How to wire my radiator fans to a toggle switch?

[focusonthefocus19]

I've been having some trouble idling in traffic, or driving in slow traffic. The car tends to over heat, and the fans don't kick on unless it's to late. It only has done that three times, but when I'm at home it doesn't get hot at all. I can't afford a mechanic, and I just want to wire the low speed, and high speed to a separate toggle switch inside the car onto the dash.

Can some one help me with this?

[Sponsor]

[elsolo]

Looking at the diagram:

The relays are controlled by switched grounds at the PCM, behind the right side of dash near base of A-pillar.

If you were to add a manual switch to ground out those control wires, you should be able to manually switch on the fans.

High speed cooling fan relay: BLK/WHT, # 31S-PA17, terminal # 17 @ PCM
Low speed cooling fan relay: BLK/BLU, # 31S-PA7, terminal # 68 @ PCM

I make no guarantees, but T-into the high speed fan relay switched ground wire and add an additional switch to ground. That way you can ground that wire with the switch or the PCM will do it for you eventually. If you use a multiple pole switch with 3 positions, you could have it on off, low, or high with one switch.

[Rob axel]

I remember seeing a diagram about using a "jumper" wire in the relay box....this would kick on the fans when the ignition is on. I located this when troubleshooting the "fan resistor" issue these cars seem to have.... it would be a quick TEMPORIARY fix...though I am not sure you would want your fans constantly running....unless your living in the desert...

[amc49]

If the fans are not switched on by the PCM you will get issues of a fluctuating idle when they come on, the PCM bumps the idle speed at same time fan comes on. Without that feature the sudden electrical load can almost kill the motor. Don't ask how I know..........

[sleepyboy]

Welcome to the forum


Here is a how to on how to get the fans to run all the time HERE.

Now that's out of the way. Here is another thread talking about the adverse effects of the fan mod HERE.

You need to fix the problem though. It sounds like your fan resistor is no longer working. It's what tells the fans to turn on. When it goes out it can cause symptoms such as what your describing. There are countless threads on it. It's really a easy fix no need to take it to mechanic. Here is a link to the part that I am talking about HERE . Also you want to check the connector that plugs into it. It's been known to get fried also. You can get the plug from the dealership, it's not too expensive. Most of us have had this problem at one time or another. I did years ago, but fixed it and haven't had a problem since. Good luck with it and let us know how it turns out.

[whynotthinkwhynot]

Now wait, you can wire the high speed hot and low speed hot together so that the fan will come on high speed whenever the computer tells it to come on low speed. That's the cheapest and easiest way to fix this issue that I know of.

I'll go search for that thread.

----> Here it is <----

Follow this guy's recommendation for an easy 10 minute/$1 fix for this issue. Later you can tie the output wires together somewhere if you want, or get a resistor and remove the jumper when you can afford the $40 resistor from a dealer. Remember, it's the cooling fan resistor, not the blower resistor.

As an electrician, I must say that they are all wrong about electricity. That circuit will create the same amount of amperage regardless of whether the resistor is used or not. The only difference is that the fans will run at a lower speed. Current remains the same, only voltage is reduced. IDK if Ford actually has it wired like that generic schematic with the 2 fuses, but bridging the 2 branch circuits is what he did whether he did it at the relay or at the wires near the resistor. Ohms law geek moment there.

[amc49]

While the thermal limiter in the resistor is fine, mine works well without it. The resistor melted up and I simply rebuilt it without the limiter along with a made up connector to avoid spending $15 for that. Total cost maybe $5. Works great.

[focusonthefocus19]

Originally Posted by sleepyboy

Welcome to the forum


Here is a how to on how to get the fans to run all the time HERE.

Now that's out of the way. Here is another thread talking about the adverse effects of the fan mod HERE.

You need to fix the problem though. It sounds like your fan resistor is no longer working. It's what tells the fans to turn on. When it goes out it can cause symptoms such as what your describing. There are countless threads on it. It's really a easy fix no need to take it to mechanic. Here is a link to the part that I am talking about HERE . Also you want to check the connector that plugs into it. It's been known to get fried also. You can get the plug from the dealership, it's not too expensive. Most of us have had this problem at one time or another. I did years ago, but fixed it and haven't had a problem since. Good luck with it and let us know how it turns out.

That's what I think it really is. I might just have to go buy a new resister pretty soon when I can afford it. The idle fluctuates when the AC is on, and some days it drops way down, and feels like it's going to die. Sorry for the late response. I've been busy due to working in the medical device field.

[a_2000_se]

Originally Posted by whynotthinkwhynot

Now wait, you can wire the high speed hot and low speed hot together so that the fan will come on high speed whenever the computer tells it to come on low speed. That's the cheapest and easiest way to fix this issue that I know of.

I'll go search for that thread.

----> Here it is <----

Follow this guy's recommendation for an easy 10 minute/$1 fix for this issue. Later you can tie the output wires together somewhere if you want, or get a resistor and remove the jumper when you can afford the $40 resistor from a dealer. Remember, it's the cooling fan resistor, not the blower resistor.

As an electrician, I must say that they are all wrong about electricity. That circuit will create the same amount of amperage regardless of whether the resistor is used or not. The only difference is that the fans will run at a lower speed. Current remains the same, only voltage is reduced. IDK if Ford actually has it wired like that generic schematic with the 2 fuses, but bridging the 2 branch circuits is what he did whether he did it at the relay or at the wires near the resistor. Ohms law geek moment there.

Careful - someone mentioned shorting out the Fan Resistor altogether - that is a current limiting resistor for the low speed operation/circuit.

Ummmm - let's take a look here again before someone misapplies this to a more expensive or important problem. Let's look at it from a practical point of view before we look at it from a theoretical point of view.

Practical -

If you put your screw driver across the terminals of your car battery - you'll get a few things. 1) Lots of sparks 2) A very hot screw driver 3) If the battery doesn't blow up and you wait long enough the screw driver may weld itself to the battery terminals. That's a LOT of current flowing through the series screw driver - probably the total cranking amps of the battery, 350 - 500 amps. For practical purposes the screw driver has ZERO resistance (it probably has something in the neighborhood of milli-ohms).

If you take a resistor with a very very large value (Mega-ohms), not necessarily a large physical size, and place it across the battery terminals what happens? Nothing you'll ever notice.

Conclusion - TOTAL resistance limits total current through a series branch.

Theory -

As mentioned you'll want Ohms Law for this one and maybe the 'voltage divider' formula. Series resistance, R total, is calculated by adding the individual resistance of each series component (parallel resistance is calculated differently). WE WILL CONSIDER A ONE FAN only CIRCUIT for simplicity.

E = I * R; OR I = E/R, Ohms Law

V Out = V in * Rx/Rt, Voltage divider formula

Rt = R1+R2+R...


So in this 'DC' circuit, we'll only look at it as steady state ('AC' can be different for many applications, especially if a device has a coil - like motors and relays).

So for the cooling fan circuit: Rt = Fan motor resistance + Fan resistor.

Let's say (it is less): Fan motor = one ohm. And Fan Resistor = one ohm also.
Rt = 2 ohms.
Let's say our voltage is 12 volts.

Ohms law says I = E/R ...or... 12/2 = 6 amps (steady state).

If you short the Fan Resistor:

Rt = 1 ohms.
Ohms law says I = E/R ...or... 12/1 = 12 amps (steady state).

So you can see that the current does NOT stay the same in the branch when the fan resistor is removed. ALSO IT MUST BE NOTED THAT THE DIAGRAM HAS two fans in parallel - Rt for the fans is now half the actual value which I believe is closer to one half ohm each fan (don't forget that parallel resistance is calculated differently than series resistance) hint: product/sum.

Extra credit - use the actual total Fan Resistance and the actual alternator voltage to calculate Current in the low speed branch of a dual fan circuit.

Now you know why the fan connectors are a 'problem' and get melted...... the rest of the story. And why the fan circuit has those big 'slow' blow fuses - it doesn't always operate in a 'steady state' (think start up current and resistance).

There is more to trying something and saying it works without knowing why it was designed the way it was. It was designed (under designed, melted connector) with many parameters in mind (or overlooked). A "tried-it-and-it-worked for me" may be a ticking time bomb for you and everyone else that does it.


The voltage divider formula will give you the voltage drop across each component. Common nodes have the same voltage.

The block diagram WAS taken directly from a FORD wiring diagram, but simplified. Tell me if I screwed something up here anywhere and I'll fix it.