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Blown ground wire - ABS brake harness cable... (Read 1245 times)
Matt_Slay
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Blown ground wire - ABS brake harness cable...
Jul 8th, 2019 at 7:16pm
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Has anyone ever seen this before??

On the rear brakes (and the front too) there is a wiring harness that has connectors that go to the speed sensor and brake pad wear indicator at each wheel.

The harness cable has 5 wires in it and a barrel connector on each end (ABS Brake Pad Sensor Harness part number is 944 612 112 00). Two wires in the cable are for the speed sensor, and two wires are for the brake pad wear indicator, and the 5th wire is a ground wire and at each barrel connector there is an external eye that bolts to the frame of the car to filter out electrical noise from the speed sensor (at least that's what I read online).

Well... something happened and the darn *ground wire* shorted out and blew through the outer shell of the entire cable. I know it's the ground wire that blew because I checked continuity on the other 4 wires that run through the harness and they all check fine on both connectors on each end of the hardness cable.

What in the world would make a ground wire short out and explode like this? Surely that ground wire does not have current flowing through it.

See my reply below for pics in the next message.


« Last Edit: Jul 8th, 2019 at 9:13pm by Matt_Slay »  
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Matt_Slay
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Re: Blown ground wire - ABS brake harness cable...
Reply #1 - Jul 8th, 2019 at 7:22pm
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Here are some pics:










  
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Re: Blown ground wire - ABS brake harness cable...
Reply #2 - Jul 9th, 2019 at 12:22am
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Hmmm, interesting.

You say...."Surely that ground wire does not have current flowing through it."

I'm guessing it does since the purpose of a "ground" wire is to provide a path to ground in an electrical circuit.  Now how much current - I don't know. 

What caused this?... Maybe something got crossed up inside the connector?  Maybe some moisture found its way in the connector?  Good question.
  
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Dave W
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Re: Blown ground wire - ABS brake harness cable...
Reply #3 - Jul 9th, 2019 at 3:32pm
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The usual cause is corroded grounds.  You need to clean ALL of your grounds.

If current can't go to the intended ground, it will flow through any wire (even logically backwards) to get to a ground.  If a high current line finds its way to a low current wire's ground, it burns it out.
  

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Re: Blown ground wire - ABS brake harness cable...
Reply #4 - Jul 10th, 2019 at 2:22am
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@DaveW - I get that, and I’ll clean the ground points as I replace this harness.

However, the baffling part of it to me is where *that much* current came from in the first place.  The ground wire in the bundle with the rest of the wires picked up a ton of current from somewhere, and the only other wires in that bundle are the brake pad wear sensor and the speed sensor.

It read that the ground wire is just supposed to catch “noise” from wherever to prevent it from messing up the signal in the speed sensor feedback. It’s basically like a shield.

Oh well, I’ve got a new one coming on Thursday and I’ll replace it and hope the same thing doesn’t happen again.
  
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T_Ryan.
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Re: Blown ground wire - ABS brake harness cable...
Reply #5 - Jul 10th, 2019 at 9:48pm
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it is kinda interesting for a hole to blow through casing. it would take quite a bit of current to blow a hole instead of just melting it.

the motor/exhaust/trans is suspended by rubber so I would check the engine grounds. the higher starter current might have traveled down the torque tube, the trans and out to the wheel. if the torsion bars are well lubed, I could see the shield being the path to the chassis ground.
  
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Dave W
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Re: Blown ground wire - ABS brake harness cable...
Reply #6 - Jul 11th, 2019 at 3:45pm
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Electricity does odd things.  If you have a bad engine ground in a Fiat Spider, then the current goes through the clutch cable and eventually causes it to break.
  

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Re: Blown ground wire - ABS brake harness cable...
Reply #7 - Jul 29th, 2019 at 8:52am
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Typically installing a fly-back diode in a DC application prevents shorting.

When a coil of wire conducts and is interrupted in a magnetic field collapses and has no where to dissipate the energy a silicone diode is used to prevent backfeed energy at instanteneous collapse.

Now how to size one (OEM) would or should publish a chart based on the energy field or resonate inductive capacitance of the wire connection.

Best practice is to add a electronic clamp between ground and the energy source. A TVS diode. See below attached.
  

what_are_tvs_diodes.pdf ( 52 KB | 5 Downloads )

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Re: Blown ground wire - ABS brake harness cable...
Reply #8 - Jul 29th, 2019 at 10:49am
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anchor
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Re: Blown ground wire - ABS brake harness cable...
Reply #9 - Jul 29th, 2019 at 12:15pm
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There exist the possibility the new harness's ordered by the original post in the pic resolved. I guess anything is possible. One tangible concept honorable mention on Rennlist ( different thread) equated to electrical insulation wear and thus causing a metal to electric grounded short.
Normally a fuse would blow when experiencing a ' bolted' fault or the latent heat would compromise the extruded insulation to weaken and then expose the copper in the conductor to corrosion (see Dave W above post) two types of corrosion exist they are galvanic stress cracking and caustic agent discreetly and prominently held as an explanation as well. Finally a solution just put put in another harness and be done as originally held but add a path to electronic isolated ground not equipment ground as the original post when " like a shield" or drain wire is connected. It would represent transistor to transistor logic ground (TTL) at a voltage nominally 5vdc or less, often referred to as signal common in a negative value dependant on the agency listing or patent authority regulations.

I sort of do not particularly advance other sites but they do serve as a legitimate reference standard. The only Avenue to discovery is testing with or without a credible sustainable set of tools and imagination with a trusted colleague. There exist a BOSCHE CAN network bluetooth interogation tool AXLE sensor. The PDF file is to large to put in here:
https://www.scribd.com/doc/164087261/Porsche-928-Factory-Manual-Volume-4
« Last Edit: Jul 30th, 2019 at 6:03pm by anchor »  

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