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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / December 2004Spark plug wires - allowable resistance
What is the allowable resistance for plug wires. Car is a 98 Subaru legacy 2.5L > What is the allowable resistance for plug wires. Car is a 98 Subaru legacy > 2.5L I cant answer that, but that is not the only consideration. What usually happins is that the insulation breaks down, and that cant be tested with a multimeter. I usually check to see if the reading is proportionite to the length as compared to the other wires, with the longest wire not being over say 30-35 k without being suspect, differant car though. What is it guys, like 10K per foot as a rule on high voltage wiring? > > What is the allowable resistance for plug wires. Car is a 98 Subaru legacy > > 2.5L > > > I cant answer that, but that is not the only consideration. What usually > happins is that the insulation breaks down, and that cant be tested with a > multimeter. >>What is the allowable resistance for plug wires. Car is a 98 Subaru legacy >>2.5L > > I cant answer that, but that is not the only consideration. What usually > happins is that the insulation breaks down, and that cant be tested with a > multimeter. like today - finally decided to do the plugs on my 86 Jimmy. They're only 4+ years old... found out at least THREE plug wires have been arcing. Whoops. Man, sometimes it's hard to stay ontop of maintenance if you have multiple vehicles and this is a beater truck... but now I feel dumb for not changing them sooner... So, I'm off tomorrow to get plug wires... Testing plug wires with a multimeter for DC resistance isn't really the same load they see in an engine. Even if they test ok, they may fail under high voltage. I've usually seen them just get cooked and cracked or they rub and chafe and start arcing through. If they aren't flexible anymore without cracking they're probably shot. Ray | "das86turbo" <das86turbo@comcast.net> wrote in message | > What is the allowable resistance for plug wires. mysterious posters ( Mr know all ) get no info fr me | I cant answer that Any electrical engineering diploma holder can answer this. > | "das86turbo" <das86turbo@comcast.net> wrote in message > | > What is the allowable resistance for plug wires. > mysterious posters ( Mr know all ) get no info fr me > > | I cant answer that > Any electrical engineering diploma holder can answer this. Bull. Every manufacturer has a different resistance spec for 'their' wires so unless the engineer has a massive data base to check he can't know. And even the average person can find the brand name and likely find what 'their' resistance is by going to the brand's website and reading the FAQ or asking. I never bother. I just measure the closest one to 1' long I can find, measure it and add or subtract from there. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's | unless the engineer has a massive data base to check he can't know. 1 thing is obvious, u don't have ? a diploma in elec engneering > | unless the engineer has a massive data base to check he can't know. > > 1 thing is obvious, u don't have ½ a diploma in elec engneering I have a degree in electrical engineering. Off the top of my head, I have absolutely no idea what the allowable resistance for plug wires is. As with many things done by engineers, one looks things up. I've got various specs, books, and documents sitting on my desk at work. I can assure you that I don't have everything memorized in my head. I could ask my coworkers, and I'm guessing they don't know either. From a purely electrical standpoint without EMI to worry about, stranded high-conductivity oxygen free copper would probably be ideal; the stuff would be cheaper than resistive plug wires. Real world considerations dictate that wires must be made with other materials/designs such that they don't screw up your radio and/ or engine computer. Resistance isn't that big an issue. The biggest problem would be if in the insulation breaks down and there's arcing between the wire and the valve cover/engine block. >> | unless the engineer has a massive data base to check he can't know. >> [quoted text clipped - 16 lines] >biggest problem would be if in the insulation breaks down and >there's arcing between the wire and the valve cover/engine block. What he said. I've always used the spray bottle test where you spray water mist on the wires in the dark and look for arcing. IMHO, over time and heat the wires cook and develop cracks that allow leakage/arcing especially in damp weather. Don't know how scientific it is but it works as a diagnostic tool. "When a legislature undertakes to proscribe the exercise of a citizen's constitutional rights it acts lawlessly and the citizen can take matters into his own hands and proceed on the basis that such a law is no law at all." - Justice William O. Douglas The same per foot as the rest of the set. If one wire is 2' long and measures 'say' 100 ohms, then a 1' one should measure 50 ohms and a 3' one 150. The readings should be a mathematical increase or decrease in proportion to the length. Every brand and type of wire has a different 'per foot' resistance reading. If they are all over the place for readings, then it is time for a new set. The boots and insulation can leak causing a miss too. I look for those at night. I open the running car's hood and spray mist the wires with water. If the wires are leaky, you should see it easily... Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's > What is the allowable resistance for plug wires. Car is a 98 Subaru legacy > 2.5L Thanks guys > The same per foot as the rest of the set. Yep. > If one wire is 2' long and measures 'say' 100 ohms, then a 1' one should > measure 50 ohms and a 3' one 150. The readings should be a mathematical > increase or decrease in proportion to the length. Those numbers would be WAY too low. I measured the DC resistance of some old cables, and it was in the 5,000-10,000 ohm range. > Every brand and type of wire has a different 'per foot' resistance > reading. > > If they are all over the place for readings, then it is time for a new > set. Yep. However - it needs to be noted that lower resistance isn't all that necessary in plug wires. They're often made more resistive with carbon cores, which is a good idea unless you like massive radio interference and other assorted electronic interference. I thought that they were simple high-conductivity oxygen-free copper wires, which I'm now told would be horrible in any passenger car. I was taught many years ago, 1k ohm per inch is max. > I was taught many years ago, 1k ohm per inch is max. For what type and brand of wires? Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Supression core wires, brand shouldn't matter, if you have more than 1k per inch, replace 'em. Solid wire cores will probably be here or there. If checked with an ohmmeter, shorted good, open bad. > Supression core wires, brand shouldn't matter, if you have more than > 1k per inch, replace 'em. Solid wire cores will probably be here or > there. If checked with an ohmmeter, shorted good, open bad. 1k per inch...10 k per foot...pretty close.... good stuff I like the night test with the engine running and the hood up and watching the fireworks under the hood on bad insulators....nice so long as there's no gas leak!! :) } }> If one wire is 2' long and measures 'say' 100 ohms, then a 1' one should }> measure 50 ohms and a 3' one 150. The readings should be a mathematical }> increase or decrease in proportion to the length. } } Those numbers would be WAY too low. I measured the DC resistance of } some old cables, and it was in the 5,000-10,000 ohm range. There are different construction techniques for ignition wires. Most wires (factory and cheap replacements) are resistive. These are 10kohm/foot or therabouts. The good wires are lossy inductors (a fine wire wrapped around a core) and only measure 100 ohms/foot or something close. You can get a lot hotter spark with these. In an old car that burned oil good wires cured my spark plug fouling problems.  SignatureFrank Ball frankb@sonic.net |
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