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Troubleshooting Electrical Problems

Contributed By: Richard Hager

Since there have been several electrical problem questions lately, I'll offer a couple tips. I lean on this group quite heavily for almost every other area of FSJ life, but electrical is "my thing", so I'll give something back here.

1) Get a multimeter !!! For the cost of buying a couple parts which didn't really need to be replaced after all, you could've got yourself a great troubleshooting tool that you'll use over and over again for the next ten years. On the FSJ and around the house too.

Go to your local electronics store and pick up their cheap $10-$20 digital multimeter. Later, when you're a whiz, then you'll know better what you want in a meter and you can drop a hundred bucks on one then.

There are only two 'tricks' to learn for easy and successful electrical troubleshooting.

a) Being able to "see" the juice (get the right 'tool' for elec.)

b) Knowing how to "break down" the problem into a series of tests which will quickly isolate the broken part/wire.

The meter solves a), and the following tips should help you learn b), to the point where you will be able to fix practically -any- electrical problem you encounter in life.

2) All the world's electricity in 4 sentences:

a) VOLTAGE is the "pressure" behind the flow.
b) CURRENT is the "volume" of the flow.
c) RESISTANCE is like an "orifice" or restriction in the "pipe".
d) There MUST be a CLOSED LOOP for current to flow.

The higher the pressure, the more flow you can get through a restriction. Put this into math-terms, and you have "Ohm's Law" :

E = IR I = E/R R = E/I

Also, deriving power: P = E x I P = E^2 / R P = I^2 x R

Basic ohm's law: You connect a 12v battery across a 10 ohm resistor. What's the current flow? I = E/R I = 12/10 I = 1.2 amps

What if it was a 24v system? I = 24/10 I = 2.4 amps

Just like draining a tank of water through a small hose. Double the height of the tank (double the pressure) and you will force twice the flow through the same restriction (resistance).

3) Electricity ALWAYS has to have a CLOSED LOOP to flow in. In other words, the energy comes out of one battery terminal, flows through wiring and a load, and MUST have a way to get back to the other battery terminal. Otherwise, you have what's known as an OPEN CIRCUIT. No loop, no flow. But in your Jeep, virtually everything has a -single- wire going to it. What gives? Where's the loop?

The "GROUND" is the SECOND WIRE. The frame and body is a giant 2nd wire to close the loop for all devices in the vehicle.

Although we tend to think of cars as "parallel" systems, where all devices are connected in parallel to the battery (true), we always have to remember that each -individual- device and its wiring is a SERIES circuit. Remember those %@$%@ series-wired Xmas tree lights? All in series. One bulb burns out and the whole string goes dead, right? Broken loop.

ANY break in the current loop will prevent power from flowing. In other words, even if the 12 volts is connected to one side of the bulb, you better check the other side; because if it's not grounded (closing the loop) no current will flow! Even if the meter reads 12 volts on the hot side of the bulb. Note, if 12v IS on the hot side, and you have a bad ground on the other side, you will see 12v on that side too! You'll have -zero- volts -across the load-. Seeing 12v on -both- sides of a load (when it is supposed to be on) does NOT mean the wiring is ok.

Note: an example of the above that you might see is the dome-light system, where the switch is NOT in the 'hot' lead, but rather in the -ground- system. With the switch open (off), you'll see 12v on -both- bulb terminals. That's normal. When you close the switch, that should pull one terminal to ground (zero volts on the meter). If both sides stay at 12v even when the switch is 'on', then the switch is probably bad.

4) Troubleshooting: This is something which is very easy to do once you've learned the knack, but I've always had trouble putting it into simple -words-

You have to think in terms of an upside-down "tree". Start at the top, and the trunk branches. Each branch branches into two more, each of those twigs branches into two more. Every one of those branches is a "path" that you follow. The real trick is in -correctly- visualizing the problem in tree-form, i.e. correctly identifying where to -start-. That's half the problem, and if you pick the right start-point, you rarely have to follow more than a couple paths before isolating the problem. Following those paths is the other half of the job.

Another way to 'branch' is by 'type' of possible problem, i.e:

a) no power? (broken wire, blown fuse, etc.) or

b) bad device? (burned out bulb, bad switch, etc.)

Let's take Jason's brake light problem as an example. We visualize the loop that has to exist in order for current to flow through the brake-lamp filaments. Let's the most basic sense, plus 12 has to flow through a wire, to the brake-pedal switch...then through another wire to one side of the bulb filament...and then out the other terminal of the bulb and back to the battery. Through another wire? Doubtful. Much more likely that the other bulb terminal is grounded right to the frame, through some metalwork in the lamp fixture.

Well, we -could- take our handy-dandy new meter and start at the very beginning, the battery terminal, and follow the +12, checking at EVERY point in the circuit to make sure we still have 12 volts. I.e, checking to see that the circuit isn't broken. But sometimes you can save a lot of time by starting in the MIDDLE, and then working either forward or backward from there. This is part of the trick of visualizing the circuit and deducing the easiest way of eliminating the most possibilities the soonest.

In this example, a good 'middle' place to start would be at the brake-switch itself. From there, we can break the problem in half, and go whichever direction our -test results- take us. To do this, we first have to hook up and -verify- our 'eyes', our test equipment.

Take your meter's negative (black) lead and make sure it's connected to frame-ground. Buying a pack of "clip leads" with your meter is a very good idea.... By the way, in many FSJ's, a lot of the dash is plastic and that ain't a ground! Make SURE you've found a real ground.

I have learned through painful experience to ALWAYS verify that the meter is working BEFORE beginning any trace. You do this by hooking up the ground, and then putting the positive lead (red) on a KNOWN-GOOD source of 12 volts. The meter should of course read 12-13 volts.

Note: never hook the leads to anything until you've made sure the meter is in the correct mode and range. For this example, that would be "DC Volts" 'mode', and probably the "20" 'range'. And ALWAYS keep an eye out for the meter's ground lead getting knocked loose while you're thrashing around under the dash! . That can waste your time too.

A good source of known-good power is the fusebox. Turn the key to ON, and you should be able to touch virtually any of the fuse leads in the box and get 12 volts.

NOTE: our assumptions can bite us in the a**. While troubleshooting my heater fan problem, I wasted a half-hour digging in the dash wiring and trying to figure out why there was no power getting to the fan-switch. I finally went all the way back to the heater fuse in the fuse-box, and was shocked to find that there was no juice -there- either! What the hell?

Turns out that this J20 does NOT put power to the heater fan when the key is on ACCESSORY !! Unlike every other vehicle I own... Keep this kind of thing in mind. When none of your results are making any sense, stop and think about your -assumptions-. Maybe you forgot to hook the battery itself back up? Maybe the battery is -dead- ??

OK, so we've hooked the meter ground up, and verified that it reads 12 volts OK from a known-hot source. Heck, since we're down at the fusebox anyway, why not stick the probe on the brake-light fuse while we're there? Check BOTH sides of it. Sometimes fuses LOOK good, but have hairline cracks in them.

Now, let's troubleshoot! Go to the brakelight switch on the back of the pedal. Two terminals. It's in -series- with the hot-lead of the circuit, and it's a normally-open switch contact. You should read 12v on one terminal (the source or battery side) and zero on the other terminal (the load or brake bulb side). Depress the brake pedal while holding the meter lead on the load-side terminal (yeah, I know, it would help if we all had 3 hands). Got juice now? OK, the switch is fine. Congrats for buying a meter. You just saved 10 bucks by not replacing a switch that was actually OK.

What? It's -still- at zero even with the pedal depressed? Make sure the switch plunger is -really- going in far enough. Sometimes just the mechanical adjustment is off. Many times you can just leave the pedal alone and push the plunger itself in with your finger. If -that- works, but the pedal doesn't, then you need to adjust the switch. If that doesn't work, then you -do- have a bad switch.

OK, if the switch -does- work, then the loop is bad somewhere between the switch and the bulb or the bulb and the other side of the battery. See? You've already eliminated one-third of the possibilities in one easy step. Everything between the battery and switch is OK. The wire between the switch and the rear-end must have broken. Either that or the ground for the sockets has "opened". -Something- has opened the loop.

But in Jason's case, it probably isn't a ground. Most vehicles just ground each socket right where it mounts, and it's unlikely that -both- would corrode open on the same day!

However, -some- vehicles do in fact connect the grounds of both sides together with a wire, and then run a single ground wire from the junction to the frame. In that case, both lights -would- fail at the same time from a single broken ground wire or corroded wire-to-frame connection. This is where investing a few bucks in the manuals and wiring diagrams really pays off!

What about the case where we did -not- find 12 volts on one side of the switch? Both sides read 'zero'. hmmmm... Well, in this case you work -backwards- instead of forwards. Move from the switch to the fusebox, and verify 12 volts on BOTH sides of the proper fuse. OK? Then you've got a bad wire or connector between the fusebox and the switch.

What about no juice on the fuse itself ?! Hey, that's what -I- found on that heater fan problem. Puzzled the hell out of me for a few seconds too! I could feel an 'assumption' tickling the back of my mind, but I didn't get it until I said to myself "'s like the key wasn't even turned on or something..."

If your key-position is proper for that fuse, but you don't have juice on either side of that fuse, then something is really messed up in your fusebox. If everything else in the jeep is working, then it's highly unlikely to be the power feed to the fusebox, or the ignition switch, because those items are -also- feeding the devices which -do- work.

It's very important to be on the lookout for clues about "common" or "individual" things. Thinking those through can be a powerful way to isolate a problem, even -without- test equipment. Sometimes you can 'logic' your way through a problem without ever touching a wire.

In a simplified sense, there are usually three feeders coming into a fusebox. One from the ON position of the keyswitch; one from the ACC position, and one 'live' from the battery. Note however that this 'always live' circuit is sometimes fed through a "fusible link"; not just a simple wire. If that link opens from an overload, those 'always live' devices will be dead. Like maybe the horn, brake lights, headlights...

To reiterate an important point: Always build a list of clues in your mind, especially common vs. separate circuits. That can save a lot of troubleshooting time. -Visualize- your wiring system, and mentally 'overlay' your problem onto it. What clues do you have? Both lights failed at the same time? Then the problem -must- be something that's -common- to both lamps, right? Then you -don't- need to check anything specific to only -one- bulb. Time saved.

Or if your radio died, did -every- other 'accesory' fail too? Then it's almost certainly the ignition switch or the ACC feed going to the fusebox. Something -common- to all those devices. Back to the brake-light problem...did only ONE side fail? Then it's almost certainly NOT a 'common' item. It must be between that side's socket and the junction where the single wire coming back from the switch splits out to the two wires feeding each side. Is that a hard splice, or a connector? hmmm...connectors are prone to corrosion....or simply getting smashed or knocked apart.

Or it could be the ground for that socket. Heck, maybe that particular bulb just burned out! You -did- check the bulb right? In any case, it's something -specific- to that side.

(note: now that you bought an electricity tool, you can easily check bulbs too, by using the 'resistance' mode of the meter, i.e. ohms. Same as checking for 'continuity' )

The above method of splitting the problem into branches, choosing the right place to start, and tracing down those branches one at a time, can be used to repair anything from a brake light to a stereo receiver to the electric water heater in your house. With less than $30 of 'tools', you can fix practically anything electrical. A meter, a set of clip leads, and a brain, goes a long long way.

5) Documentation! Buy the wiring diagrams! If you're going to own the vehicle more than another week or so, it'll be the best 20 bucks you ever spent (besides your new meter of course ).

6) Get a can of DeOxit D5. It's wonderful stuff. It'll cure almost any kind of 'noisy' switch or volume control, or connector which is intermittent due to corrosion. And it helps a lot in preventing them from corroding again.

7) Get an ARRL Handbook, of any year. An old $5 one will work fine. There is an excellent basic electricity and ohm's law teaching section in it, as well as all kinds of other useful info, like wire size/capacity tables, antenna info, etc. etc..

8) Remember the LOOP ! Every device is in a -loop-, coming from, and going back to, the battery. If your power-seat isn't working, the loop is OPEN. Broken wire? Blown fuse? Switch contact isn't really closing? Motor winding burnt open? SOMETHING is open. The loop is broken.

I hope this will save someone a little grief and frustration. Electricity is really very very simple. It's only confusing if you've never done it before. Like, to me, the thought of changing a t-case is terrifying! Once you've learned to use your meter and worked through a couple of troubleshooting cases, you'll feel all-powerful! It adds a whole new bag of tricks to your personal 'toolbox' of capabilities.


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