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

By , Photography by Mark Hamilton,

Cooling Fan
M.A.D. likes to use two relays to power up one large electric cooling fan. Even though the ISO relays are rated at 20 amps for this application, experience has shown a large fan motor will overheat a single relay. Installing two relays in parallel removes stress loads from each relay and improves reliability with redundant parts. (One relay per fan is sufficient for systems with two small fans.). Fans' relays are typically actuated via an engine- or radiator-mounted thermostatically controlled switch, but you may wish to install an extra manually operated switch on the dash as a fail-safe device.

Mounting relays near the front of the car makes it easy to route high-amp current directly to the headlights. The factory headlight switch and beam-select switch need only energize the relay for it to send full battery power into the front lighting system harness. This removes the load from the dash harness and is recommended for old under-wired musclecars or any vehicle upgrading to high-power halogen lights.

Fuel Pump
This circuit delivers maximum current to the fuel pump and enhances safety by using a factory GM oil pressure switch. With the ignition in "run," engine oil pressure closes the switch to complete the circuit from terminal "I" to terminal "P," which delivers power to relay terminal 86 and to the ground via terminal 85. This energizes the relay, allowing battery positive power to flow directly through terminals 30 and 87 to the fuel pump. When the engine stops running, oil pressure drops, the switch opens, and the relay de-energizes. This provides automatic fuel pump shutdown after a crash or engine failure, and also prevents the fuel pump from running if the ignition is left on. Wire the pressure switch's "S" terminal to the ignition "start" circuit to provide fuel pump pressure during engine cranking, and/or prime empty carb float bowls.

Mount the relay as close to the battery as practical, and install a fusible link two wire-gauge sizes smaller than the high-amp power feed fire. Wire the pressure switch using generic 1/4-inch female-blade terminals or a three-prong, weather-resistant, molded-on, 56-series connector assembly: GM 12101928 (AC-Delco PT200) has a black PVC connector-shell; slightly more expensive GM 12085529 (AC-Delco PT139) has a high-temp TPE shell.

Many cars experience voltage drop in the main-feed wire that delivers power to the dash area. The more dash switches that are turned on, the greater the power drop. This can cause the voltmeter at the dash to display a significantly lower voltage than the actual battery voltage. You may think there's a problem with alternator output, when in fact the alternator is working fine. The voltmeter can be made to show exact battery voltage by wiring up a relay as shown here. Be sure to route the wire from relay terminal 30 directly to battery positive, and that no other circuits use this wire.

Mad Enterprises
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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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