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Electric Fans and Relays

Easy Fan Install

Douglas R. Glad Jul 19, 1999

Step By Step

It is important to select a fan that won't begin stalking you and require a restraining order. One that effectively cools the engine is also helpful. In some cases, the engine won't have a provision for a water pump-driven fan. LT1s and the LT4 in our '66 Chevelle, for example, have the new-style camshaft-driven water pump requiring electric fans. To select a fan, clearance is the first consideration. The Flex-a-lite Double-Puller used in this installation has two fans and a total size of 15x131/2 inches with a 21/4-inch depth. The Chevelle has 6 inches of clearance between the radiator and the water pump and 41/2 inches to the upper radiator hose; any fan up to 41/2 inches in depth will fit. The fans draw nearly 20 amps, pull 2,500 cubic feet of air per minute, and mount on the engine side of the radiator. The big considerations in fan selection are: 1. Will the fan fit? 2. Will it handle the cooling needs of your engine? 3. Is a thermostatically controlled model necessary, or will there just be an on/off switch? And 4. Do you need an air conditioning relay? The Chevelle is the most basic of cars, requiring a simple fan; we used a low-profile dual-prop puller with a manual switch under the dash and cut a small notch in the lower right side of the fan housing to clear the lower radiator hose.

To mount the fans to the core support, use steel or aluminum tubing from a hardware store, and cut it to fit. This tube was painted black for aesthetic reasons; this fan required three mounting bolts.

Disconnect the battery. Install a switch under the dash and run one 18-gauge wire through the firewall to the relay; the other wire should be connected to a key-on power source in the fuse block.

Two (or more) small fans at radiator--use only one relay for each fan.

The relays we used on the Chevelle are standard Bosch ISO models that can be used in a variety of applications. The numbers and relative positions of the terminals are an industry standard; the wire colors on the pigtails are not. The kit supplied by M.A.D. Industries will have different color wires leading to various terminals than a parts-store version, even though the relay will be the same. Because of this, it is important to read the numbers on the relays to wire the fans correctly.

Terminal #30 needs a high-amp 12-volt source. The Chevelle had a bus bar on the horn relay near the radiator connected directly to the battery with a large 10-gauge wire. The hot side of the relays were connected to the bus bar, which was an excellent alternative to hooking the relay directly to the battery. Do not use the ignition system as a 12V source.

The ideal place for the relay installation is on the core support near the battery (or other 12V source) and the electric fan. This provides the shortest distance between the two and will allow maximum current to reach the fans. We used two universal relay kits from M.A.D. Enterprises to connect our fans to power.

This installation necessitated two relays for the two fans. On the pigtail, splice the wires from terminal #86 on both relays together, and attach them to the single wire from the dash switch. Splice both #30 terminals together and use a 14-gauge wire to connect them to the battery or constant 12V source. For this application, splice and ground both #85 terminals. Terminal #87 for each relay will be attached separately to each fan with a 14-gauge wire.

Our test Chevelle will soon be fitted with a larger 2,800-cubic-feet-per-minute-pull fan for those hot August nights. The Black Magic electric fan measures 18x16 inches and is 41/4 inches deep, making it the largest electric fan Flex-a-lite provides. In an application such as this, two relays can be used for a single high-current load. Simply use the existing relays, splice the terminal #87 wires together, and hook them to the single fan. Where a single relay might get hot and burn out over a period of time, the second relay is cheap insurance. CHP

Burning down your dashboard does nothing for your reputation, not to mention your old Chevy, and connecting high-amperage cooling fans to an under-dash power source is asking for a meltdown. One solution is running rigid conduit and zero-gauge welding cable to a knife switch from Frankenstein's lab. Or, you could just run a small 14- or 18- gauge wire to a micro switch and use a relay to handle the load.

The idea behind fan relays, or any other relay for that matter, is to run the largest amount of current to the load as possible. To do this, the shortest possible amount of heavy-gauge wire must be used between the battery and the load. The role a relay plays is simple: It acts as a heavy-duty conductor between the fan and the battery. A switch inside the car operates the relay instead of the fan and carries a small amount of current (0.08 amp) to trip it. This accomplishes several things:

1. Long lengths of heavy-gauge wire cause a substantial voltage drop; a shorter wire between the battery and the fan reduces this drop and improves performance; 2. Control computers and other devices sensitive to large amounts of current can be employed to control the relay; and 3. Any light-gauge wire attached to an under-dash switch can be used to control the relay.

The most difficult part of installing electric fans properly is the wiring, and there are several different ways to wire relays depending on the trigger that's used. Switches used to control relays can supply either a ground or power to the relay to activate it. CHP will take you through a typical electric fan and relay installation and cover basic relay operation to make your job easier.


Fife, WA 98424
M.A.D. Enterprises
Springville, CA 93265
(559) 539-7128

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