Making an Electric Fan Temperature Switch

The mechanical fan robs you of 5 to 10 hp at highway speeds. The engine is powering the fan for no reason, the air moving at 20 mph and up is more than adequate to cool the rad. An electric fan is only on when you need it. I've purchased an electric fan and found two radiator fan temperature switches to use. The radiator fan temperature switch turns on the fan when the rad gets hot and off when the coolant cools down.

  • 1980 VW Rabbit 1.5L 203 +/-5 degF on, 194 degF off (Wells SW533, Standard TS-151) M22 x 1.5 threads
  • 1991 Subaru Loyale 1.8L 203 +/-5 degF on, 194 degF off (Wells SW554, Standard TS-183) M16 x 1.5 threads
  • 1987 Buick Regal Grand National 3.8L Turbo 210 degF on, 205 degF off (Wells SW555, Standard TS-136) 3/8 x 14 threads

I oringally purchased a 1984 Mustang temperature switch (235 degF) as I want the engine to run hotter: warm up faster and be more efficient (use less gas). Unfortunately, it is meant to be mounted in the block. My 1992 2.3L has no temperature sensor mounting hole in the block or thermostat housing so I had to make an inline one - basically a metal tube with a bung soldered to it so that the coolant temperature switch can screw into it.

The problem with the Mustang temperature switch is that the water flowing through the rad hose reaches only 210 degF past L - the far end of the temperature gauge scale (C NORMAL H). I want the fan to come on at around 200 degF (M on the gauge) and the two 203 degF temperature switches are the closest to the ideal values that I could find. They turn on at 203 degF and turn off after the water has cooled to 194 degF. This is only needed for stop and go driving as anything above 20 mph provides enough air to cool the radiator.

To make an inline temperature sensor, you cut the rad hose and clamp a metal tube with a bung soldered on between the two pieces of the rad hose. I found that a standard air hose adapter worked well for the bung. I picked up a used upper rad hose from the local wreckers for $1.99 as all the parts places didn't stock one and had to order it from their warehouse! I wanted to have two upper rad hoses: one for experimenting with and the original one for when my mods leaked (and they did until I fixed them).

The inside diameter of the rad hose is the same diameter as a sink trap which I also happened to have in the junk drawer! There are two sizes of piping for sinks as I found out. When I first tried soldering to the chrome sink trap, the solder wouldn't stick. So I tried JB Weld to hold the bung in place and it looked great until I checked it after driving to the local grocery store. When the coolant was hot, JB Weld softened and you could peal it off of the chrome. I swapped out my modified rad hose for the original and purchased a brass colored sink pipe - after cutting and drilling it, I found out that brass sink pipes are 1/4 inch larger in diameter than the chrome ones! I also tried welding and brazing but the sink traps are too thin to work with.

So back to square one, using the original chrome sink trap but this time grinding the chrome off to reveal the brass pipe underneath which we can solder to.

First you drill a small pilot hole and then use a hole saw to carefully drill a hole for the bung

Carefully grind off the chrome plating on the sink trap in preparation for soldering

Here's the tools you need to solder: Propane torch, solder paste, solder and workbench

Apply some solder paste onto the part to be soldered

Preheat the part so that the solderpaste melts

Tin the sink trap with solder by heating the sink trap and applying solder so it flows evenly and is shiny. You may have to apply more solder paste

I cleaned the threads on the bung using my wire wheel and acetone, then applied solder paste to it, and preheated.

Then I tinned the bung with solder. You want to have an even coat of solder on the threads. This is where the previous cleaning is important!

Sorry, I don't have a picture. I screwed the bung into the sink trap and held it in place with a C clamp than slowly built up the solder on all sides. Go slow here and just heat up enough of the bung to just barely flow the solder in the immediate area. You will need a LOT of solder! This is a picture of the finished tube, bung, cut rad hose and temperature switch in place during testing for leaks. It will take a couple of tries to properly solder and seal the bung. Use teflon tape on the threads of the temperature switch to aid in sealing.

The temperature switch is grounded through its outer case. Unfortunately, the rubber rad hose insulates it from ground! So I had to drill and tap the bung for a 6-32 stud. After it was tapped, I used an extra long screw with LOTS of teflon tape on it for sealing. I used a nut to lock the screw to the bung. I will be using a dremel tool with a cutoff wheel to cut off the screw head so it becomes a short stud.

Here's the schematic for the fan wiring. The automotive relays have standard numbering for the pinouts. The color code unfortunately is not standard. You can read more about automotive relays here.

Alternative Approach - Buy a Kit!

An alternative is to do what CK suggested:

To install an electric fan in the ford ranger is easy. I did it with no problems. Just buy this kit from jegs and you run a few wires and set the module to the temperature and you're set.

Another simple option is to purchase a Zirgo fan controller kit, they have several models ranging from $50 (fixed temp of 185 degF) up to $150 for an adjustable one with a digital readout. I went to the local parts store and they had a few generic ones there also ranging from $65 (fixed temperature 185 degF) to $100 (an adjustable one).

After fiddling around for a while, I purchased an adjustable fan control module for $65 that came with a temperature sensor. Mounted it and then just adjusted the exact temperature using the adjustment pot. Next is replacing the mechanical fan with an electric fan

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Copyright February 2011 Eugene Blanchard