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How To Wire Cooling Fans, Headlights, Fuel Pumps & Voltmeters - Custom Wiring You Can Do

By , Photography by Mark Hamilton,

Are your old musclecar's headlights too dim? Want to add a new high-current accessory without overtaxing the existing electrical system? Perhaps you need to activate an electric fan or nitrous system by means of an automatic switch. You could accomplish these tasks by rewiring the entire circuit using heavy-gauge wire, but that's time-consuming, adds needless weight, and is electrically inefficient. A far better solution is to use a relay.

A relay is a type of heavy-duty remote-control switch able to handle high-current accessories, yet capable of being actuated by substantially less current. Relays are installed between the power source and the electrical accessory requiring on/off power. When the relay is energized (turned on), the high current required to operate the accessory flows from the power source, through the relay, and directly to the part.

Supplying high-current devices requires heavy-gauge wire; the longer the circuit the larger the required conductor diameter. In 12-volt automotive electrical circuits, even small amounts of resistance can cause significant amounts of voltage drop. Circuits with long wire length and multiple switches and connections inevitably have a certain amount of voltage drop caused by normal resistance buildup; deterioration from vibration, high temperatures, and corrosion only compound the problem. Relays provide a solution by shortening the required length of the heavy-gauge power delivery wire from the battery or alternator to the load.

Relays are typically energized through an operator-controlled dash-mounted switch, or by remotely mounted pressure-, vacuum-, or linkage-actuated switches. With only a small amount of current needed for the relay-actuation side of the circuit, you can use a low-amp on/off switch and small-gauge wire (even on long runs) to connect the on/off switch to the power source and relay. The diagrams on these pages show just some of many typical relay uses, as well as how to hook up the universal ISO relay sold by M.A.D. Enterprises, Jacobs Electronics, and other electrical suppliers.

Using a relay as a heavy-duty remote-control device ensures delivery of full power to high-load electrical components: Your headlights will burn brighter, your electric fan will run faster, and fuel pumps will operate at their peak efficiency. Voltage drop at the dash harness is reduced, and overall switch and electrical system performance and longevity are enhanced.

How It Works
This diagram illustrates the function and operation of the versatile ISO-style "1 Form C single pole/double throw" (SPDT) relay. It can be used to control both normally open (NO) and normally closed (NC) circuits, or it can act as a change-over device to switch current from one user to another. The low-current switch controlling the circuit sends current through the pull-in winding terminals (85 and 86). This energizes an electromagnet that pulls a movable arm, closing the NO high-load circuit (30 to 87). When the controlling switch stops current from passing through the magnetic winding, spring action pulls the movable arm in the opposite direction, completing the NC high-load circuit (30 to 87a).

The ISO relay's high-load circuits are rated at 40 amps NO/20 amps NC for resistive loads, and 20 amps NO/15 amps NC for inductive or lamp circuits. Other ISO relay variations may have only NO or NC circuits, but because of the terminals' universal numbering scheme, you can always figure out their function and how to wire them.

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My car died the other day... I was messing with the fuse box inside the engine compartment and got it to working but if I turn on the headlights. The car would stalled. A buddy told me, maybe the headlight shared wire with the engine power and maybe the headlights electrical wire got short. Sending the headlights power source to the engine and cause it to stalled. Is it correct? I think I need to do something similar to this. Separate the two wire into two parts. I don't know if it has a bad relay but don't know which relay it can be...

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