Coil-to-Distributer Intermittent?

I've been trying to chase down an idle-speed miss. Finally read to use an old fashion timing light to narrow down electrical issues with individual spark plug wires. Well, long story short, could detect mis-fires on multiple plug wires...then I hooked the timing light to the coil lead. Seems it sometimes (frequently) "blanks out" for short periods...coinciding with noticible misses.

Anyone have an idea what might be the root cause? Bad coil? Bad connection between coil and distributer? Any help is appreciated...as always, thanks in advance!

Guess I forgot to provide critical statistics. It's an '89 wagoneer limited (XJ), 4.0L six, AW4 tranny. No engine work at 260k on the clock. Hope somebody has an idea. I replaced plug wires, coil wire, plugs, distributer and rotor within the last year (replaced all at once).

Ohm check the plug and coil wires. I've induced an idle miss, using a non stock coil to cap wire. Wipe out the cap with a clean cloth and get the carbon dust out. Open the hood and let it idle in the dark, look for high voltage shorts.
Separate the coil from the module and clean the corrosion off of the contacts. Push the contacts together carefully (don't break them) to induce some more pressure (they do relax over the years) for better contact. The coil is fairly high amperage, it moves a good bit of current. Volt tests sometimes don't tell the whole story, they are just an indicator often.
Check the in voltage to the module (larger yellow wire), with it connected and disconnected and see if you have much line loss through the connectors or splices.
Ohm test your CPS.
I must have spent a few hundred hours in the last (nearly twenty years) trying to find the idle miss in this or that Renix. Sometimes I'd succeed for awhile.
Grounds can also be important, the ignition coil ground, the head to firewall and the dipstick brace ground. Even the dash ground, drivers side, under the dash near the courtesy light.
More often it is a compilation if little things that add up. Though cleaning my grounds and putting a stock coil to cap wire in seemed to have the more profound affect on my idle, everything after that just made it a tiny bit better.
Mine is smooth now and thumps rather nicely.
Pull a couple of plugs, check the gap and for fouling. Clean them with a brass brush, re-gap and reinstall. One thing about the Renix is when they are working right, the plugs last forever.
If you used anti seize on the threads clean it off and next time use copper paste. Use very little and avoid getting it on the sealing cone or the washer. Definitely keep it away from the electrodes.

Wow...lots of good information there. Maybe you can help further with those of us that are electrical engineers (I'm a civil engineer).

How do I go about "checking the voltage to the module", both connected and disconnected?

How do I "ohm test the CPS"?

I will clean stuff and get a stock coil-to-cap wire (mine is after-market now), and will clean the inside of the cap.

I have had some ground issues in the past..mostly with rear lights and turn signals, and know they can be culprits for multitudes of problems. Will check/clean those.

Thanks again for the good information...I hope I can chase it down!

Don't even ohm test the CPS.
I usually agree to troubleshoot rather than replace, but that damn sensor is
capable of so many things. First while it's idling fine, wiggle or move the connector. They are well noted for problems as well. IT's a three pin connector coming up from the rear drivers side of the engine, the very rear.. Also that wire is often pinched or rubbed about 8 inches above the sensor if it hasn't been strain reliefed right. Mine was actually wedged in tight against a transmission line and rubbed through. That will kill everything.
I would lean heavier on the connections or burned insulation.

Flyfisher said:

Wow...lots of good information there. Maybe you can help further with those of us that are electrical engineers (I'm a civil engineer).

How do I go about "checking the voltage to the module", both connected and disconnected?

How do I "ohm test the CPS"?

I will clean stuff and get a stock coil-to-cap wire (mine is after-market now), and will clean the inside of the cap.

I have had some ground issues in the past..mostly with rear lights and turn signals, and know they can be culprits for multitudes of problems. Will check/clean those.

Thanks again for the good information...I hope I can chase it down!

Click to expand...

The large yellow wire going to the ignition module comes from the ignition switch, through more than a few connectors and splices. Unplug the connector to the ignition module and with the key in the run position, do a voltage test to battery ground. Test your battery across the poles. The difference between the two readings will tell you how much line loss you have. A couple of volts is typical.
Put a small cut in the insulation on the yellow wire, plug in the connector, start the motor. Do a voltage test from the cut to the battery ground. Much below ten volts (actually 9 or so) and the coil isn't going to produce a very hot spark. 6 volts or less and the motor will do some serious misfiring. These voltage numbers aren't definitive (or absolutes) but indicators. Low voltage (pressure) or low amperage (flow) will cause low high voltage spark which can cause misfires.
If you pull a plug, open the gap to around 0.060 and ground the plug, start the motor and look at the spark closely, you can see a mostly blue spark crisp with little splatter. Do the same with a plug that has low high voltage and it will look thicker, yellow and splatter.
This test is best done wearing rubber gloves, with your crotch well away from the fender.
The advice you got to inspect the CPS wire was good advice. I covered my CPS wires with plastic cable sheathing and cable tied them to the speedo cable a long time ago (on my last three XJ's). Learned that lesson the hard way.
The Crank position sensor should ohm test to 200 ohms plus or minus 75 ohm's. When they get near the outer tolerance edge they can cause misfires and other intermittent problems. The CPS I have in mine now reads 292 ohms hot, but starts fine and runs good, Renix is more of an art than a science. I'm a firm believer in the if it isn't broke don't fix it philosophy.
The connector between the coil and the module (under the coil), is out in the open and has a tendency to corrode. When it corrodes it can get hot from resistance, which can relax the connector. Worth a look, it only takes a few minutes to separate them and a torx driver. I usually pull the whole module/coil and separate them on the workbench. Grease the screws/threads before you put it back in the engine compartment and it will be even easier to pull and separate next time.

Thanks again for the great how-to advice. I should have time tonight or tomorrow night to run through the tests. I'll post here how they come out!