Quick question - ohm readings on coil
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Cross-Country
Joined: Mar 2012
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Quick question - ohm readings on coil
I have a 1996 F350 7.5 with a no spark thing going on. I ran it to town week before last, and when I got home it wouldn't idle very long before dying. Now it won't start - no spark detected. I have good fuel pressure - both tanks. I'm getting a good electrical signal to the injectors. Coil is energized. I took the coil out and I'm getting 3.8 ohms between the primaries, and nearly 35.900 ohms primary to secondary. I checked it with two multimeters. What do you guys think. Is this coil out of range?
Cargo Master
Joined: Mar 2007
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Checking the resistance of a coil never has worked for me. It can tell you it's no good, but it cannot tell you its good.
All coils work on power and ground. When the ground is removed, there is spark. I always test an ignition system with a test light. One lead to the power side of the coil, and the other lead to the grounding side. When the engine is cranking, the test light should flicker. If it is not, then you know it's the trigger side of the ignition system. If it does [and there is no spark] then it's the coil. Weak coils can be tricky to figure out. This is where experience comes into play. The more coils you check, the more you know what to expect.
All coils work on power and ground. When the ground is removed, there is spark. I always test an ignition system with a test light. One lead to the power side of the coil, and the other lead to the grounding side. When the engine is cranking, the test light should flicker. If it is not, then you know it's the trigger side of the ignition system. If it does [and there is no spark] then it's the coil. Weak coils can be tricky to figure out. This is where experience comes into play. The more coils you check, the more you know what to expect.
Logistics Pro
Joined: Nov 2013
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From: Mojave, Ca
Those numbers are alittle suspicious, but not enough to condemn it without doing a side by side comparison with a known-good coil. The numbers I have memorized is 1.5-2 ohm on the primary and 8,000-10,000 ohms on the secondary.
If I was in your position, the first thing I would check would be the grounding signal on the negative side of the coil.
With everything hooked up and ready to start, use an incandescent test-light and back-probe the negative side of the coil connector. Connect the other side of the test-light to the battery + terminal. Try to start the engine. Does the test-light flash rhythmically as you're cranking the engine?
If I was in your position, the first thing I would check would be the grounding signal on the negative side of the coil.
With everything hooked up and ready to start, use an incandescent test-light and back-probe the negative side of the coil connector. Connect the other side of the test-light to the battery + terminal. Try to start the engine. Does the test-light flash rhythmically as you're cranking the engine?
Hotshot
Joined: Jul 2020
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From: Near Salt Lake City
Correct
Those are the first words out of most technicians mouths
A simple test light on the coil negative looking for a blinking light while cranking the motor over has been the test for years and years
That "blinking light test" tests the stator or "profile ignition pickup" in the distributor and key on power to the coil
The coil should be .4-2 ohms on the primary windings and 5-10k ohms on the secondary windings
Fun stuff
Those are the first words out of most technicians mouths
A simple test light on the coil negative looking for a blinking light while cranking the motor over has been the test for years and years
That "blinking light test" tests the stator or "profile ignition pickup" in the distributor and key on power to the coil
The coil should be .4-2 ohms on the primary windings and 5-10k ohms on the secondary windings
Fun stuff
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Cross-Country
Joined: Mar 2012
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I attached a test light between the + battery and the neg. coil wire and got no flashes. I tested the injectors with a noid light a few days ago, and they were flashing. Does the PIP trigger the ignition and the injectors? I've had this truck for 12 years, and this is the first time it has not started. I'm having to crash course this stuff. I don't want to be one of those guys that throws parts at it.
Logistics Pro
Joined: Nov 2013
Posts: 3,928
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From: Mojave, Ca
Yes. The injectors are fired off of the PIP signal. Do you have 12v at the red/green wire on your ignition coil with the ignition on? If so, I suspect your ICM (ignition control module) by the driver side hood hinge is faulty. It should say Ford or Motorcraft on it. Aftermarket, especially duralast, are known to be faulty out of the box or have a very short life span.
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Hotshot
Joined: Jul 2020
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From: Near Salt Lake City
I attached a test light between the + battery and the neg. coil wire and got no flashes. I tested the injectors with a noid light a few days ago, and they were flashing. Does the PIP trigger the ignition and the injectors? I've had this truck for 12 years, and this is the first time it has not started. I'm having to crash course this stuff. I don't want to be one of those guys that throws parts at it.
Alligator clip on the negative battery post
Positive lead on the negative side of the coil looking for a blinking light (you're looking for positive voltage at neg side of coil)
If you hook up to the + battery post, you will dee NO blinking light
Cargo Master
Joined: Mar 2007
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It does not matter where the other end of the test light is on. There is power and ground at the negative side of the coil [when the ground is removed at the coil, the power from the power side of the coil will eventually get to the negative side of the coil and with no ground present, the light will turn on]. Just try it before saying it does not work.
I always hook the test light at the coil because you want to know if there is power and a grounded trigger at the coil. I really don't care if there is power at the battery. I need to know if there is power at the coil. Sure, you can use the battery but a second test will need to be done. Yes, that second test with the test light is faster than writing this sentence.
I always hook the test light at the coil because you want to know if there is power and a grounded trigger at the coil. I really don't care if there is power at the battery. I need to know if there is power at the coil. Sure, you can use the battery but a second test will need to be done. Yes, that second test with the test light is faster than writing this sentence.
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Cross-Country
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Here's the follow up: I did all of the tests you guys mentioned, and got no signal from the ICM. I got a new Motorcraft ICM and put it in. Still didn't start. I checked the connections and everything was good. I checked the spark from the coil and got a very weak spark. I put a new coil in it and plugged in the old ICM - no start. I put the new ICM on with the new coil and it started right up.. Idles smoother than I have ever known. Haven't driven it yet. The tough part was finding some heat sink paste in our small town. Finally got a small syringe of it at a cell phone repair shop. Appreciate all of the help.
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