How to test plug wires?
09-23-2004, 05:00 AM
#1
Racer
Thread Starter
How to test plug wires?
Hello from Australia,
My service kit has arrived from Pelican - was a good saving 'till the customs man hit me for $198.00 Aus duty and tax - Ouch.
Question. My 90 C4 has 190,000 k and records say leads changed 40,000 ks ago but they are not all the same - some look older - so I suspect previous owner was duped and paid for 12 new leads when only a few were replaced.
So, what are the resistance figures per "foot" or other distance for porsche spark plug wires?
If I know this I can use a multimeter to do a simple resistance test.
Finally, I do realise I can do a comparison to see if any are below the average but it would be good to know what the numbers should be.
Thanks again,
Michael Gibney
My service kit has arrived from Pelican - was a good saving 'till the customs man hit me for $198.00 Aus duty and tax - Ouch.
Question. My 90 C4 has 190,000 k and records say leads changed 40,000 ks ago but they are not all the same - some look older - so I suspect previous owner was duped and paid for 12 new leads when only a few were replaced.
So, what are the resistance figures per "foot" or other distance for porsche spark plug wires?
If I know this I can use a multimeter to do a simple resistance test.
Finally, I do realise I can do a comparison to see if any are below the average but it would be good to know what the numbers should be.
Thanks again,
Michael Gibney
09-23-2004, 07:49 AM
#2
Burning Brakes
Join Date: Feb 2003
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Michael
Resistance isn't the only factor that determines if a plug wire is bad although an important one. If we were only concerned with low resistance we'd be using the thickest solid or stranded core wire we could find which would give us almost negligible resistance over the longest length spark plug wire. If we did, even the best radio filter wouldn't be able to remove the EMF generated from a running engine. What happens over time to a spark plug wire amoung the resistance going up is the insulation breaking down due to heat and carbon tracking. So even though a wire may show a resistance within spec the insulation could be breaking down causing arcing to occur. This is a static analysis, when the wire is subjected to hi voltage pulses a whole new set of rules come into play.
Hope this helps
Bill
90C2
Resistance isn't the only factor that determines if a plug wire is bad although an important one. If we were only concerned with low resistance we'd be using the thickest solid or stranded core wire we could find which would give us almost negligible resistance over the longest length spark plug wire. If we did, even the best radio filter wouldn't be able to remove the EMF generated from a running engine. What happens over time to a spark plug wire amoung the resistance going up is the insulation breaking down due to heat and carbon tracking. So even though a wire may show a resistance within spec the insulation could be breaking down causing arcing to occur. This is a static analysis, when the wire is subjected to hi voltage pulses a whole new set of rules come into play.
Hope this helps
Bill
90C2
09-23-2004, 06:18 PM
#3
Addict
Rennlist Member
Rennlist Member
Darth is correct - insulation breakdown is the typical end-of-life failure mode. Research threads on ignition wires. I like the night test where you fog the area with a pump-spray bottle set to "fine mist". You will see a light show if you have leakage. Keeping fresh plugs extends life if ignition system by ensuring that the sparkover voltage is as low as possible. If you don't want to do the mist test, just use you hand and start touching near the fittings on the high-voltage leads. Just make sure somebody that is CPR trained is available to secure the test and tend to your immediate needs. You are probably on borrowed time if any wires have 190 K on them. Mine are still original, and are fine at 52 K (miles in my case).
