Pinpointing a wire break location? AC power technique?

[JHZR2]

This thread is related to my work on my new 93 300SD. Trying to fix the alternator D+ circuit, which I have a thread on. This is a technique question, so I think it’s worthy of its own thread.

I know there’s no conductivity on a length of wire that’s a few feet long. I can access both ends. I know there is at least one discontinuity based upon multimeter testing.

I’ve tried a tone generator, but since it’s such a small closed area, they really just don’t work. If the sensor tip is in the same horizontal plane as the signal generator, the unit will make noise and result in a situation where we really have no idea what’s what.

My thought is that I think one can use an AC non-contact tracer to find presence of an AC voltage through layers of wire insulation. Thus my next approach would be to take AC power, through a variac, and put some small amount (30-50VAC?) on the one end of the wire, to see if the non contact tester finds it.

I’ve seen some videos where someone has a hot lead just sitting someplace, and the non contact tester picks up the voltage right to the end, then rapidly goes off. That’s what I’d like.

So I’m thinking:

AC outlet - switch - variac - ac wire hot only - alligator clip - D+ cable - unplugged connector.

When I run the non contact tester on the D+ cable it should see the AC voltage up to the point where the break is. Then I can open the harness there.

The risk is if the cable insulation and the harness insulation is enough to make it impossible to sense.

Skin dielectric when dry is around 50v. So keeping the AC voltage less than 50 should make it relatively safe. The NCT avoids handling any wire. Granted I’m doing this on my back.

Any thoughts on this approach? Any better ideas on how to pinpoint the spot that it is discontinuous in the harness that is currently closed and sealed up?

Thanks!

 

[BigCahuna]

If you know where your getting voltage, why not run a new wire and disconnect the suspected bad section?. The bad section probably is at a spot where there's a severe bend ,or the harness moves when the car is running. Either way you do it, you'll have to splice something together.,,

 

[Jetronic]

Yes, splice in a new wire, it's the least disrupting to the rest of the harness.

if the broken wire is contacting another damaged wire, the AC could be disastrous.

 

[pcoxe]

If you are able to put tension on the wire, the insulation will stretch where the broken conductor is. I found a broken wire in a door jamb connector like that. Don't overlook a badly crimped connector pin either. Many bad connections are at the terminals.

 

[JHZR2]

If you know where your getting voltage, why not run a new wire and disconnect the suspected bad section?. The bad section probably is at a spot where there's a severe bend ,or the harness moves when the car is running. Either way you do it, you'll have to splice something together.,,

I don’t. That’s the issue.

The D+ and the alternator output B+ are in a harness with an outer cover for many feet. I don’t have any way to know where the break is so that I can splice in new wire.

Yes, splice in a new wire, it's the least disrupting to the rest of the harness.

if the broken wire is contacting another damaged wire, the AC could be disastrous.

I’ve verified that it’s only the D+ wire. It’s not shorted to anything else, and I can even take the ac down to 12v which my non contact tester can sense.

Totally agree that I need to put a new wire, the question becomes where. The wire ends in a circular harness connector, under a heat shield, under the main exhaust downpipe. I’m hoping it isn’t coming from that end. The problem is I don’t know where the break or breaks are.

Taking this harness apart under the car would be impossible. I’d have to cut the wire inside the car and push a new wire through the firewall I think.

If you are able to put tension on the wire, the insulation will stretch where the broken conductor is. I found a broken wire in a door jamb connector like that. Don't overlook a badly crimped connector pin either. Many bad connections are at the terminals.

I can’t. It’s combined with the alternator B+ wire in a loom. That’s the issue. I’d like to try to see where I need to cut into the loom to splice to the remaining hood wire. I’m hoping it’s not far from the alternator end. The whole thing is well supported. REALLY hope it’s not at the connector end!!

 

[spackard]

I understand why you want to know, but can you:
- Take the loom apart just on the back side of the connector, enough to give a hard tug to the D+ ? If it breaks, then keep pulling it out of the loom and you'll know within a half-inch where the break in the loom is. The broken wire is a measuring stick.

- If it doesn't break, try a hard tug at the other end of the loom. (What're the odds it's exactly half-way from either end?). Then, just run a new wire. Cut most of the old wire and discard.

There's a tool, TDR (Time-domain reflectometer) used for finding opens, and they've come a long way, but they're normally used when there's large volumes of earth to be removed.
That said, if all adjacent wires don't connect to a transistor or sensitive circuit, you could probably get an old ignition coil and fire it into D+ and listen for where the high voltage arcs to ground. May even be able to see a spark, in a dark garage. Seems to be a lot more work than doing a hard tug.

 

[JHZR2]

I understand why you want to know, but can you:
- Take the loom apart just on the back side of the connector, enough to give a hard tug to the D+ ? If it breaks, then keep pulling it out of the loom and you'll know within a half-inch where the break in the loom is. The broken wire is a measuring stick.

- If it doesn't break, try a hard tug at the other end of the loom. (What're the odds it's exactly half-way from either end?). Then, just run a new wire. Cut most of the old wire and discard.

There's a tool, TDR (Time-domain reflectometer) used for finding opens, and they've come a long way, but they're normally used when there's large volumes of earth to be removed.
That said, if all adjacent wires don't connect to a transistor or sensitive circuit, you could probably get an old ignition coil and fire it into D+ and listen for where the high voltage arcs to ground. May even be able to see a spark, in a dark garage. Seems to be a lot more work than doing a hard tug.

Probably can pull hard. There’s no real risk in it…

 

[JHZR2]

I understand why you want to know, but can you:
- Take the loom apart just on the back side of the connector, enough to give a hard tug to the D+ ? If it breaks, then keep pulling it out of the loom and you'll know within a half-inch where the break in the loom is. The broken wire is a measuring stick.

- If it doesn't break, try a hard tug at the other end of the loom. (What're the odds it's exactly half-way from either end?). Then, just run a new wire. Cut most of the old wire and discard.

There's a tool, TDR (Time-domain reflectometer) used for finding opens, and they've come a long way, but they're normally used when there's large volumes of earth to be removed.
That said, if all adjacent wires don't connect to a transistor or sensitive circuit, you could probably get an old ignition coil and fire it into D+ and listen for where the high voltage arcs to ground. May even be able to see a spark, in a dark garage. Seems to be a lot more work than doing a hard tug.

I pulled on the end of the wire and without much of a tug, it broke just a bit up. Maybe 3-4”.

I made a strategic cut in the loom just a bit up and found the alternator B+ wire was perfect, but the D+ wire had the usual insulation failure.

I could cut the loom further up, but since the B+ wire is perfect, and D+ is the same potential as the B+ wire anyway… and they are the only two in the loom, I’ll probably just heat shrink butt splice it right there… assuming that it’s continuous to the pin at the connector…

 

[JHZR2]

Well I fixed it.

I wanted to push a wire back up the loom, but it didn’t go easily. So I heat shrink/butt spliced to a spot on the existing wire that I continuity tested to the OE connector. I ran it in parallel through the support, it is about a foot long run.

The battery light now illuminates, and the alternator excites quicker than before… though not instantly. Need to figure that out. It goes off after the battery recovers to about 12v, but the alternator doesn’t necessarily wake up for a little bit.

 

[brianl703]

I pulled on the end of the wire and without much of a tug, it broke just a bit up. Maybe 3-4”.

View attachment 87105
View attachment 87106
View attachment 87107

I made a strategic cut in the loom just a bit up and found the alternator B+ wire was perfect, but the D+ wire had the usual insulation failure.

View attachment 87108View attachment 87109

I could cut the loom further up, but since the B+ wire is perfect, and D+ is the same potential as the B+ wire anyway… and they are the only two in the loom, I’ll probably just heat shrink butt splice it right there… assuming that it’s continuous to the pin at the connector…

Those pictures look so familiar...like what happens when you use PVC insulated wire in an engine compartment or some other place where it gets hot. The plasticizers migrate out of the PVC and it breaks just like that.

 

[Donald]

Probably can pull hard. There’s no real risk in it…

TDR is used by electric companies and contractors trying to find an underground break and determine where to dig. Very accurate.

 

[JHZR2]

TDR is used by electric companies and contractors trying to find an underground break and determine where to dig. Very accurate.

Unfortunately I suspect that tech is $$$$$…. Is the $50 version available?

 

[brianl703]

Unfortunately I suspect that tech is $$$$$…. Is the $50 version available?

If you have an oscilloscope, you have most of what you need for a TDR.

 

[meep]

nice work jhzr2.