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Brian Cameron's Avatar
 
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Yet another green wire thread: coax specification?

Replacing the coax “green wire” from the CDI box to the connector for the short wire (which actually is green). The short wire is ok but the one in the harness needs replacing.

The question is, which coax wire is the right (or least bad) spec for this? Early S Man suggested RG174 (but not heat rated) or RG196 (teflon jacket) here:
Green wire source

I recently ordered some RG196AU because heat, but looking at the specs it is physically much thinner (it’s about about 2mm diameter) than the existing green wire and harness wire.

Will it be able to handle whatever current is required in this application?

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Old 03-13-2018, 07:46 AM
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That inner core diameter nets a CSA of 0.062mm^2 which is the equivalent of approximately a 30-28 AWG wire. I wouldn't go any smaller than a 22 AWG for that particular circuit.

I know the color is not green, but this is the wire I would use to replace the one in the harness:

https://prowireusa.com/p-576-20-awg-x-1-shielded-cable.aspx

And one of these for terminating the shield:

https://prowireusa.com/p-1088-shield-terminator-with-22-gauge-lead.aspx
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Old 03-13-2018, 08:28 AM
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I would try it. This is just an AC signal going to a a transistor. Extremely low current. The coil in the distributor is a single strand of very fine wire.

-Andy
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Old 03-13-2018, 06:50 PM
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i've been running my car for 2 years now with early s man suggestion coax wire. Yes it is much thinner than the stock green wire, but it runs just fine.
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Old 03-13-2018, 10:13 PM
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Jason, wiring diagram states 1mm (18 AWG approx) so you may have a point, but OTOH maybe 1mm is just the smallest wire the factory kept in stock... and overbuilding certainly never hurt anything.

Andy, good point, the hall transistor might need to transmit only a very small current to the base of the the next transistor, assuming the latter is controlling current to the coil primary. Hard to tell from the diagram though

Wreckah, did you use the RG174 that Warren (early s man) suggested? Or the RG169 he mentioned was three times more expensive but more heat resistant (which is why I bought it)?

I guess another alternative might be RG142 which has a .94mm core, exterior diameter of about 5mm (0.2”), and FEP (similar to PTFE) jacket rated to 200C. But the solid core would be less flexible than the stranded alternatives, I’d worry it would fatigue and break if I bent it one too many times.

I think maybe I’ll pop open the CDI box when time permits, to make sure the coax shield or core have nothing to do with the big fat capacitor, or the emitter/collector of the thyristor, which are hiding in there. Hopefully those are connected to the brown/red and white wires (both 1.5mm). If so I’ll take a chance on the skinny wire unless Loren chimes in and says don’t do it.

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Old 03-13-2018, 11:13 PM
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Hi Brian,

i've been looking through my emails but no record of any online purchase, so i must have bought it locally in electronics store,
in my historical notepad file, it says i bought RG-188A/U or RG-180/U .... coupled with a AMP timer junior connector.

cheers, Jan
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Old 03-13-2018, 11:33 PM
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Quote:
Originally Posted by Eagledriver View Post
I would try it. This is just an AC signal going to a a transistor. Extremely low current. The coil in the distributor is a single strand of very fine wire.

-Andy
the polarity is still very important because of the slope of the signal.
one side has a very quick rise time, the other is much longer and "MOVES".

if the engine runs the same after the fix, the polarity is good, if the timing is way off or it wont start, its backwards.
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Old 03-14-2018, 02:55 AM
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There is effectively little current. The system works on a VR effect creating a low voltage AC signal with the reluctor wheel passes the coil. That smaller diameter wire will work great.
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Old 03-14-2018, 04:08 AM
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I have some longer random length of the the green coax

Lorne M.
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Old 03-14-2018, 08:23 AM
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https://image.ibb.co/c5X64c/20171027_155845.jpg

Trace from 81sc. Note sharp falling edge of signal. This is what triggers the CDI to fire.

As others have noted if the polarity is reversed, so is the waveform. So the trigger point becomes the shallow ramp you can see on the left hand side in the pic. This edge and shape changes with rpm. So the timing is over advanced and erratic.

Last edited by Sub8; 03-16-2018 at 12:48 PM..
Old 03-16-2018, 12:43 PM
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