More obscure vacuum leaks can easily be found with the aid of a propane torch. Warning: Do not ignite the propane torch when you are using it to track down vacuum leaks! You are not using the heat of the propane torch in this procedure; you are using a small amount of the flammable gas to detect vacuum leaks. Start the engine, and open the valve on the propane bottle just enough to begin the flow of gas. While the engine is running, pass the unlit torch over all vacuum lines, past the intake manifold gaskets, and even near the vacuum brake booster if your rig is so equipped. Should you pass the nozzle of the torch near a vacuum leak, the idle speed of the engine will increase because the propane will be sucked in through the vacuum leak and burned as fuel inside the engine. Listen carefully for a rise in engine rpm as you work over the engine compartment. Use caution near the engine's air intake because even though propane is heavier than air, the engine rpm will increase if propane is drawn through the carb or throttle body.
Adding Electrical Relays
The standard 12-volt automotive relay (left) is made to handle up to 30 amps continuously and should be used whenever you add off-road lights, compressors, or another high-demand accessory. A relay works like a remote switch that you mount on the 12-volt positive wire between the battery and the accessory you want to power. By using a relay you can keep the amount of wire between these two points as short as possible. A relay effectively decreases the voltage drop at the accessory so that it actually receives a full 12 volts. You still have to run a12-volt wire to a dash-mounted switch to control the relay, but the switch wire carries a very small amperage that will not affect the amount of power available to run the added accessory.
The Nissan relay (right) functions the same as a standard relay but is designed to handle far more amperage. Though the Nissan relay is designed to use a special connector, it will accept female spade connectors and is commonly available at most parts stores to use in custom wiring jobs that will see more than 30 amps.
Wiring a Relay
Most relays will have four or five male spade terminals to distribute power, and fortunately for us relays usually have a wiring diagram imprinted on the case to show you where to make your connections. Using this standard automotive relay as an example, we see that relays have two sides: One side is controlled by you through a low-amperage circuit (a dash-mounted switch), and the other side is a slave-side that makes the high-amperage connection when you energize the low-amperage side. In simplest terms the low-amperage side is an electromagnet that creates a field when you run 12 volts through it. This electromagnetic field pulls the high-current switch into position, completing the circuit from the vehicle's battery to the accessory (lights, compressor, whatever). This is why relays "click" when you energize them. To wire this type of relay, connect the terminals as follows.
30: To battery positive with 30-amp fuse; use 10-gauge wire
85: To vehicle ground; use 16-gauge wire
86: To dash-mounted switch that supplies 12-volt positive source; use 16-gauge wire
87: To accessory positive you want to control; use 10-gauge wire
Automotive Soldering
Other than marine use, nothing is as tough on electrical wiring as off-road trucks. Knowing how some of you drive, we bet your 4x4 may be tougher on its 12-volt system than any boat could be!
The best electrical connections are those that never break and don't introduce a lot of resistance (wasted electricity) to the system. For our money that means soldering all connections on essential components instead of using the cheap and easy crimp connections. If you haven't yet figured out the simple aerosol-can-lid trick to hold your wires, take a look at the photo and kick yourself later. Use 50-percent tin and 50-percent lead rosin core solder for automotive wiring. Don't use acid core solder.