 12v source so the fan does not keep running after shutting down the truck. The AC Request will also shut down when the truck is shut down.

My second relay simply controls high/low based on the state of the Air Conditioning system. If the compressor is off, then the relay goes to low speed state. If the compressor is on, then the relay goes to high speed state.

The motor sounds like it is pretty robust. On low speed I get the impression that it pulls about 15-20 amps. On high speed, I get the impression it is about 30-40 amps. It has been measured to pull up to 60 amps when on high and spooling up.

Now for my questions:
I've read that to prolong the life of relays, you are supposed to put a capacitor over the coil contacts. If this is true, how is this sized and what type do I use?

I've read that a free-wheeling dc fan motor becomes a generator when power is not applied. In order to prolong the life of the motor, diodes are put in series to keep power from going back through the system. However, if the fan is controlled through relays, when power is not being applied to the 12v+ wires, the relays have broken contact, so voltage cannot flow. If diodes are needed, how do I size them and what type?

When AC is turned on and fan starts from a stopped state and is trying to spool up, it pulls a lot of power. Could I add a capacitor between relay 2 and the diode to build up enough energy so that I don't need to purchase a 75amp relay and stay with a normal automotive 30/40 amp relay?


This is the third time to type this up and as I was typing, found an error and made an attempt to fix it...if something looks off...that may be why.
 

tcmtech

Banned
Most Helpful Member
Actually they make a thermostatically controlled fan clutch that gradually increases the fan speed as the sensed temperature goes up. Its still mechanical like the one you have and looks nearly identical other than being a bit bigger.
Just ask the parts guys about it.
They are typically refereed to as a heavy duty or towing fan clutch. They work very well!
The last one I put in was on my Ford 460 and it cured the the belt squealing and air ripping sound problem! I didn't notice power loss from the fan kicking in being I built it to top 400 hp. But it shredded belts when towing heavy loads.

Its afar simpler install than the electrical method too! Those electric fans kill the older smaller stock alternators because the vehicle charging system was not designed to run them.
The Taurus had a 100 amp alternator stock because of that.
 

ccernst

New Member
My ranger came with a clutch fan...when it kicks on, it is not pleasent. Trying to crawl through a parking garage with a manual while the thing is clicking on and off is maddening.

OEM replacement for the Ranger is a 95amp model. The 3.slow engine is only rated at 145HP when it was new.
 
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tcmtech

Banned
Most Helpful Member
We have had several pickups over the years that had the same fan clutch style as you have. There is a far smoother operating one thats variable and not full on or off.
It solved our problems! The vehicles stayed cool and we never had that full tilt fan roar either!

The single stock electric fans work on the smaller cars with V6 and I4 engines but cant keep up with anything bigger. I know several people that tried using them in pickups and older vehicles just to do what your doing.
They ended up buying the bigger and rather expensive after market fans in order to handle the heat load.
Your design idea is practical and should work though. Its a basic version of the electrical control system on most ford cars.

I am not saying to not try it but it could become a time consuming and expensive learning experience real fast!

The after market variable rate clutches are not that expensive, My super duty one for my 460 was about $120 and its the biggest baddest one they made and was designed for Ford commercial truck engine applications.

I would suspect the one for a Ford Ranger would be considerable less.
 

ccernst

New Member
If you have a link for that type of clutch, I'll look into it. All I can find are Hayden type where they either on/off or spin at 80-90% of the water pump speed when "on" or 20-30% when "off"...which means it will always be on when I don't need it.
 
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