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Tips for hunting down corroded but not broken wires.

posted Jul 11, 2016 19:06:11 by Chris
Had a great hunt last week and had to share it. I know we have all had the misfortune of trying to hunt down a green booger in a wire. We had a 2014 KW in the shop missing the RT Rear turn signals and brake lights. KW and PB trucks now incorporate a central electronic control module and a solid state device called a chassis node.

There are at least 10-12 fuses that supply power to both units and after we verified we were missing power at the chassis node on one pin the hunt was on.

We had a more or less dead open wire. Showed OL with an ohm meter and it wouldn't light a power probe green or red light. But I would register about 40mv at the chassis node connector. It had a good 12v supply from the fuse box. With a meter in the node connector and a guy watching it we started wiggling the harness down to the node connector. Sure enough as we got close to the green booger we started seeing the voltage jump around getting as high at 300 mv. It's not much but it was enough. We got it narrowed down to 1-2 feet of harness before we lost the reading on the volt meter ( we broken the wire ). Ripped open the harness and found our little green friend.

We have seen time and time again how unloaded circuit voltage testing can bite us in the butt. In this case it saved ours. The green booger was such a large resistor it was killing all but 40 mv to the end connector. A loaded circuit would have shown a full 12v drop with nothing left over. Another tech had said "have fun finding it, just put a harness in it". Well I did have fun finding it and showing my padwon a new trick.

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5 replies
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Noah said Jul 11, 2016 23:45:09
Nice find Chris. Those sneaky bastards can be a real pain. Pretty cool you were able to read any voltage considering the ohm meter showed OL.
Have you ever tried the power probe short finder?
Massachusetts, USA
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Chris said Jul 12, 2016 02:40:51
I don't have one. Thus the low tech approach I had to take. Trying to find a green boogie in 20' of harness on a truck isn't easy. But usually easier than changing the entire harness. I do need a short finder though.
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AndyMacFadyen said Jul 12, 2016 05:22:26
I have a US-Pro wire tracer which I picked up at a very low price,a very nice item but so far it has not been as quite as useful as I thought it would be. Last week I had an old Rover in with loss of coms on the OBD2 socket. My home made OBD2 breakout box came in handy, it turned to to be a simple loss of power on pin 16. I had no luck with tracing the bad connection in the end I just Scotchloked a jumper wire from the DVD player supply which comes from the same fuse. A very quick and very dirty repair but it is a non-critical wire under the dash and the car will be in the crusher by this time next year.
[Last edited Jul 12, 2016 05:24:19]
"Rust never sleeps"
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Chris said Jul 12, 2016 16:43:39
Andy do you have a photo of that breakout box
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AndyMacFadyen said Jul 12, 2016 21:15:50
The breakout was nothing clever just an OBD2 socket cut off a junker and and a plug cut off a junk chinese fault code reader. The car the socket came from only used the K and L lines and power and grounds so I just joined them pin for pin with pig tails so I can easily scope the K-line and look at the powers and grounds or use the +12v and ground for other test purposes.


Since last week because found it so useful I bought a proper one from Germany that has all 16 pins connected. It arrived yesterday and seems good quality.

[Last edited Jul 12, 2016 21:18:03]
"Rust never sleeps"
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