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Capacitance in wiring loom
r1_pete - 6/12/11 at 11:52 AM

I have just completely re wired my E Type, front to back, all wiring is new thinwall, all connectors soldered and double insulated, i.e. heatshrink then the moulded plastic covers.

The reason for rewiring it was 1, the old wires were cloth covered and tatty after 44 years, 2, it just looked crap with all the mods in those years, and 3 most importantly I was getting a very slight current drain which I put down to capacitance in the loom made worse by the breaking down of the insulation.

Before I installed the new loom I tested everything for shorts and continuity, connected it to the battery, absolutely no current drain at all.

When I put it in the car, and connected everything up, all worked fine, and I didn't even blow a fuse, but, now with the ignition off, there is definately no component which could cause this, I am seeing a .0019 Amp drain, I know it isn't a lot and is negligable but I want to understand why.

Could this be capacitance between the loom and the body shell?, a lot of the loom is tie wrapped to chassis rails, or does anyone have a better theory?

Cheers.

Pete.

westf27 - 6/12/11 at 12:01 PM

could be a slight leak through the alternator diode

steve m - 6/12/11 at 12:44 PM

How much current does the clock use?

r1_pete - 6/12/11 at 01:02 PM

Thanks for the ideas guys,

I'll try pulling the alternator connection off.

It doesn't have a clock, and I havent put the radio in, as that has a perm + to preserve the presets.

HowardB - 6/12/11 at 01:58 PM

that's something about 6kohm resistance path, are there any other diodes in circuit? Or any sensors that are always on?

hth

v8kid - 6/12/11 at 02:14 PM

If you had a 50ah battery it would take 3 years to drain it at that leakage. I really don't think you have a problem!

No capacitance effects with DC, reverse leakage with diodes plus resistance at light bulb holders etc easily accounts for this.

Cheers!

matt_gsxr - 6/12/11 at 03:43 PM

Why should capacitance cause a current drain in a DC circuit. Surely it would charge up and stay there?

If you have a 40Amp.Hr battery it will take 2.5years to discharge it.
Leakage from alternator diode (or other diode) or perhaps some leakage from condenser (if it has failed).

Stott - 6/12/11 at 04:36 PM

What does your meter read when open circuit? ie is it zeroed?

1.9mA is more likely a meter error unless you have a high quality one

Hth
Stott

snowy2 - 6/12/11 at 04:59 PM

A few years ago i traced a fault to the dirt on the battery top the current was actually tracking between the terminals. cleaning the battery top cured the problem.

r1_pete - 6/12/11 at 05:16 PM

Thanks all, I did the ah calcultions I know its not really a problem, I was just anoyed after spending ages re wiring it I had a similar but much lesser problem.

I use a needle AVO, not a digital meter, and at anything above the minimum amps scale it doesn't read.

Two wires in paralell will have a capacitance effect, hence my theory....

MikeRJ - 6/12/11 at 05:42 PM

quote:
Originally posted by r1_pete
Thanks all, I did the ah calcultions I know its not really a problem, I was just anoyed after spending ages re wiring it I had a similar but much lesser problem.

I use a needle AVO, not a digital meter, and at anything above the minimum amps scale it doesn't read.

Two wires in paralell will have a capacitance effect, hence my theory....


With a DC level a capacitance will only draw current until it is charged to the same voltage as the supply (which will take a tiny fraction of a second in the case of a car wiring loom and a car battery). Any current draw after this period will be down to leakage, not capacitance.

I suspect alternator diode leakage will be the primary culprit in your case.

r1_pete - 7/12/11 at 05:58 PM

Got home a little early tonight so did a quick test, no alternator connected, no current flow, now I'm happy with the rewire.

Thanks everyone.

PSpirine - 7/12/11 at 06:14 PM

Really don't mean to put a minor downer on your rewire, but I'd watch out for connections if you soldered everything.

There's a reason why automotive wiring is ALWAYS crimped and never soldered. Soldered joints aren't flexible and can suffer from vibration induced fatigue..

In 99% of the connections it probably won't be a problem for years (if ever), but if you get any intermittent electrical gremlins popping up in the future, I'd always have a look at the soldered joints first. Just a heads up!


Bet you're glad to see the end of that task though (I sure was when I did a full rewire! )

r1_pete - 7/12/11 at 06:24 PM

Yes I understand that, thats why I've put 20mm of heatshrink over things like spades, before sliding the moulded insulators on. Most of them are clipped to the chassis too, so hopefully will be ok.

Cheers.