wgebb

Hey guys, '96 4.0 here. Could anyone give me details on properly testing ignition coil and its circuity? My service manual gives some instructions that I do not understand, and refers me to other procedures which aren't in the book. I'm also unaware of my ignition coil's brand, because it was replaced by the first mechanic I took this vehicle to, and I was not knowledgeable about our beasts back then.

I have confirmed no spark to distributor coming from coil. With a friend cranking the key, I used 12V test lamp with one end hooked up to negative battery terminal and probe hooked up to positive and negative terminals at the coil, both failed to light up the test lamp. My manual says depending on the make, resistance for the coil should be around 1 Ohm. Placing my ohmmeter across the coil's terminals read resistance of 88Ohms.

I have received from advice in a previous thread to trace power upstream from the coil with the test lamp, but as far as I can tell it's hooked up directly to the battery.

DFlintstone

That's about I have, hope it helps! (CPS's) fail and having a spare if changing it doesn't help wouldn't be a disaster)


That's one I don't have nailed down. "Primary" is the 12 volt feed through the coil which is "broken" by (points opening), or the ICU in this case. When that magnetic field collapses, many more winds in the "secondary" (high voltage), part of the coil, creates the say, maybe 35,000 volt spark.

So that primary should have very low resistance (one ohm?) and not be shorting to ground or the secondary. It's my guess when they state below, "12-13,00 ohms on the secondary" that they mean from the coil output terminal to ground.

I do like that bit about seeing if the primary voltage changes at the negative coil terminal while cranking, is a sign that the CPS is telling the puter' to (tell the ICU) to fire the coil. Maybe someday I'll get a fine "pig-tale" wire in there to "back-probe" mine to see what it does all hooked up.
COIL.
Roughly 1 ohm on the primary and 12,000-13,000 on the secondary.
Can't always find a bad coil with a resistance test though. They sometimes hold together fine on the low voltage from the DMM but short like crazy under high voltage.

You should have battery voltage (12-13V) on the positive primary coil connector and about half that on the negative while cranking. If the positive and negative primary connectors show the exact same voltage while cranking, the ECU isn't trying to fire the coil. That's often a bad CPS

wgebb

This is great information, thank you. The thing is, the CPS is new and I installed a Mopar part. My mechanic also replaced the Cam Sensor. It's possible my multimeter is bad. It's digital, which from my readings on this forum I've learned is not the ideal instrument.

I would love some information on primary vs. secondary parts of the coil. I see an output (the main output going to distributor), the input (wiring harness) and the two terminals. I'm college student and a mechanical engineering major, not an electrical, so I'm not really sure of the difference between the primary and secondary windings in the coil, considering I see just two terminals on my unit.

wgebb

Additionally, I'm sure this is not a fuse issue because my A/C works very well, and I have swapped the A/C fuse with the ASD fuse and had no difference in behavior (crank no start).

When I placed my ohmmeter across the terminals of the coil, the leads were still on, so I will update after using this info to test some more. Confusing to me though is why you ask if the primary terminals of the coil were "unplugged". Are you referring to the sensor? Or the wires that connected to the terminals via nut?

CCKen

My two cents:

wgebb

Thank you for that helpful diagram Ken. Before I set out to use these testing procedures today I wanted to try and start the car, to make sure it was still giving me crank no start condition. To my despair the Jeep started right up. I could let it idle for a long time, or drive it around until it dies again, but today I noticed a sizable coolant leak that hasn't been there before, so I don't really want to take it out.

CPS (Mopar), CamPS (Airtex) and coil (unknown) are all new. ECU is new to me-pulled from a junkyard, has been flashed and gives no fault codes. I'm frustrated at this point because sometimes the vehicles behaves properly-I drove for an hour on the freeway last Monday- and sometimes it will start then die within seconds, and sometimes crank and crank to no avail. Spark from the coil to the dizzy is intermittent.

Just confusing for me because it seems like the ECU is out of whack. I can't say for sure it's not the coil because I haven't tested it fully. But the part is new, replaced when I first broke down.

Also out of curiosity, what are the two terminals that are located below the ignition coil mounting screws for? Seems one wire goes to the PDC and the other to the battery. I thought these were the ignition coil terminals, which is why I thought it was hooked up directly to the battery.

CCKen

The two large cables at the coil mount studs are ground wires. One should go to the battery NEG post and the other to the alternator. A short ground wire should go from the battery NEG post to the right inner fender


There should be a Red cable that goes from the battery POS post to the PDC and one going to the starter. A positive cable goes from the alternator to the PDC as well.


You should have a gaggle of wires at the stud for the oil dipstick tube, and at the left inner fender.


Bad grounds can cause lots of problems. You should remove the battery NEG post connector then disassemble each ground point and polish the wire ring terminals, then secure. them