o/t (kinda) will this work?

Ok, so I got a truck that has what I believe to be a short somewhere in the rear lighting system.

Shortly after I bought the truck, it had this very problem, so i rewired the bed (has a flatbed w/ lights in headache rack) and corrected what I believe to have been the cause of why the wires rubbed and shorted.

Fast forward a few years, and the problem seems to have resurfaced, so naturally, I have visibly checked for bare wires and bad connections.. Can't seem to find any.

Why do I believe I have a brake light short? Well, because the "plug" on my brake switch has now melted, and it has popped a fuse. If I stay on the brake long, I can smell the wire getting way too warm. I have a good idea as to which is my problem, however I can not visibly see a problem.. (but it seemed to start shortly after a repair was made, so I'm thinking something in that repair is the problem)

I believe a "amp clamp" would probably help my diagnosing, but I don't have one, nor the money to invest in one this second.. What I do have is time, and a lot of "assorted items" in my shop..

My thought was to use a ammeter to try to "isolate" my problem area.. If I were to add a couple "leads" and probe connections, it should show what part of my circuit is drawing the larger amps, right?? (yes, I have a way of "isolating" all the lights from one another at a junction box like would be used on a semi-trailer)

So, would my thought of using a everyday old ammeter work to show me what light was doing the heavy draw? (I don't see why it wouldn't, so thats why I am asking for opinions.. Hate to spend time trying it to find out it will not help)


Thanks for your opinions in advance,

Brad

Just might have a hot wire feeding to ground and too heavy fuse or one with tin foil ???

Yes, that should work.
Also- years ago when I had to find why a fairly new cab tractor was blowing fuses- I hooked a circuit breaker in place of the fuse. Then as I worked along the wires I could hear the "snap" of the breaker, just wait a minute for it to reset, then resume.
I had gone through a package of fuses and was getting nowhere, getting frustrated, and thinking of wiring that circuit direct, but afraid that I couldn't unhook it before things melted, when the circuit breaker idea came to me.

I have done just as you discribed with an intermittent short, watch ammeter and manipulate wires until meter jumps. So you should be fine, I also like the circuit breaker idea.

Try this: Connect a turn signal flasher, or circuit breaker in the place of the fuse. Turn on the switch. Take an ordinary compass, and follow the wiring harness with it, from either end. As the current pulses, because of the flasher, the compass needle will jump to the left or the right. As you follow the harness, passing over the short will cause the needle to swing the opposite direction. Follow back to the spot where it changed direction. Investigate there.

Where are you located? I had a mystery short in my motorhome a few years back and after 3 shops and $1600 later, it was still there. I bought a Snap-On fault detector and found it myself. If you are somewhat close, I could help trace it with you.

(southern Indiana)

I like to substitute a test light for the fuse. Light will stay lit, without overheating the circuit, until you find the short. Could it be something wrong with your brake controller?

I like the sound of CCer's idea with the flasher and magnet. Can't wait for something to go wrong so I can try that.

Good luck
Paul

Thanks guys.

I have in fact put a circuit breaker in as of the moment.. The problem still is if I sit at a stop sign/red light too long, my brake lights will eventually cut out, so I want to correct the problem.

1655, thanks, but I am in southeast lower MI, near Toledo Ohio, so it's a bit of a haul.

The compass trick sounds like a pretty slick one, and I may in fact try that after I isolate the "leg" in which has the problem.

The truck currently don't have a brake controller. I have one to install, just have not got to it yet.

When I bought the truck, within a couple months of driving, I found it had this problem (on a long trip with the 4-ways on, it started to eventually smoke).. with the rear lights disconnected, it did not trouble. The bed was originally wired with "flat 4 wire" like you would see on utility trailers and the like, and had rubbed through where the wires "turned". I rewired with double insulated (extension cord type) wire. I have a feeling it is rubbed bare again up inside the headache rack where the wire turns (rough roads tend to cause excess vibration.. and it was 5-6 years ago now), but I can't see inside where it makes the "turn" and I hate to cut and yank the wire to find that's not my problem..

Thanks again guys.

Brad

The cheaper multi-meters can read amperage, alligator clip it in.

Break the connector under the dash, measure amps. Check the positive feeds for resistance, you may be chaffed into the frame rail some where. Fab up a quick harness from under the dash to the taillamps bypassing the original wiring, then directly to the taillamp harness. If problem goes away, it is in that section of harness.

Or with this jumper harness hook your bulbs right to it, measure amps, then go back to your original wiring and masure amps. Huge difference should equal a ground condition somewhere in that section.

I am not a wiring pro or an electrican, but so far most of my electrical issues I have figured out, with my multi-meter. So far, knock on wood...

Let us know what you find.
Rick

If you have a junction box in the system, just unhook them all, then run a test lead with a light fuse or circuit breaker in it, and put power to each leg out of the junction box til you find the one that is drawing too much amperage. You can use a multimeter, but the one I have is only rated for 10 amps for X seconds every so many minutes, as I imagine all but the very top-end units would be.

If you have room in the J/box I would also run a hot lead back to it, stick the lights on a relay, and then the relay would be the only thing that your brake switch would power.

Also, what kind of plug do your lights have? Check the 3-way plug on the back of your lights if it has the sealed lights like semi-trucks run. I have had a connector corrode and feed power between the tail lamps and the brake lights.

To avoid smoking an expensive ampere meter when trouble shooting, I use a 0.01 ohm shunt and a VOM that will read milli-volts. The shunt is made from a lenght of #14 ga solid copper wire (one wire from Romex will work) 47 1/2" long which is equal to 0.01 ohms. Attach spring clips to each end of the shunt to connect into the circuit you want to measure the ampere draw.

To measure amperes connect the test leads from a VOM to each spring clip of the shunt and measure the voltage drop in milli-amperes. From ohms law I=E/R, since the resistance is 0.01 ohms; 10 milli-volts = 1 amp, 100 milli-volts = 10 amp, 200 milli-volts = 20 amp....etc.

Now - if there is smoke it is not your meter.

Unhook the power and us a small battery charger and watch the meter.

If you can't exactly locate it, sometimes you can wire around it. Rewire and cut the other stuff out on both ends. Dave

Thanks again guys,

I took the time today to work on my lighting.. I made a meter as i previously described.. I put it on the "input side" of the brake switch and showed what I thought was rather low amperage (hooked it to one light before testing on the truck to get a feel what each bulb should show).. The gauge showed very close to 4x what one light showed, which threw me for a loop..

I noticed it took about 45 seconds for the terminal on the "live wire" to be too hot to touch.. I completely disconnected the brake switch and used my amp gauge setup as a "jumper".. showed a slightly less amperage, and after nearly 3 minutes of the brake lights being on I could still touch the terminal that would have previously been too hot.

I hooked my brake switch back up, and noticed I lost nearly 3/4 of a volt across the switch..

Being this is the 6th 80's era Ford I have owned, I dug into my stashed parts and found a new in the box brake switch I had bought somewhere along the line.. I installed it, and checked voltage from one pole to the other.. about .01 difference in volts.. Held the brake down for a few, and terminals seemed to stay cool, and not trip the breaker, so I'm calling it problem solved.

Hopefully, that's it.. I'll find out after a few days I'm sure.

Brad