Technical Need Short & Broken Wire Finder Tester - where's the quality stuff? - Advice?

If you have a 10 amp circuit breaker, you can install it in the suspect circuit. If you don't have the inductive ammeter, you can use a small radio (do they make them anymore?) tuned to a dead spot on the AM band. You will be able to hear the pop in the speaker when the breaker breaks as you move it closely along the loom. Note that this will only find shorts to ground that blow the fuse.

While I know how the short checker works, I've never used one to find a short. I wired my current project, but don't have the front lighting finished. It made the car moveable, which was my goal. It worked fine then all of a sudden, it started blowing the tail light fuse any time I pulled the light switch out. I have the loom for the front wiring tie wrapped to the radiator support. I checked for any bare wire ends in this loom and found none. That leaves the tail lights or the dash lights as causing the problem. I have a connector in the rear that isolates the tail, license, and stop lights. Disconnected it, and the fuse no longer blows plus the dash lights work. Found a poorly made bulb that allowed one of the power leads to touch the bulb body; instant short. That's the method I've used to find shorts. Figure out what all is in the circuit, figure out where the loom runs, and see if you can easily isolate sections of the loom.

 

You're making a series of ***umptions that may not be true, and at least one that isn't true....

You have three possible scenarios; one, the wire is shorted directly to ground. Two, the wire is shorted to another wire which gives a path to ground. And three, the wire is open. In only the first case will this method have any possibility of working. Let's examine each one...

One. In this case the harness has been damaged, allowing a bare wire to go to ground. While testing should detect this, this should be visible by physical examination, no testing needed.

Two. The wire is shorted to another wire. No help here, as the 'detector' will simply show current in both wires. Detecting a thermal 'hot spot' may be difficult as it will be very localized and if the subject wire is wrapped by other wires you probably won't find it.

Three. With an open, no current will flow and the detector won't read anything.

And you're ***uming that an overcurrent device will 'limit' current, which isn't at all true. They're designed to remove power when current exceeds a set value. But how quickly they do this depends on the type. A fuse reacts very quickly, which is why they're commonly used. Circuit breakers (of the thermal type typically used in automotive) can be much slower, I've seen a 15A breaker hold 20 amps for up to five minutes before opening. But as current goes up, they react faster. Still not as fast as a fuse though. But the thing to remember is they don't limit current, they simply disconnect it, hopefully before wire damage occurs. You already have a damaged wire, too late....

How much current will there be? Using your values from above, it's a simple calculation; I = E/R. Plug in 12 for the volts and .03 for Ohms and that gives 400 amps! Now, there are a number of factors that will likely reduce this number but not enough to prevent at least some damage, and even though this amount will be of a very short duration, you will still have an arc generated each time the breaker makes and breaks the circuit. I'll also note that asking a thermal breaker to continuously try to clear a short like this will result in the 'on' time growing ever shorter while the 'off' time increases, and the breaker will fail sooner or later. I would suspect that you would burn a #18 wire in half after not that many cycles.

Like I said, there are no magic bullets... I hate splitting harnesses open too, but sometimes there's no help for it...

 

Crazy Steve, thanks for the "400 amp" calculation, I forgot to include that for comparison.

Also, I used my 20a manual reset ATC circuit breaker and it took 1/2 to 1 second to trip whereas the fuse was "instant."

SOLUTION FOUND!

IT WAS the Coolant Temperature SWITCH! (2-term, SPST) It had finally failed permantly and the 12v connection was pure short to ground.

When I did all the "disconnect stuff until short found" diagnosis, I initially got an indication of alternator field which was unlikely and also incorrect. Then and issue with the harness seemed to match all the evidence.

What I failed to mention but did take into consideration was the fact that I DID NOT disconnect the Coolant Temperature Switch which I glossed over and thought it was a Sending Unit. (Being a Sending Unit like in a 60's car, this would "never" be a short.

I try to "Think 100 Times", then Drill Hole. At least:

• I didn't buy a tool I didn't need, and
• I didn't horse off and start tearing looms open (whew!)

QUESTION: Why so many critical systems on this one 20a fuse:

• Ignition
• Alternator
• Radiator fan
• Main Relay

I'm beginning to think the intent was to shut down the engine if there was any failure in one of these critical systems.

 

I just noticed that you are including broken wire in the description. These testers will not find a broken wire. They rely on current flowing in the circuit. That doesn't happen with a broken wire. Finding a broken wire is best done with a continuity check using either resistance of the dead circuit, or voltage if you are testing a live circuit. It is starting where you know the circuit is open and moving to a point where you find it to be good.

 

There is a cl*** of "proximity testers" that might work but I couldn't find a definitive answer. One thing, there are AC prox probes that glow where you bring them near an open AC outlet. As far as DC goes, maybe they can work as the probe is approaching. Again, I'm speculating.