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shh-the robot is sleeping
 
Join Date: May 2010
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Multimeter help needed for spark plug wire resistance

I am working through an at speed / acceleration stumbling issue that gets worse when warm. Im investigation if the 4 year old plug wires are all still good but have practically no expertise with multimeters. 3000 ohms is printed on the wires. I selected the meter as shown below:



Wire 1 resulted in 1676 displayed (dont know the units)
Wire 2 showed 1346
Wire 3 showed OL

I wonder if three is an open loop because the car starts and runs well.

Am I doing this correctly?
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Old 09-12-2018, 03:13 PM
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You need to select the 20k setting in the Ohm area. Rotate the dial 3 clicks counter-clock-wise. Currently you are measuring "a voltage drop across a diode" and the reading makes no sense. It will read zero when you touch the leads and a real voltage if you connect the leads to a diode in the right way.

OL is open-loop. This means there is no connection (or infinite resistance) between the leads. When this shows up while you have the leads on either end of a spark plug wire it's definitely bad.

Ingo
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Old 09-12-2018, 03:46 PM
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shh-the robot is sleeping
 
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Ishmitz, Thanks - Missed your post and just came back from the garage. I had the leads on each end of all six plug wires. All six were showing between 1300 and 1600 units of something. Perhaps nothing based on your comment above. However, #3 this time did flicker between OL and 1500-1800 or so.

Understanding I may be on the wrong setting, can this still be indicative of one bad wire?

Should I test all six again on 20K?
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Old 09-12-2018, 04:07 PM
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Yes, re-test and then check if the reading changes due to poor contact between test lead and wire or because you manipulate the spark plug wire. I am assuming you disconnect the wire at the distributor and from the spark plug and measure the center contact at each end, right.

There is an internal resistor in the connector of the spark plug wire that is 3k Ohm. If there is poor contact internally this could cause the meter to jump.

However, even if the meter reads correctly the wires could still have internal issues. Remember the resistance measurements happen with very little voltage and very little current. In operation there is high-voltage and this behaves differently. If can have an internal pathway that only shorts the high-voltage.
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Old 09-12-2018, 04:22 PM
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Quote:
Originally Posted by ischmitz View Post
Yes, re-test and then check if the reading changes due to poor contact between test lead and wire or because you manipulate the spark plug wire. I am assuming you disconnect the wire at the distributor and from the spark plug and measure the center contact at each end, right.

There is an internal resistor in the connector of the spark plug wire that is 3k Ohm. If there is poor contact internally this could cause the meter to jump.

However, even if the meter reads correctly the wires could still have internal issues. Remember the resistance measurements happen with very little voltage and very little current. In operation there is high-voltage and this behaves differently. If can have an internal pathway that only shorts the high-voltage.
Yes, I disconnected the wire from the plug and distributer. I made contact with the leads on the metallic circular clip, for lack of a better word, at each end.

Ill retest tomorrow at the correct setting. Thanks again.
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Old 09-12-2018, 04:32 PM
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I doubt four year old wires are bad but who knows.

Here is how you check. Wait until it is DARK. Start the motor, no lights. Make sure your eyes are adjusted to the dark. Look at your motor. If the wires look like neon lights, your wires are toast. You may also see some arcing if the cap or coil is causing a problem.

FWIW, I had a similar issue, it was the rotor. Check closely for carbon tracks on the rotor.
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Old 09-12-2018, 04:52 PM
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Quote:
Originally Posted by Trackrash View Post
I doubt four year old wires are bad but who knows.

Here is how you check. Wait until it is DARK. Start the motor, no lights. Make sure your eyes are adjusted to the dark. Look at your motor. If the wires look like neon lights, your wires are toast. You may also see some arcing if the cap or coil is causing a problem.

FWIW, I had a similar issue, it was the rotor. Check closely for carbon tracks on the rotor.
Just replaced cap and rotor last month.
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Old 09-12-2018, 05:30 PM
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Looks like you had it pointed to continuity. You want it to be set for resistance as Ingo pointed out. You were probably getting an audible "beep" when you took the readings?

Last edited by cabmando; 09-12-2018 at 06:09 PM..
Old 09-12-2018, 05:47 PM
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To test the wires, set your meter to Ohms. If it doesn’t say Ohms on your multimeter, look for the Greek symbol for Omega. Connect one lead to one end of the wire, and the other lead to the other end of the wire. What you’re checking for is called continuity. High resistance will indicate there is damage or corrosion somewhere in the wire. Refer to your wire's manufacturer for acceptable resistance ranges. For example, my Magnecor plug wire resistance is 98 ohm per cm, 3K ohm per ft. +/- 10%.

* Generally, if the measured resistance on standard plug wires is greater than 7,000 - 10,000 ohms per foot, the wire is defective and should be replaced. Some high end wires (like Magnecor) may be considerably less than 7,000 ohms per foot.

Finally, while keeping the meter hooked up to the wire, begin bending and flexing the wire while you monitor the voltage. If the voltage fluctuates, there is a break in the wire.

Plug wire problems can result in misfiring, hard starting, poor fuel economy and even the acceleration stumbling issues you have experienced.

Good luck!
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Last edited by David E. Clark; 09-12-2018 at 06:04 PM..
Old 09-12-2018, 05:59 PM
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The spark plug end unscrews. So if you are getting a different reading on one do an unscrew and reconnect. Don't worry too much about the details, just as long as all the leads ohm readings are much the same. On my 20 year old set I snipped 10mm off the ends and reconnected the plastic, spark plug end, and they are perfect.
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Old 09-12-2018, 06:24 PM
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shh-the robot is sleeping
 
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Updated readings on 20K ohm setting:

#1 2.80
#2 2.78
#3 OL
#4 2.98
#5 2.97
#6 3.03

I am really surprised on #3. The car starts and purrs beautifully. It is hard to believe there is no continuity.

Am I missing something here?
Do the rest of the resistances look acceptable?

The stumbling usually gets most noticeable at full operating temp - 10 to 15 minutes of driving.

Again testing was from spark wire tip at distributer to tail at plug end.
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Old 09-14-2018, 10:20 AM
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well - this wire is definitely not good. It might still get some weak spark depending on how small the gap inside the wire is. But this needs to be replaced, no question.
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Old 09-14-2018, 10:42 AM
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#3 OL is an indicator that the conductor has broken inside the wire or one of the ends has lost its connection with the core conductor. Even with a broken wire the spark plug may still fire in some instances. The ignition creates enough voltage to jump a gap as evident by the design of the distributor and spark plug both having gaps that the electricity has to jump forming the spark. so, effectively when you have a break in the spark plug wire conductor you have increased the total gap the electricity has to jump. At low power settings where the cylinder pressures are low it is easier to jump the gap so the ignition is still capable of firing the plug, but you can then have a misfire at higher loads as the energy to jump the gap increases with cylinder pressure.
Old 09-14-2018, 10:47 AM
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shh-the robot is sleeping
 
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Thanks all, if any one has a good used #3 wire laying around, Im interesting in purchasing it. I would hate to buy a whole set as this one is only 4 years old.
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Old 09-14-2018, 12:00 PM
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As I previously mentioned. The plastic end unscrews. Check the ohms on the plastic resistor and check the ohms on the cable. It it's the cable try chopping 10mm off the end ans screw it back on. Easier than replacing.
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Old 09-14-2018, 02:15 PM
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Bill - you are correct again. It is the plug connector. That is all I need.
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Old 09-14-2018, 06:44 PM
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Are you sure that the red probe is actually touching the threaded part of the connector?
Does the wire itself ohm ok?
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Old 09-14-2018, 07:17 PM
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One of mine died too. I don't know why just one died of old age. Pleased it is an easy (cheap ) fix.
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Old 09-14-2018, 07:31 PM
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Quote:
Originally Posted by pete3799 View Post
Are you sure that the red probe is actually touching the threaded part of the connector?
Does the wire itself ohm ok?
Pic is not the best but, yes for the probe and yes regarding the wire.
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Old 09-15-2018, 04:02 AM
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For the very thrifty:

You can file the top off the plastic part (better if you turn it off with a lathe) and the whole thing comes apart. Usually the resistor and the spring just need a good clean. Seal back up with epoxy.
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Old 09-15-2018, 07:15 AM
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