Conductive rubber?

Chuck41

Chuck41

Member
Here's a disturbing discovery. The fuel gauge was acting up and I traced it to an 83-ohm leak to ground somewhere in the sender wire. That seemed odd since the car has 100% new wiring harnesses. Adding to the oddity was the fact that turning on the headlights changed the gauge reading. I finally traced the short to the bundle coming through the firewall -- the one that has the pear-shaped cover on it. Now the fun part -- when the rubber connector for the sender wire was squeezed tight against the connector for the high beam indicator, a high-resistance short occurred. Check out the photo -- that connector has a resistance of 359 ohms from one end to the other. All the other black ones were similar, but the larger, brown connectors were not conductive. This is a new wiring harness from C&G.
 

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Chuck41 said:
Here's a disturbing discovery. The fuel gauge was acting up and I traced it to an 83-ohm leak to ground somewhere in the sender wire. That seemed odd since the car has 100% new wiring harnesses. Adding to the oddity was the fact that turning on the headlights changed the gauge reading. I finally traced the short to the bundle coming through the firewall -- the one that has the pear-shaped cover on it. Now the fun part -- when the rubber connector for the sender wire was squeezed tight against the connector for the high beam indicator, a high-resistance short occurred. Check out the photo -- that connector has a resistance of 359 ohms from one end to the other. All the other black ones were similar, but the larger, brown connectors were not conductive. This is a new wiring harness from C&G.
Click to expand...
484641
 
I’m confused. You say you have a short to ground, but you confirm that by describing a high resistance to conductivity. I confess I don’t know squat about electricity, but I do know that a short and resistance are two different things. In any case however, I would suggest that bullet connectors, as well as any other type of splice should not be grouped in close proximity to each other.
 
You're right, "short" was the wrong word - that implies a zero resistance path to ground.

The rubber bullet connectors are acting like partial conductors rather than complete insulators as they should. So the sender wire was being grounded through the high-beam light bulb.

Everything is packed tight in the firewall bundle cover -- it's impossible to not have the rubber bullet connectors touch each other.
 
I should add that I have a package of individual rubber bullet connectors (not part of a wiring harness) that are fine -- they have infinite resistance and are perfect insulators. Very strange.
 
Ah, now I understand. And I also had missed your reference to the firewall cover. Perhaps the connectors you have are defective, as they may very well be if NOS, hard old rubber containing carbon black! They are certainly not worth much, but you might find C&G to be appreciative for a heads up on the problem.
 
You will find radiator hoses compressed under the radiator clamps are also conductive. Compressing the carbon in the black rubber decreases resistance.
 
I was complaning about a miss on my Harley to another guy, he bent over and moved the fuel line away from the sparkplug boot, miss went away.


Bruce
 
Terry said:
You will find radiator hoses compressed under the radiator clamps are also conductive. Compressing the carbon in the black rubber decreases resistance.
Click to expand...

That would definitely explain what I observed. I think I'll replace the 4 or 5 connectors in the firewall cover and call it good. Nowhere else are they compressed tightly together.
 
Note that some rubber molding compounds use what's known as "Lamp Black", a carbon based powder, to give the rubber compound a black color. This can make rubber components conductive to some degree depending on how much of this black powder is mixed in the rubber compound. We encountered this on some of the product manufactured by the company I once worked for.
 
I recall a tale told on another forum, seems a fellow was going away for a couple weeks and was concerned about theft of his pride and joy parked on the street or wherever. As part of a stealth deterrent he removed the coil wire, substituting a similar length of black rubber vacuum hose in its place. It looked identical to ignition wire or near enough to the casual observer. Of course the idea was, the engine wouldn’t start without a coil wire.

He said he returned home and by then had forgotten all about his ignition coil wire substitute. It started right up, and ran fine. Hm.
 
Further confirmation that carbon black can cause low resistance in connector housings. I saw it occur when it was not well distributed and instead was in nodules in some parts my company made. Interestingly, they didn't fail initially. But voltage surges created each time the circuits energized gradually created carbon tracks between them until they broke down.

A similar failure may occur when reground plastic is used in connectors. This is not recycled stuff. Instead, when the parts come out of the molds, attached to the parts are plastic runners formed in the channels which feed the melted plastic to each mold cavity. The runners are ground up and reused which is normally OK. But if the grinders are not properly maintained, the blades can produce metal filings which get into the reground plastic. The result is the same as with poorly distributed carbon black.
 
As a kid I was given a pair of wheels on an axle that was off a ww2 bomb trolley. There was a statement on the side of the tyre to the effect that they were electrically conductive. I assume to prevent static buildup.
 
Electronic assembly areas use rubber mats grounded to prevent static damage to components.

Maybe using shrink wrap on the connectors would solve the problem.
 
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