The Long and the Short of It

The '02 Ford F150 comes in with no brake lights and no turn signals. First rule is usually, verify the complaint. There were no working brake lights. Even the third brake light at the rear of the cab was not working. Turn the key on and neither left or right turn signals were working. There also was no turn indicator on the instrument panel. No third brake lamp and no turn indicators made me think checking fuses would be a good idea. When I say *check*, I mean *test*.
There are a lot of different pieces in a fuse block.

Relays, diodes, cartridge fuses, blade fuses and a circuit breaker are all part of the underhood fuse block on this Ford. 

Now, I said the symptoms so far make it worth looking at fuses. I'm not going to worry about relays, diodes or circuit breakers at this stage of the game. I wanted just fuses. Cartridge fuses are designed so that you can look through a clear plastic window on top and usually tell if the fuse is okay or not okay. This one is okay. 

The look-and-see approach works most of the time. If things get questionable the top cover is removable for better surveillance. Just be careful not to break the plastic. The cartridge fuses looked okay in this underhood block so I wanted to test the blade fuses. You can also view inside a blade type fuse but you have to remove the fuse from the block in order to see if it is good or bad. 

If you start pulling blade fuses just to look... well, there are a lot of blade fuses and even if you pull one at a time it can easily be placed back into the wrong slot. I had a Jeep Grand Cherokee in just a couple days ago the problem turned out to be caused by an ABS fuse placed into the wrong space and the ABS fuse slot empty. There is also the problem of just looking for a fuse marked *Turn Signal* (in this case) and only looking at that one. Many fuses aren't that easily identified and if you do find a fuse marked for the system it could be a good fuse and the one causing the problem is a completely different fuse. You need to check all the fuses when you are testing and since the only part of the fuse readily accessible with the blade fuse in place is the shiny blade tip then you use a circuit tester and probe that tip. Do that and you won't misplace a fuse, you won't pull a lot of fuses and you will know if there are any blown fuses on other circuits causing a problem. 

First, grab a circuit tester suitable for probing the blade tips. I have a lot of circuit testers. 

 I even have a couple I made when I was a kid. Humor me. 

My favorite tool for this particular purpose though is my old Power Probe II. The leads connect directly to the battery, plenty of cable to go anywhere on the truck, a good ground lead at the probe end, switchable power or ground available to the probe tip and audible for when the circuit indicator isn't visible. I know, sounds like overkill but depending on where the diagnostics take us it could be a real time saver. 

Back to the problem at hand. I started with underhood fuses. I wanted to power to as many fuses as feasible prior to testing. Most fuses are either battery powered (hot all the time) or ignition powered (powered with key on). There are nuances, like battery power that times out. Or ignition power with key on accessory instead of *on* position. Or even ignition only in crank (start) position. In this case though I would have either battery power (brake lights) or key on ignition (turn signals) most likely bad. Turning the key on and testing blade fuses with the probe was all I needed to get started. Just touch the probe tip to each of the two blade tips of each fuse. If one side is hot (power), the other side should be as well. If one side is hot and not the other, bad fuse. The LED on the probe turns red for power, green ground. 

All the underhood blade fuses tested good. All the underhood cartridge fuses looked good. Time to move inside under the instrument panel. 

Found! Bad fuse. 

Technically in Fordspeak, that is the second fuse block (F2) and the #13 fuse. So fuse F2.13

Remember that for a test later. Just joking. If you were going to pull a wiring diagram you could pull up the circuit diagram for F2.13 and look for likely areas for shorts. 

Even though the brake lights and the turn signals worked after replacing the fuse I still was looking for why the fuse was blown in the first place. Fuses are there to protect the circuit. If the fuse blows there was some sort of overload because those fuses don't wear themselves out. This ain't my first rodeo though and it is a truck. The area where a short most likely to take out brake and turn lights would be is the rear. Especially if there is a trailer hitch and a trailer plug back there. 

I put a bar in place to hold the brake on. I slid under the rear of the truck with brake lights on and the plan became one of carefully examining the wire harness at the back of the truck while wiggling and pulling to try and induce a short if it did exist in that area. 

I found nothing with the brake lights on. I turned only the right turn signal on and tried again but only in the area under the right turn. Nothing. Okay, may as well try the left turn before I move on to something else. BINGO!

Fuse replaced. Short repaired. Longer arms and I could pat myself on the back. 

I can walk you through a wiring diagram. It wasn't needed here but it would still be interesting. 

Thanks!

Kenny@ggauto.repair

G&G Auto Repair