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How to create a variable two speed electric fan control panel for a Taurus two speed fan.

My idea was to have a Flex-A-Lite variable speed controller (#31165) run the Lo speed side, and have a Hayden adjustable thermostat (#3653-H) turn on the Hi speed side if the temperature got too high.

It took a couple tries to get the schematic just right, the final rendition worked perfectly. Two relays for the Hi speed side (split’s the load between the two relays), one relay as an isolation switch for the fan ground wire (one side for the fan ground wire from the Lo speed controller, the other one for a direct ground for the Hi speed side), the fourth relay is to isolate the fan power wire coming from the Lo speed controller.

Here is the schematic that works.

On the original I only isolated the fan ground wire, it wouldn’t work properly, don’t use this!

First I laid out the parts and figured the area I needed for them. Next I formed an aluminum panel and mounted the parts to the panel. (Note: this does not have the fourth relay on it yet.)

Used a piece of cardboard to determine area available.

Mounted to panel.

I mounted the control panel under the battery tray, I had to bend the side panel mounting the MAXI Fuse to a 45 degree angle for clearance. (Note again that this pic does not have the fourth relay mounted yet.)

Mounted to battery tray support.

I put the wires from the controller going to the fan in braided cover, and routed them between the battery and the radiator support, and then across the top of the radiator support.

Spade connectors to connect wires to fan wires.

Extra insulation over connections.

Then finished the wires with braided cover.

I run a 165 degree thermostat in my 347, so I set the Lo speed side to come on at about 170 degrees, starts at 60% power and ramps up to 100% if temp goes up by 10 degrees. I set the Hi speed side to engage at about 195 degrees, turns off at about 185 degrees. When the Hi speed thermostat reaches it’s set temperature, it creates a ground circuit that energizes the coils in the relays.

This switches the fan ground wire from the Lo speed controller to direct ground, isolates the Lo speed fan power wire from the controller, and sends full battery power to the Hi speed side power wire of the fan. You cannot have power going to both of the fans power wires at the same time or it will burn out the fan. The system worked beautifully!

The Lo speed controller turned the fan on at 172 degrees, temp stayed right there and the fan would slowly cycle on and off. The fan would start up slowly (I doubt it ever made it to 100% as the temp gauge never fluctuated), run about a minute, and then slowly come back to a stop. Then it would cycle again, the cooling system is working perfectly on just the Lo speed side!

I manually turned off the Lo speed controller in order to set the thermostat for the Hi speed side. The temp climbed to 195 degrees and I adjusted the thermostat to start up the Hi speed side, the fan came on full power (think hurricane) and the temp came back down, switched back to the low speed side and started the cycling again. Next, I checked to make sure the Lo speed controller would turn on when the AC was turned on, yep, worked perfectly, and it held the temp right at 172 degrees even with the AC running. I'm a very happy camper!

I sourced the Flex-A-Lite controller # 31165 from Summit $98.

The Hayden adjustable thermostat I’ve had for about 6 years, I used it to control my old cooling fan, I think I paid maybe $20 for it.

The two linked relays I used for the Hi speed side, I got from Summit, #VIA-80238, $9.
The other two I already had.



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