Air compressor line faillure......

[Ncbob]

Here's a question for our more astute Electrical Engineers on the Board...

The braided air line from the top of my compressor to the AD-9 air dryer failed the other day (noticeable leak) and when I removed the line for replacement I noticed a black spot where the leak was.  It strikes me that it is an arc burn where it had been loosely ty-rapped to a 1/4" air line overhead in the engine compartment....hmm...that's strange.

We'd been suspicious of perhaps a bad starter since the engine wouldn't crank and put a meter on the starter while cranking...it turned out that the batteries were dead. So I charged them (8D's) separately with a shop type charger (having removed the + lead while charging).

I'm including all this information in the hope that someone might tell me ...why an arc between the supply hose and an air line...they're both ground potential? Could it be that when the compressor is working there's a sufficient static charge in the hose that it would try to go to ground? That doesn't make sense even to me...because the compressor and the dryer are frame grounded...

The Bus gremlins are loose again!

Bob

[Prather]

One of the things you are told  to check when replacing the compressor is  build up in the discharge line that can block the air flow. I suspect the black you are seeing at the leak came from inside the air line and not a arc.

Prather

[Ncbob]

Prather, both the compressor and the line were replaced less than a year ago.  I just looked at the interior of the air line when I read your post and it's as clean as when it was new.  Sure don't wish to knock holes in your offering...might be the case in an older line...but not the case here.

Thank you for your post.

Bob

[Len Silva]

It could very well be that you have a bad engine ground and when trying to crank, it just finds the best path it can.

Take a voltmeter and connect one lead to the neg battery post and the other to the engine block.  Watch the meter while cranking.  Any reading at all would indicate a less than perfect ground.  Anything over a few tenths of a volt indicates a problem that needs fixing.  It,s not a static issue.  It seems that the 1/4" line may be a better ground than the starter.

Len

[FloridaCliff]

Bob,

My 1st guess was the same as Prathers!

Seperate the lines with some non conducter and with the engine running mesure between them for a difference of potential with a VOM.

Cliff

[Dallas]

Bob,

Just as a thought, what kind of condition are the ground straps betwixt the engine and the body in?

If they are questionable at all, change them.

If, and this is just a surmise, the engine/body ground junction is "iffy", when you hit the start button you'll be trying to pass a high amperage burst through whatever path is available. If no single point is handy, the voltage will take multiple paths and since you have a metal braided sheath on the compressor on the engine, going to ground at different points on the body, .......... ZAP!

It could also be, and this is another surmise, an open neutral somewhere in the 120V system, although looking at your coach, I don't see that being much of a problem.

Again, this is just my rambling brain fumbling for an answer that covers what you describe.

Dallas

[Ncbob]

You could be closer than you think, Dallas....I found a not too tight ground on the house batteries and sense that I might not be getting all the charge available from that monstrous alternator back there.  Before I do much more, other than replace that $80 air hose...I will lift all the grounds from the battery banks and wire wheel the metal and the lugs and reassemble with shake proof lock washers.

In addition I will ty-rap a piece of heater hose around the braided hose where it might come in contact with anything metallic.

Thanks for the replies, guys...as always a wealth of information.

Oh, NAPA is ordering me a coupling so we can cut the old hose and swage the coupling in for a spare.  It's not just me that might need one in the future.....

Bob