I have a misfire. I used a code reader to see that it was in cylinder 4. How do I use an ohm meter test the wire. My book isn't very descriptive on using the tool. What setting should I have it on? And which holes should I plug the wires into? The book says to look for around 7,000 ohms. Will that show on the screen?

 

Use the selection on meter that looks like the greek symbol Omega. You place one lead on one side of wire and other lead on opposite end. If you are reading 7000 ohms that is a horrible amount of resistance. Good plug wires should have no more than 50-200 ohms per foot of wire. IMO you should just switch your cylinder 4 wire with say cylinder 3 wire and see if misfire changes to #3 cylinder. You can also spray wire with water and watch in the dark to see if you can locate arcing wire.

 

Hey Wheelsup, from your posts, you obviously have way more experience than I do with these things, but I remember from school (which was a long time ago!) that we were told that approx. 8k ohms per foot give or take a couple thousand ohms was a good check. So I checked an oldbook I had (printed in '92) that says a good range is 1000 to 10,000 ohms per foot. Are the wires constructed differently nowso thatthey will have a different resistance reading? Could the "good" range be more narrow now...or is my info bad?
...I don't mean to hijack this thread, but I hope the question is relevant.

 

IMO resistance checks on passenger car plug wires is pointless. You really are not going to be able to find the misfiring plug wire with resistance check. Best way to find misfiring plug wire is by using test light and putting alligator clip on ground and jamming point through top of boot on coil or cap with engine running, the misfiring cylinder will not change engine idling characteristics. After pinpointing cylinder you can then swap wires btwn cylinders and rule out wire and move on or replace. BTW I have mainly most of my experience with racing wires and stuff, and can tell you that mine are a constant 50 ohms per foot, just like the nice MSD 8.5 wires, even the cruddy Accel 8.8 wires carry less than 400 ohms per foot. I have'nt tested a passenger car wire ever, but my wires for drag car are made with Delphi Packard wire witch is used by some other wire manufactures for passenger cars. 8000+ ohms per foot is about what you would get from 1 foot of steel clothes hanger.

 

Ahh, that makes sense. After double checking my book, and reading your reply, I can see where the resistance check might not be a valid test.It said inmy book is that spark duration lasts about 1.5 to 2 milliseconds at about 1000 volts, but the voltage needed to initiate the jump across the spark plug gap can run from 5000 to 30,000 volts in the first 30 micro-seconds. (The book mostly covers Ford EEC-IV systems - first generation stuff). With that much electrical pressure, any problems may not show up withan ohms check that just uses the battery in your tester. I've heard about shorting the plug wire with a test light as you suggested. I've never done that before, but that's something to keep in the old gray matter for future use! Thanks forthe info...

 

No problem, ground testing is my best friend at shop, it helps me locate cylinder misfires without ever craking into OBD connector. Quick, easy, reliable. I would like to see if car would run with insulated clothes hangers though.