I have a 1989 F150. It blows the turn signal fuse regularly. Also on the same circuit is the reverse lights that haven't worked for a while. I actually unplugged the pigtail from the reverse light switch to see if that portion of the wiring was bad. Still blows fuses.

So should I get rid of all the loom to determine a problem. How do I troubleshoot this? I'm not a moron, but have never liked electrical so I don't know how to determine if there is a problem with a meter.

Is there a place (Like painless performance) where I can just purchase a rear harness?

 

No one makes harnesses for these old trucks that are plug and play. Not sure if there is a NOS dealer around with one on the shelf, but not likely.

If it blows the fuse right away, you have a chance of finding the problem. If it's intermittent, it's going to be more difficult. The first thing to do is look at the wiring at the rear bumper area. A lot of people cut the harness to add trailer wiring, and their wiring methods are not that great. If you have a mess back there, the first thing to do is clean that up.

Also look up front at the transmission area. You were already there and that was a good troubleshooting test that you did, but I would still check the wiring carefully up there, especially if the exhaust has been modified, It's very easy for some wires to get melted against the exhaust system up there, and like you said the back-up lamps are on the same circuit. I think you eliminated the circuit after the back-up switch, but you still have a hot wire going from the fuse down to the back-up lamp switch that could have a problem.

There is also the slight chance it could be one of the front turns or wiring also.

 

Definitely intermittent. It'll sometimes go 2 weeks. Then 3 fuses in one day.
Everything on this truck has been modified at some point. I hope it's not the front wiring.

I guess I'm pulling wires to check. Can I use a continuity tester on this? However I assume a ground wire will always show continuity to the frame right? So if I unplug and start with the hot wires and they show continuity to the frame that's a problem? Just not exactly sure how it all works.

 

You will get some readings to ground if the bulbs are in place. Your meter will read through the wire, through the filament in the bulb, and then to ground in the bulb socket, and then back up through the metal in the truck to where you have your negative meter lead attached.

Since it's intermittent, it's going to be very difficult to find. Trying to use the meter is the same as troubleshooting with the fuses. When you are checking with your meter, it may be fine that day and not be blowing fuses. The best way, but the most time consuming, will be to visually run over all the wiring that pertains to this circuit.

One thing I just thought of, I am wondering if you can go to the store and get a universal light bulb and temporarily wire it across the fuse that blows. Run it up and tape it to the dash were you can easily see it (up high). Wire it in putting one wire on one side of the fuse, and the other wire on the other side of the fuse. This will work best with a plastic light socket with two wires, not the metal kind with one wire.

For a test, put a blown fuse in place. Put the turnsignal on, see if the temporary light blinks, I am thinking it should. Then step on the brake, the light should light again. Then put the truck in reverse, the light should glow again.

Put a good fuse in place, the light bulb should not light up. What I am thinking you could do is drive it, and as you do different things, you can watch when the fuse blows(the light lights up). If it suddenly blows when you put the truck in reverse, then we know it is something to do with the reverse circuit. If it suddenly blows when you touch the brake pedal, we then know it's something in the wiring to the rear only, since the brake lights and the rear turns use the same wires and same bulbs. If it blows only when the turnsignal is activated, then I think I would put another fuse in it and try it again, making sure to touch the brake pedal often. If it never blows with the brake pedal, but always blows with the turnsignals, then I would lean toward it being up front somewhere.

 

Dave,

Thank you very much. I'm digging into this this coming weekend. Part of my problem is this the primary vehicle for my 16 year old. So I don't necessarily get all the data. Even though he is a pretty good mechanic. He is buying his own car soon.

I'll get the bulb etc. I'm pretty sure it's only turn signals, but I could be wrong. Brake lights are not on the same circuit. I believe its signals and reverse light. We've been driving it without the reverse lights plugged in, and it's still blowing fuses. So I'll visually inspect all the wiring.

 

The brake circuit having it's own fuse hadn't dawned on me. Did you know the rear turnsignals use the same wires and the same bulb filaments as the brake lights? If the brake light fuse has never blown, that might rule out it being a problem with the rear bulbs/sockets/wiring.

 

Here is a diagram for a 1986, but a 1989 is wired much the same. I bet even some of the wire colors are the same. Fuse positions are probably different, but as you said, the stop circuit and the turn flasher circuit are on separate fuses. You can also see how complex the turnsignal switch is, and how it uses the same bulbs and wiring for the rear stop and the rear turn functions.

http://www.garysgaragemahal.com/turn...ard-lamps.html

 

Interesting. I have some more exploring to do. Thank you very much. One thing I forgot until now is that the reverse lights even when plugged in do not function. They share the fuse, however the switch at the trans is currently not plugged an still blows fuses.

So given that anything else? I really appreciate the assistance. This poor truck has been abused and neglected. Bringing it back and making it reliable has been a chore.

Also while thinking about this. And wires that need to be checked. How do I check the turn signal switch on the column? Do I have the dreaded clock spring thing? No airbags.

 

The clock spring thing would only be for the horn and the cruise if you have it. If the brake fuse has never blown, I would concentrate on the turn flasher and it's wiring, and the reverse light wiring from the fuse to the switch on the side of the trans. The only wire I see in the column that could cause a problem would be the wire from the flasher leading up to the turnsignal switch. It would be nice to somehow separate the reverse wiring from that fuse, and then see if the fuse still blows. You could drive it that way for awhile for the test, since you said the reverse lights do not work anyway.

 

Thank you. Diving in deep this weekend.

 

Ok here I am. I found the reverse light switch is not functioning. I did run a wire direct from the battery to the wire at the switch and the lights work.

I then started looking for continuity. When I went to the wire that goes to the fuse box it grounded.

so what I mean is I had an alligator clip on the engine ground to the cowl. This way it was all the same. Well when I plugged the red into the harness at the switch on the trans I had continuity. Although when I turned the key to one I got 12 volts. It did this up to the fuse block disconnecting as I went. So basically at this plug I had continuity. Also back from the fuse block as well. You can see the red probe on the "top" side. Seems to me I have a dead short however I could be wrong. I'm just not sure. Did I do this right?

 

You can't put your meter in ohms mode and check for continuity when the circuit could possibly be powered. You always need to make sure the circuit is dead. And even if the battery cable is disconnected, you can't take a continuity reading from the hot side of the fuse back with the key on. This will give you a reading back through many circuits in the truck, giving you false readings.

Is it possible to find that splice in the purple/orange wire and temporarily separate the back up light circuit from the turn circuit?

 

It's not in Ohms mode when powered. Mine meter has a buzzer for continuity. I have the fuse pulled. The black on the meter is connected to ground. When I put the red probe into the harness it beeps. Going straight to ground. Again this is with the fuse pulled and no power on the truck. So at tha connector that I posted a picture of sitting on the fender is the closest connector to the fuse block. The meter beeps. Also on the other end (fuse side) at the spot, where the posted picture is, it goes to ground.

again this is hard to convey I appreciate your help!

 

I would put the meter in ohms. That continuity thing on your meter is not very accurate when reading very low value resistances.

 

I'll do that. However I shouldn't be reading anything right? That with fuse pulled and black probe to ground.