GM F-Body Electric Coolant Fan Specifications and Modifications

By William Weissman (tplsz87@cs.com)

Overview

General Motors engineered these cars to run HOT!! The primary coolant fan does not engage until 222° or higher (depending on application) and the secondary coolant fan until 243° or higher. It’s been shown that at these high temperatures, air and fuel burn more completely and therefore produce less hydrocarbons. However, these temperatures severely reduce engine life and kill performance. So why not just install a lower temperature thermostat and call it done? Simply because a lower temperature thermostat only opens sooner. In stop-and-go traffic, the engine’s temperature will continue to rise unless there is sufficient air flow to the radiator. Assuming the A/C is turned off, have you noticed that on the open road, the temperature is acceptable, but when you hit city traffic, it climbs to astronomical levels?

Objective

The fan turn-on temperature must also be lowered. The true ideal system would incorporate a simple, straightforward design without disabling GM’s system. I will explain this later.

So let’s say that you install a 160° thermostat and a switch that engages the fan at 176° (or so). It seems like the ideal situation — until the first cold day. Then, the engine will never reach full operating temperature because the fan is running — when you don’t want it to. The heater will only work marginally, at best. Of course, you can change your thermostat and fan switch twice per year, but do you really want to do that? How about a typical fall or spring day when it’s cold in the morning and warm in the afternoon. What do you do then? Is there a practical solution?

First, let’s examine how GM designed their coolant fan turn-on system. Then I will explain how it can be modified in the "Recommendations" section.

Note: For those F-bodies with only one coolant fan, the following information referred as "primary coolant fan" is applicable. In the "Recommendations" section, there is a sub-heading specifically for single fan cars.

Primary Coolant Fan Turn-On System
Factory Circuit Operation

On TPI engines, the primary fan is controlled by the ECM and A/C head pressure. The ECM grounds the coolant fan circuit when the coolant temperature sensor indicates the temperature is greater than 222° or when the A/C head pressure is greater than 232 psi and the vehicle speed is less than 35 or 40 mph (depending on application). When this circuit is grounded, the fan relay is energized and voltage is applied to the fan.

Secondary Coolant Fan Turn-On System
Factory Circuit Operation

On some applications, the secondary fan is controlled by the ECM, A/C pressure switch and a temperature sensor, similar to the primary fan. On other applications, the ECM is bypassed and only a temperature sensor is used.

Aftermarket Products

Here is a sampling of products that will modify the factory settings. There are others, but they essentially accomplish the same thing.

Hypertech makes a fan switch (Part No. 4026). Available through Summit Racing (800) 230-3030 for about $45.

Features:

Replaces the factory switch.
Fan turn-on at 176°, off at 166°.

Hypertech makes a fan switch (Part No. 4028). Available through Summit Racing (800) 230-3030 for about $45.

Features:

Replaces factory switch.
Fan turn-on at 200°, off at 185°.

Hayden makes a fan turn-on relay (part no. 3652). Available at Pep Boys for about $25.

Features:

Thermal switch mounted in radiator.
Fan turn-on at 185° and off at 170°.
Can be wired to augment factory switch, not replace it.

Hayden makes an adjustable fan turn-on relay (Part No. 3647). Available at Pep Boys for about $39.

Features:

Adjustable turn-on from 160° to 210°.
Can be turned on automatically when AC is engaged.
Can be wired to augment factory switch, not replace it.
Can be manually turned on with a dash-mounted switch.

Derale makes an adjustable relay (Part No. 16759). Available through Jegs (800) 345-4545 for about $42.

Features:

Adjustable turn-on from 140° to 260°.
Adjustable turn-off from 2° to 20° below turn-on temperature.
Can be turned on automatically when A/C is engaged.
Can be wired to augment factory switch, not replace it.
Can be manually turned on with a dash-mounted switch. LED indicator.
If running when ignition is turned off, the fan continues to run until turn-off temperature is reached.

Recommendations

Overview

We know that heat shortens the life expectancy of engines. The temperature that GM operates their engines is acceptable for lower emissions, but a killer for performance and longevity. I found it imperative to lower the operating temperature.

Through five years of research, primarily trial and error, I believe that I have designed a system with the most versatility. I call this my "Total Fan Control" system.

For most performance enthusiasts, the approach is to lower the operating temperature by using a 160° thermostat and either running the fan constantly or using a turn-on switch at approximately 170° to 175°. This is acceptable and fully functional, except in the winter months or during emissions testing. If we follow this approach, the engine never reaches full operating temperature in the winter months and the heater works marginally at best. I suppose you could change to a 180° or higher thermostat and change the fan operation back to stock, if you’re willing to do this twice per year. I do not find this solution acceptable. Also, think about the ambient temperature on a typical spring or fall day. In the morning and night, it’s cold. In the afternoon, it’s rather warm. On these days, it would be ideal to use the 160° thermostat and modified fan control in the afternoon and a 180° with factory fan control on those cold mornings and nights. If there were a way to run "Summer Mode" and be able to change to "Winter Mode" at the flip of a switch — that would be ideal. This was my goal — and I’ve accomplished it.

In analyzing all the products on the market, I came to the conclusion that none of them would meet all my criteria. It would take a combination of products and a little ingenuity on my part. In analyzing the factory coolant fan wiring diagrams, I decided that I would not disable GM’s wiring scheme in any way for two reasons:

(1) For those who live in states with a yearly emissions check, wouldn’t it be nice to revert back to the factory settings at the flip of a switch?

(2) By supplementing the factory system, there are actually two systems working independently of each other, the factory system and the Total Fan Control system. If the Total Fan Control system were to fail (in four years, it never has failed me), the factory settings, which have not been disabled, will engage the fan(s).

The information I share with you is a culmination of my research. I think you will agree that this is the best and most versatile system available today.

Thermostat
Of the numerous experts on tuned port injection engines, Greg Carroll of Carroll Supercharging Company and Myron Cottrell of TPI Specialties stand out. Greg Carroll states that 160° is too cold for these engines. He suggests a 170° thermostat. Myron Cottrell says, "We have found that 170° is a better temperature for all-around driving. The computer in your car is designed to provide a correct fuel mixture at whatever temperature the motor is controlled. The 170° thermostat provides the best compromise between power, economy and wear." I have found that with a 170° thermostat, I do not need to change between a 160° for summer and a 180° for winter, but that I can use it year-round.

I used to use the 160°/180° thermostats in combination with my coolant fan system and a stock chip for about four years with no problems. I did grow tired of having to change it twice per year, though. I knew of the 170° thermostat, but to the best of my knowledge, TPI Specialties was the only source. For $10, it was not worth it. Last year, I discovered the AC Delco part number for this thermostat (about $4) and have been using it ever since. On those cold mornings, the heater actually works. It never really worked effectively with the 160° thermostat (luke warm at best). And as for those hot days, think of this; with a 160° thermostat, what do you think your normal operating temperature is with the A/C off? I’m willing to bet that it’s not between 160° and 170°. It’s probably around 170° to 185°. Even with a 160° thermostat and both fans running, the engine will only cool to around 170° to 185°. Therefore in the summer, it’s irrelevant whether the thermostat is a 160° or a 170°.

Before installing the thermostat, modify it by drilling four 0.150 inch diameter holes in the thermostat body. The holes allow a constant restricted flow of coolant through the engine which prevents hot pockets from forming. Although GM does not list a 170° thermostat in their application charts, it is available through any GM dealership. Order AC Delco part number 10220957.

Primary Coolant Fan – Dual Fans
Now that I’ve decided to use the 170° thermostat, which fan switch should I use?

I chose the Hayden thermal switch for a variety of reasons. This switch allows for an additional path to ground (Option #1a outlined below). The Hypertech switch does not because it replaces the stock switch. This additional path to ground affords many other options that aren’t available with the Hypertech switch.

For example, by installing a toggle switch in series with my new circuit, I can turn off this circuit. This is important on those cold winter days when you want your engine temperature to be warmer and only the second fan is needed. The Hypertech switch allows no options as with the Hayden switch. Once the Hypertech switch is installed, your stuck with it – your fan will turn on at 176° and your heater won’t work.

I must mention that the Hayden kit contains several parts; a relay, the thermal switch, and a fuse holder. Only the thermal switch is used for the primary fan circuit. The relay may be used on the secondary fan circuit as outlined later.

The wiring diagram is simple and straightforward. The optimum location to insert the brass switch is about three inches below the top radiator hose. This allows the switch to "see" the coolant temperature as it enters the top of the radiator where it is hottest.

Option #1a
What I have done is added an in-line toggle switch to this circuit. By turning the switch off, control of the first fan is accomplished the way GM wanted it. By turning it on, the fan will turn on when coolant temperature reaches 185° (fixed turn-on point of the brass switch) and turns off at 170°. Imagine that — off at 170° — and we’re running a 170° thermostat. It couldn’t be more perfect. I call this switch "Auto 1".

Option #1b
In addition, if you want the option of turning on the primary fan at the flick of a switch, it’s as simple as adding another parallel circuit (spliced into the same location — the green/white wire at the coolant fan relay). Essentially, this circuit is used as a "manual override" and I call this switch "Fan 1".

Option #1c
An fan indicator LED can be wired as confirmation that the fan is on. This is also shown in the wiring diagram. Secondary Coolant Fan – Dual Fans
I use the Derale adjustable relay for my secondary fan turn-on. For the same reasons outlined above, I did not select the Hypertech switch. I did not select the Hayden adjustable switch (I used to use it until I found the Derale unit) because it does not have an LED mounted on the unit, nor does it continue to run when the ignition is turned off on those hot summer days. By allowing the fan to run after shutdown (usually only 5 minutes or so), it helps to prevent the coolant from continuing to heat up. If you start your car back up within 10 to 20 minutes of shutdown, the temperature is higher than before shutdown. The Derale unit helps to prevent this, the Hayden does not.

According to the Derale installation instructions, the fan wires, (+) and (-) are connected to the unit. This necessitates disabling the factory circuits. I do not recommend this. Instead, what I’ve done is connect the fan wires, (+) and (-) to a SPDT relay (explained in detail later). The SPDT relay then energizes the fan by grounding a parallel circuit installed between the coolant fan relay and ground (just like the circuit I added to the primary fan circuit). The Derale unit wired in this manner allows full function of the factory setup.

Option #2a
What I have done is added an in-line toggle switch to this circuit, similar to Option #1a. By turning the switch off, control of the first fan is accomplished the way GM wanted it. By turning it on, the fan will turn on when coolant temperature reaches 195° (adjustable). I call this switch "Auto 2".

Option #2b
In addition, if you want the option of turning on the secondary fan at the flick of a switch, it’s as simple as connecting the A/C turn-on wire on the Derale unit to a toggle switch. Why not wire the A/C turn-on to the A/C compressor? The factory setup will turn on the fan when A/C pressure exceeds 232 psi. Because my circuit does not disable the factory setup, the A/C will still engage. Therefore, we can connect the A/C turn-on circuit to a switch that acts as a "manual override". I call this switch "Fan 2".

I occasionally drag race my car. In between runs, I want to cool the engine as much as possible. This is when I turn on "Fan 1" and "Fan 2". In the event that the thermostatically controlled switch fails (it’s never happened to me) and/or the Derale unit fails, then you can turn on either fan at the flick of the switch.

Option #2c
A "fan on" indicator LED can be wired as confirmation that the fan is on. This is also shown in the wiring diagram.

Primary Coolant Fan – Single Fan
Now that I’ve decided to use the 170° thermostat, which switch should I use?

I recommend either the Hayden thermal switch or the Derale adjustable switch. Your personal preference will dictate which is right for your application.

Both switchs allow for an additional path to ground (Option #3a outlined below). The Hypertech switch does not because it replaces the stock switch. This additional path to ground affords many other options that aren’t available with the Hypertech switch. Once the Hypertech switch is installed, your stuck with it – your fan will turn on at 176° and your heater won’t work.

Let’s examine the Hayden and Derale switches in detail.

The Hayden thermal switch closes the circuit at 185° and opens at 170°. These points are fixed and non-adjustable. The optimum location to insert the brass switch is about three inches below the top radiator hose. This allows the switch to "see" the coolant temperature as it enters the top of the radiator where it is hottest.

The Derale unit can be adjusted to turn on from 140° to 260°. It can also be adjusted to turn off from 2° to 20° below the turn-on setting. This unit also has an "on" LED, and most importantly, if it is running when the ignition is turned off, it will continue to run until the preset turn-off temperature is reached. By allowing the fan to run (usually only 5 minutes or so) after shutdown, it helps to prevent the coolant from continuing to heat up. If you start your car back up within 10 to 20 minutes of shutdown, the temperature is higher than before shutdown. The Derale unit helps to prevent this, the Hayden does not.

Option #3a
What I have done is added an in-line toggle switch to this circuit. By turning the switch off, control of the first fan is accomplished the way GM wanted it. By turning it on, the Hayden or Derale switch will be armed. I call this switch "Auto".

Option #3b
In addition, if you want the option of turning on the fan at the flick of a switch, it’s as simple as adding another parallel circuit. Essentially, this circuit is used as a "manual override" and I call this switch "Fan".

Option #3c
An fan indicator LED can be wired as confirmation that the fan is on. This is also shown in the wiring diagram.

Summary – Dual Fans

Modes of Operation
I have addressed these problems stated in the Objective section — and have solved it. It does not take just one aftermarket product, but a combination of two products installed with a modified wiring scheme. What I have designed is a system that will run the engine as cool as possible in the summer, and at the flip of a switch, warm enough in the winter to allow the heater to function effectively. Also, how about that dreaded time each year — emissions check — when we all wish our cars would magically return to the stock temperature. My system is so versatile that this can also be accomplished at the flip of another switch. All this without having to change the thermostat.

Oh, Those Hot Summer Nights.
When the ambient temperature is high, turn on "Auto 1" and "Auto 2". This will engage the primary fan at 185° and the secondary fan at 195° (can be adjusted to your preference).

Brrrrrr!!! Those Icy Cold Days.
When the ambient temperature is low, turn on "Auto 2" only. This will engage the secondary fan at 195° (can be adjusted) and the primary fan at the factory setting (about 222° depending on application). In the winter, I have found that the secondary fan is sufficient.

Wanna run???
At any time, one or both fans can be manually turned on by switching "Fan 1" and/or "Fan 2".

Yes, Mr. EPA, my car IS stock!!
At any time, both fans can be returned to factory specifications by turning off all switches.

Summary – Single Fan

Modes of Operation
I have addressed these problems stated in the Objective section — and have solved it. It takes one aftermarket product installed with a modified wiring scheme. What I have designed is a system that will run the engine as cool as possible in the summer, and at the flip of a switch, warm enough in the winter to allow the heater to function effectively. Also, how about that dreaded time each year — emissions check –when we all wish our cars would magically return to the stock temperature. My system is so versatile that this can also be accomplished at the flip of another switch. All this without having to change the thermostat.

Oh, Those Hot Summer Nights.
When the ambient temperature is high, turn on "Auto". This will engage the primary fan at 185° (Hayden) or at your pre-selected temperature on the Derale unit.

Brrrrrr!!! Those Icy Cold Days.
When the ambient temperature is low, turn off "Auto". In the winter, I have found that the fan never turns on when in factory configuration – it just doesn’t get warm enough.

Wanna run???
At any time, the fan can be manually turned on by switching "Fan".

Yes, Mr. EPA, my car IS stock!!
At any time, the fan turn-on temperature can be returned to factory specifications by turning off both switches.

Additional Part

SPDT relay
Required with secondary fan or single fan (adjustable switch only)
This is a 30-amp single-pole, double-throw relay. In addition to the relay that’s supplied with the Hayden kit (Part No. 3652) with the thermal switch, there are two others available. AutoZone stocks a Blazer SPDT relay and costs about $4, but it does not come with a wiring harness. Bosch’s SPDT relay costs $9 and it comes with a wiring harness. I prefer the Bosch because there is a noticeable difference in quality. In addition, GM uses Bosch relays in their cars. Unfortunately, I cannot seem to find the Bosch relay locally. It can be purchased from Crutchfield (800) 955-3000 (Part No. 120E5000).

If you would like wiring diagrams for your specific application and/or information on custom built switch panels, please supply the following information:

Year, Make and Model, Engine Size, TPI, TBI or Carbureted, Single or Dual Fans, Which products and options (e.g. Option #1a, #2b, etc.) most interested in, your street address, and e-mail them to:

William H. Weissman
tplsz87@cs.com