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Choosing an Electric Fan Control: By Jim Clark (The Hot Rod M.D.)


By Jim Clark (The Hot Rod MD)

For many years the most effective means of cooling the engine at slow speeds and when idling was with a multi-blade fan attached to the front of the water pump pulley.  It drew air through the radiator when the forward motion of the vehicle was too slow to create enough airflow through the radiator.  The addition of a shroud helped to funnel the airflow through the full face of the core thereby increasing the cooling effect.  However, it is not necessary for the fan to operate at times when the vehicle is running at highway speeds.  If it did, the engine would most likely never reach proper operating temperature and the parasitic loss of power could significantly lower operating efficiency.

To remove this power drain a fan clutch was added to disengage the fan so that it freewheeled at highway speeds when the airflow was sufficient.  The installation of big engines in small engine bays created a problem though; room for the traditional fan/shroud combo was limited.  The most popular solution for this was to install an electric fan and thin-line shroud in place of the mechanical unit.

Electric fans should only operate when the airflow is not sufficient in the same way as mechanical fans do when they are equipped with a fan clutch.  Electric fans don’t use a mechanical means of disengagement though, they are just turned on or off with either a temperature sensing switch installed in the coolant flow or by a toggle switch controlled by the vehicle operator.

Components to accomplish this can be bought at the local parts store but the job is made easier when one of the many fan control kits is utilized.  We chose to use the fan control kit from Spal to control our Spal 16-inch fan on our Cool Craft custom shroud installation.  It is offered in two different models; designed to engage at either 185°F. or 195°F.  Deciding which one to use requires the answers to a number of questions about your cooling system.

First you must determine which type of cooling system that you have.

Closed-type cooling system.  The closed-type system doesn’t vent excess coolant to the outside when the heat causes expansion of the coolant.  It has a coolant reservoir that collects coolant that has expanded with the rising temperature of the coolant and would otherwise overflow from the system.  When the system temperature drops, the coolant is drawn from the coolant reservoir back into the radiator by suction created by coolant contraction thus maintaining a completely filled radiator.

This cycle occurs when the coolant reaches a certain pressure in the radiator regulated by the spring holding the overflow cap closed in the filler neck.  The pressure of the expanding coolant overcomes the tension spring in the cap and allows the excess coolant under pressure to overflow into the expansion tank, or recovery tank through the overflow port in the filler neck.

In a closed-type system care should be taken when checking your coolant level in the radiator.  After initial fill up, the radiator is full and the cap is installed, all level checking should be done at the expansion tank.  As the cooling system goes through it’s heating and cooling cycles, coolant needed to replace air trapped in the system will be drawn from the expansion tank.  This is where you add coolant if the system is low.  Removing the radiator cap will result in loss of system balance and it will take another heating/cooling cycle to reestablish this balance.

Open-type cooling system.  Early vehicles with narrow radiators placed the tanks at the top and bottom of the core.  The filler cap was placed on the top of the top tank.  The radiator was filled with coolant to within one to two inches from the top of the tank to allow for expansion of the coolant.  When these systems were pressurized by adding a pressure release cap to the radiator, excess coolant created from expansion was vented to the outside.  An over flow catch can was installed instead of dumping this excess coolant onto the ground.  However, the coolant is not drawn back into the system as it is with the closed system.  In street applications, an upright radiator (top and bottom tanks, with the cap on the top tank) represents a compromise that will work, as long as the car is not operated at sustained high RPM, like those seen in racing.

Radiator Caps

The radiator cap is often overlooked in common engine overheating problems.  It is designed to add pressure to the cooling system and increase the boiling point of the coolant.  That is accomplished through the use of an internal spring-loaded pressure relief valve.  Which one to choose depends on the type of system your vehicle is equipped with.
antifreeze chart
This is a typical pressure release-type radiator cap with a spring-loaded internal cap designed to lift when reaching a predetermined pressure within the cooling system.  Early cooling systems would use a cap that released at 4-lbs. while most modern hot rod cooling systems are set for 15 to 20-lbs

Top Mounted Radiator Cap Location
In a cooling system, a higher pressure equates to a higher boiling point for the coolant.  Higher coolant pressures also transfer heat from the cylinder heads more efficiently.  It is recommended that a radiator cap be used with the highest pressure rating that the radiator is designed to accept.  The coolant will typically only build to 16-18 PSI, due to expansion up to 200°F.  However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI.  Once the radiator cap has opened and vented coolant, the engine will not cool down until it has been turned off and more coolant added.  The radiator cap is basically a "safety valve", so always use the highest-pressure radiator cap that the radiator will tolerate.  If you are unsure of the pressure rating for your radiator, check with the manufacturer for the maximum recommended operating pressure.

Cross flow Radiator Cap Location

Crossflow radiators with the tanks located on the sides of the core are used in closed systems.  The radiator cap should always be located at the highest point of the cooling system, and on the low-pressure side (after the radiator core).  Cross flow radiators mounted higher than the engine are ideal because the cap is on the tank that is connected to the water pump inlet.  This configuration offers three advantages:  1—The cap is at the highest point of the system, allowing any air to migrate to the area just below the cap.  In the event the cap vents due to excessive pressure, the air will escape first.  2— This area has the lowest velocity within the system, allowing air to separate from coolant even at high engine RPM.  3— The cap is located on the low-pressure (suction) side of the system, so it is unaffected by the pressure generated by the water pump.

Water Flow Thermostats

The thermostat is a temperature-controlled device installed into the cooling system that opens and closes to regulate the flow of coolant into the radiator.  It opens the valve in the thermostat allowing coolant flow at a predetermined temperature and closes halting flow when the coolant temperature drops.  Thermostats are available in a range from about 160° to over 200°.  Older vehicles with open-style cooling systems and classic engines usually use a lower temperature thermostat.  These engines operate at lower temperatures and coolant loss can occur in an open system if the system overheats.  Newer vehicles equipped with closed systems are operated at higher system temperatures to aid in reducing emissions and do not lose coolant because it is drawn back into the system when it cools.  Any aftermarket thermostat housing that mounts the radiator cap directly above the thermostat location, or that mount the radiator cap in the top coolant hose, are not recommended.  Both of those housing styles are poorly designed, and will push coolant out of the cap at high RPM.

Coolant Flow Through The System

Older vehicles used low-pressure radiator caps with upright-style radiators.  At high RPM, the water pump pressure would overcome the radiator cap's rating and force coolant out, resulting in an overheated engine.  Many mistakenly believed that these situations were caused because the coolant was flowing through the radiator so quickly, that it did not have time to cool.  Some added restrictors or slowed water pump speed that prevented the coolant from being forced out, and allowed the engine to run cooler.  However, vehicles built in the past thirty years have used cross flow radiators that position the radiator cap on the low-pressure (suction) side of the system.  This type of system does not subject the radiator cap to pressure from the water pump, so it benefits from maximizing coolant flow, not restricting it.

A common misconception is that if coolant flows too quickly through the system, that it will not have time to cool properly.  However the cooling system is a closed loop, so if you are keeping the coolant in the radiator longer to allow it to cool, you are also allowing it to stay in the engine longer, which increases coolant temperatures.  Coolant in the engine will actually boil away from critical heat areas within the cooling system if not forced through the cooling system at a sufficiently high velocity.


Standard full-size hoses should be used to ensure maximum coolant flow.  Using smaller "AN style" hoses can decrease flow that could inhibit proper cooling.

Pulley Ratio

For street applications, the water pump speed must at least match the crankshaft RPM, to a maximum of 10% faster than the crankshaft speed.

The Choice for our Application

Our application, a ’32 Ford highboy roadster, equipped with an upright-style open-type cooling system has to cool a big Buick Nailhead V8 with a small grille opening.  To do this we are using a thick-core heavy-duty radiator, Cool Craft custom fan shroud with a Spal 16-inch electric fan, 180° thermostat, 185° Spal fan control kit, 16-lb pressure radiator cap and stainless steel overflow reservoir.

We settled on this combination after considering all of the previously mentioned factors.  The key player in keeping our system operating within the proper range is the fan control.  Ours engages the fan at 185°F and shuts off at 165°F.  This allows the cooling system to operate within the proper range without overheating at slow speeds or when idling and while avoiding any significant loss of coolant.

Spals’ fan relay harness kit includes: a fan wiring harness with relay, fuse holder and fan thermostat (sending unit) 185° or 195°.  Complete illustrated installation instructions make the installation simple.

Wiring diagram includes the following instructions:
Mount the relay in a secure place in the engine compartment away from heat sources or on the panel with the main fuse block…whichever is most convenient.

Once this is completed, connect the wires per the diagram and notes below.

Red: Connect to the red wire of fan pigtail with pre-terminated yellow crimp.

Gray:  Connect to thermostat socket (sending unit) with blue ring crimp connector.
Yellow:  Connect to positive battery terminal using the fuse holder and yellow crimp connectors per diagram.
Orange:  Connect to ignition switch +12 vdc when engine is in run position.  (Hook to constant +12 vdc for the fan to run continuously when the engine is hot even when the ignition switch is off).
Black:  Connect ring terminal to chassis ground.
Fuse Holder:  Connect fuse holder inline per diagram within 12-inches of the battery using ring terminal or equivalent.
*Note: On medium profile fans use a 20-amp fuse; on low profile fans use a 15-amp fuse.
The thermostat (sending unit) supplied with the kit is an OE type that is designed to mount in the cylinder head of the engine.  However any mounting in the water jacket is suitable.  In our Buick Nailhead we mounted it in the water manifold on the engine block side of the water flow thermostat.  The sensor is 3/8-inch pipe thread so an adapter may be needed in some applications (1/2-inch adapter included in the kit).  Do not use Teflon tape on the sensor or you will get poor electrical contact and inaccurate temperature readings.
Grey wire is connected to the thermostat (sending unit) terminal with the blue ring crimp connector provided. Depending on the model, the thermostat (sending unit) triggers the fan on at 185°F. and off at 165°F. or on at 195°F. and off at 175°F.  A toggle switch can be wired into the system to manually override the cycling of the fan. 
Pre-wired connector plugs into fan connector and the red wire is routed to the relay via an inline crimp connector.  Black wire from plug is connected to chassis ground.
Our open-type cooling system needed an overflow tank to catch coolant when the system heated up causing expansion.  We chose this Speedway polished stainless steel tank that would fit alongside the radiator within the hood confines of our narrow engine bay.

SPEEDWAY Stainless steel expansion reservoir tank with screw-off cap

-Heavy Wall
-Billet Screw in Cap
-Clear Neoprene Seal

Application:  This tank is designed to catch overflow from the radiator.  It cannot be pressurized.

Info:  2" outside diameter, 1⁄4" nipples-works with any radiator.  Stops loss of coolant and prevents pulling air into system when cooling off.  Polished


-Body height: 14"
-Neck and cap height: 1"
-Tube lengths: Short 1-3/8: Long 2"
-Overall length: 17"
-Mounting holes are 9.375" center to center

Installation Note:·        Short tube is for the radiator overflow hose

Long tube is for the overflow / dump hose (vent for the tank)

Tabs for mounting the tank were welded onto the left side-strap of the radiator. A MIG welded was used to minimize heat buildup during the process.  Short screws could have been used without penetrating the radiator core tubes because the outside rows are usually capped off in the tanks.  Check with the radiator manufacturer to confirm this before assuming this is the case in your application. 
Top bracket was installed at least 2-inches below the top of the core.  The bottom bracket placed to lineup with the bottom mounting-tab on the tank.
This diagram shows the routing for the overflow hose from the radiator filler neck into the overflow tank and the excess overflow tube inside the tank.

Filler neck on the radiator has this overflow port that allows the excess coolant to escape when the coolant expands from engine heating.  This happens when the radiator caps inner cap lifts due to the pressure within the system.

Overflow tube within the overflow tank allows the excess coolant to be dumped onto the ground should too much coolant accumulate
Overflow tank fits within the hood line of the engine bay for those who wish to run side-hoods.


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