Electrical Odds and Ends FAQ

Most of these are reprints of posts I have made in various on-line forums over the years.  These questions come up time and time again, so I thought I would write them up here and save myself some serious typing.  As I come across some of the other posts I have written, I’ll add them to this page.

Why should I put a fuse in both the positive AND negative leads of my CB radio?

Why fuse the negative lead? VERY good question. I’ll try to explain.

Vehicles use the chassis/frame for some of their DC grounds. The negative lead of the battery is attached to the chassis and usually the engine block. The case of the starter motor is often times the ground connection so it gets its ground, back to the battery, via the block. This works fine.

What would happen if the chassis/ground connections between the negative lead of the battery and the starter became bad….or, more likely, started to get some resistance in it? The starter would have more trouble turning over the ending due to the voltage drop across that part that has a bad connection (aka., resistance).

Now…go one step further. You mount a radio in your vehicle. The power leads go directly to the battery (as they should) or to a nearby aux fuse block that was installed by you to handle your extra goodies.

The antenna is mounted on another part of your vehicle and its mount is grounded to the vehicle’s body/frame (in order to work properly). The coax connects the CB radio chassis and the antenna together….and the outer coax braid is at ground also (but it is covered with an insulative coating).

OK…back to earlier in the explanation. You have a heavy current carrying device…like a starter….or maybe a light bar that is grounded to the vehicle body for the negative connection for that batch of off-road lights it is holding up. If the negative battery cable that goes to the block/chassis starts to go bad, the DC current flowing from one of these items…let’s say the light bar), can find a ground through the body of the vehicle to the antenna mount and down the antenna coax and through the CB chassis/circuit board ground and through the CB negative power lead to the battery itself.

When that happens, you might very well have 30~50 amps of current flowing through the CB radio’s chassis, trying to get to the negative terminal of the battery. If it were the starter motor that found its “other ground path” through here, you would have a couple hundred amps flowing.

The end result is that your CB chassis, with those tiny copper circuit board runs on it, can become a fusable link and burn open due to the excessive current flowing through the ground circuitry of your radio chassis. So…you burn out the guts of your radio.

By putting a fuse in the negative lead of your CB radio (or ANY other electronic device that is attached to the battery), it will blow open and stop the high current flow before it does any permanent damage. (just like the positive lead fuse will blow open if you shorted the hot lead to ground).

If you don’t believe this, please feel free to stop by my house. If you volunteer your vehicle and radio, I shall be happy to do the “faulty ground wire” scenario on your vehicle. You can turn on the lights or crank over your engine and watch your CB radio smoke.

What size wire do I need for my project?

This should help you determine the proper wire size for your electrical project (CB radio, off-road lights, electric air compressor, etc.).

How do I calculate current and watts and stuff like that?

Electrical power is measured in watts.  Watts are calculated by multiplying the current used by the device times the voltage that powers the device. When we are talking about a typical Jeep electrical system, we normally are dealing with a 12 volt electrical system.

Example:  A small electric 12 volt air compressor uses 14 amps of current when operating.  So, the wattage of this compressor is 12 x 14 = 168 watts.

We can determine the current of a device if we know the wattage of it.  This is done by dividing the watts by the voltage.

Example:  An off-road light is rated at 130 watts.  So, the current drawn by the light would be 130 / 12 = 10.8 amps.

How do I get my factory fog light circuit to stay on when I turn on my high beams?

On the fuse panel (behind the glove box) are two relays.  Remove the one on the right (closest to the passenger side of the jeep). Bend the pin back labled #1 and your factory fog circuit will work when your high beams are turned on.  Many folks, like myself, don’t worry about fog (that would require moisture and we don’t have that in Arizona).  So, we replace the factory fogs with off-road lights and use them on the trail.

If you have a newer TJ and the above relays are not where I said they would be, then you need to go here to find the information. 

What is a test light and how do I use it?

This should help you better understand and use a test light.

Why do I need a power relay in the circuit? 

This true story should help explain why it is necessary.