Auto Electrical Fan Controller

Thread Starter


Joined Jun 1, 2009
First time post. I've been googling and reading up on other automotive forums for the past few weeks and I've found a lot of tutorials and how-to's...but they don't get too in depth on the electrical parts side. I've had a few electronics classes, so I think I know enough to get started, but I need some help to finish.

My goal is to create a fan controller that will be rock solid that will not break down in 2 years when I'm on the highway.

Background: I have a Ford Ranger that has been pretty good, but has the 3.0L engine that is a bit underpowered. There has been a bunch of guys that will buy kits and be on their way, but they use probes that they stick in the radiator with an adjustable on/off controller. I just don't like the idea of using a setup that other automotive manufacturers won't use.

So using what I've found online, I think I've come up with this design:

A little explanation on what's going on. Going left to right, I have the truck battery going to a relay. This relay is the on/off for the whole system. It is controlled by one of two signals. A temperature switch that has a high resistance until 200 degrees. Once 200 is reached, resistance is lowered allowing electrical flow energizing the relay coil. Once fans cools the engine until 185 degrees, resistance goes back up, stops electrical flow, and the coil de-energizes. No matter what temperature the engine is at, if the Air Conditioning is turned on, the first relay must be turned on. I had initially thought of diodes to connect both 12v switched and A/C Request, but after reading a bit on here..I could use a transistor or even another relay...but I'm not sure what would be better.
Past the first relay, the second relay basically switches the output for the fan motor. The fan is a 2-speed with three wires: ground, low speed, high speed. This relay is controlled only by the A/C Request. The fan is more than enough power to cool the engine on low, but when the AC is on, high speed should be selected to get more air past the condenser lines.
The fan is supposed to pull less than 20 amps on low. On high, it pulls about 30-40amps when up to speed, while getting up to speed, it is said to pull upwards of 50-60. I have yet to find hard information on the fan. This fan is found in Ford Taurus cars and Lincoln Mark-VIII cars.

So that's how I want it to work, but here are a few things I'm unsure about.

1) I've read that a capacitor over the coils of a relay is supposed to help protect the relay. I'm sure you guys know this, but how big of a capacitor are we talking about? I've tried reading up on the types of capacitors, but I still can't get a handle on which type would best suit this application.

2) To allow either 12v switched or the A/C Request line to run the first relay, what would be the most cost effective way? I could throw a small relay in there. I think a transistor of some sort would work, but wouldn't know how big of one to use. Diodes may work, but again, not sure of size or type.

3) I've also read a free-wheeling fan motor, when not consuming power, will generate electricity. Some have put diodes on the fan lines to keep the electricity from going back into the controller. Is this really necessary? If the first relay is off, both of the fan's power cables are in a not connected state, so there wouldn't be anywhere for the power to flow, other than through the relay contacts.

4) Since the high-speed setting consumes so much, some people have been buying 75amp relays, but they are 20 bucks each. 30/40 amp relays can be had for about 5-8 bucks. Would adding a capacitor between relay #2 lead 87 and the diode act as a buffer so that when the initial draw happens for high speed, it does not pull so much power through the relays all at once? How would I size that? On a 12v system, would I have to worry about the time it takes to charge up the capacitor?


Joined May 23, 2009
ok, if that,s strictly a temp ON/OFF switch at a certain temperature, then your good to go on that part of the circuit.
There are still other things to consider, a few years ago I tried to come up with something similar to what your doing but I bypassed the whole sensor deal. I drive an older Dodge Caravan 3.0 L that are notorious for overheating and then a gasket blows. I do mostly city driving and a lot of idelling during deliveries, so I just came up with a simple circuit that turns on my fan at half speed when the vehicle is going slow or comes to a stop. It's been working great for years and the temp gauge stays steady all the time.
This method may not be for you, but it might spark some other ideas for you.

Thread Starter


Joined Jun 1, 2009
After looking at it for a bit, I realized my design was still controlled overall by the temp switch. so if I start the truck up and want A/C, I'd have to wait till it warmed up. Revised it a bit.

WOW 40Amp fan! I'm no automobile electrician.. But 40 amps is a lot of juice... I really maybe wrong.. Maybe 40 amps is the fuse..

A normal radiator fan sucks about 5~6 amps.. And that's powerful.

With 50~60 Amps on the fan.. Your battery and alternator will go bananas..
A regular alternator gives you 90 Amps.. My honda civic 1993 gave me around 70Amp. This means when you start your car it'll draw 80Amps for the starter.. But if we add this all up.. Fan 50amps + Starter 80Amps + Headlight including power windows etc etc 15 Amps this would be a total of 145 Amps!
Meaning the car will not start when the fan is on....Then again maybe I'm wrong and only know about old cars..

This 50Amps could also mean that it could draw 50Amps when you hold on the blade or when it is under extreme load (example bad bearings)...But under no load and normal operation it should pull 5~7 amps..

In any case I modified my car fan in such as way that my condenser fan is pulse width modulated.. Same principle with the CPU processor fan. Fan will turn with variable speed.. Meaning that my fan doesn't go at max speed when it's not needed. I did this to prolong the life of my radiator and condenser fan...

There are many kits available for doing this..
Here is one of them..

And of course with a little patience it can be self designed. Using this principle..

Thread Starter


Joined Jun 1, 2009
looking at the a1electric spal site, the 16" fans for the upper two performance fans pull 22a and 13a respectively. However, those fans are still 140+ bucks and then add shipping. Then they want another $130 for the controller.

I did another quick search on how much power this Taurus fan would pull, but did find this site that made compelling arguments:
If the Taurus ran a 30amp relay, then Ford must not have regarded the initial power pull as too much. I'm still interested in using a capacitor to help with some of the initial power draw.

I think if I built my own controller, use a junk yard Taurus fan (easy mounting in the ranger)...I think this could be done for about $100. Good reference site for knowing the typical ampere rating...

In any case continous current must be below 30Amps.. You can measure it by using or borrowing an ameter from a friend..

If it really does consume 30 Amps continously.. Just buy some other radiator fan that does the same trick but less Amperage.. Too much ampere draining for a fan is overkill for your battery or alternator.. It's a waste of precious gas and energy in short...

So for your controller you can design it using the previous principle... But instead of using a small BJT transistor.. You may use something like an IGBT or MOSFET to do the switching.. If you do it all by yourself 100$ is a good budget.. You might even be able to do it with 60$.


Thread Starter


Joined Jun 1, 2009
Okay, so I went out to a pick-n-pull to check out the Taurus electrical system. It was shocking. :)

On a '95, the fuse block had a 50amp fuse. An older than 95 had a 60amp fuse, and a newer one had a 40amp fuse. Looking at the alternators, they were 130amp units. I think that puts my idea of using one of those fans to rest!

If I end up doing this, it'll be a single speed fan with amps in mind.


Joined Oct 2, 2009
I also have the ranger.. same 3 liter engine.. but i was thinking of replacing it because it keeps on stalling and underpowered too.. I am just not sure if the ford ranger parts like another engine will be easy to spot.
Last edited: