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Variance in Primary Ignition Coil Resistance Measurements / Time to Replace Coil?

To Whom It May Concern:

Greetings and salutations to all! I am having some trouble with my trusty ol’ 99 Chevrolet Suburban LT and need some assistance, please.

My truck refuses to start unless she gets a boost. Once that initial boost is given, she runs fine. And she’ll start on her own again…and again…and again - unless left to sit for several hours (i.e. overnight).

The Bus (the nickname affectionately given to my Suburban) is equipped with a new Optima Yellow-Top deep-cycle battery. The alternator is fairly young at about 1.5 to 2 years old. I just replaced my spark plugs and wires. The dashboard voltmeter reads 12-volts with the key in the ON position. I figured that ruled out those three sources of trouble.

I measured the resistances of the primary and secondary windings of the ignition coil; the secondary coil read 5.85K, rock-steady; however, the primary coil read from 0.1 to 0.8 ohms in succession. The primary coil reading would start at some random point in the aforementioned range and climb to its’ peak of 0.8 and fall to 0.1, repeating the process ad-nauseum. There is a consistent variance in the resistance reading I am receiving from the primary winding. I’m thinking a voltage is being built and stored by the primary coil and released upon reaching the bias-point established by the winding’s insulation. I could be wrong, though.

Ultimately, I am an amateur at fixing my own vehicle; please forgive any errors, mistakes or misconceptions I have. Attribute them to naiveté. I am wondering if anyone reading this would be gracious enough to share some knowledge with me and tell me whether or not I need to replace the ignition coil. Or the ICM, or the distributor cap or the distributor rotor or anything in the electrical path of ignition that is keeping my baby from starting on her own. :disappointed:

I do appreciate everybody’s time spent reading this and any and all help provided. Thank you. :+1:

  • southsidesmoka

Are you making these measurements with all wires disconnected from the coil?

By ‘refusing to start’, do you mean it cranks and cranks, but won’t catch? Or that it makes no sound? Or it only clicks?

Because it will start with a boost, I suspect you have one of two things going on; a parasitic drain or compromised connection(s).

A parasitic drain can easily be verified. Slowly draining your battery over time of non-use until it reaches a point it cannot adequately power everything well enough to start.

A bad connection can appear good once it’s warmed up by current flow and voltage drop heating up the connection. Left to stand overnight, it becomes bad enough to prevent starting.

1 Like

Coils usually fail hot before they fail cold so I think you are on the wrong track there.

Does your engine crank normally, slowly or not at all?

If it is slowly or not at all you have a charging system problem or a parasitic drain.Or poor ground connections.

If it cranks normally, you have a spark or fuel problem.

Remove the cap and rotor from the distributor and check for carbon tracks on the inside of the cap and pitting on the end of the rotor. The tracks look like cracks look like cracks and will allow a warm dry ignition system to work, but in the presence of moisture like damp overnight air, convey the spark to ground instead of the spark plugs.

Try turning the key on in the am and waiting 4-5 seconds before starting the engine. If that works you have a fuel delivery problem, could be leaking fuel injectors or a weak fuel pump.

Yes sir, all wires are disconnected from the coil during testing. She cranks and cranks and cranks, but won’t catch. When started with a boost, she is slow to catch.

Again, if it starts with a boost, always, that limits the possibilities to the battery (bad or discharged) or the battery connections.

When you jumper the battery, usually one connects to the battery clamps, not to the battery terminals themselves, as they are not accessible. Thus starting with the jumpers could indicate a bad connection.

A discharged battery, as has been mentioned, could mean a parasitic drain on the battery, or a bad battery that is discharging internally.

Use a voltmeter to measure the voltage on the battery before you try to start the car, and during start attempt. Touch the probes directly to the battery terminals, not the clamps. If the voltage is good, repeat, measuring at the terminals.

With readings this low, the pressure from the meter probes can cause this much variation easily. If you use clips to connect and don’t move anything, do you still get this variation?

edit: by “don’t move anything” I mean put the coil and meter down on some surface and keep you hands off. Too many times I have gotten false readings by relying on the meter probes and my hand pressing them onto a contact.

When you set about cleaning the connections at the battery and the negative (-) lead is disconnected, take the time to trace the hot wire (from the + terminal) down to the starter, and disconnect and clean that contact, too. Also, there is a ground connection from the engine to the frame somewhere and both ends of it should be cleaned, too.

I have a hard time believing it’s the battery since it’s turning over strong. Turning the car over via starter motor is by far the largest drain on the battery… If that’s working fine then the battery should be fine.

The battery [I]is[/I] only a few weeks old and the vehicle has not been driven since it’s installation; it has however, been started with a boost and let run for a number of moments. She’s never cut-off on her own, either.

Yet it starts with a boost even though it’s cranking. Pretty sure this vintage is HEI. They, in my experience, are quite sensitive to good bus voltage. You can get a starter motor to turn over just fine but only have 10 volts while it’s turning. HEI doesn’t like that…the boost from an external battery is enough to get it going.

And by the way, if you DO have a parasitic loss or have run your new battery down cranking, you should trickle charge it all the way back up. Otherwise, you’re running on low charge state and new or not, it’s probably going to struggle starting the truck…

Nope . . . fairly conventional ignition system

Separate distributor and coil, definitely not the older HEI

I doubt any auto made would be designed to fail if the battery dropped to 10 volts. That is something that occurs every time the starter is cranked in cold weather.

What is HEI? High energy ignition?

The threshold for PCM operation is generally around 10 volts, It is designed to shut down for self protection if the voltage is very low.

OK, thanks for clarifying. I certainly trust your opinion and facts!

See how the coil is on top of the distributor cap

Hmmm…battery voltage is steady at 12-volts across unconnected post terminals located on the top of the battery. The side post terminals in use give the same, consistent reading. It is a new battery with clean, corrosion-free terminals.

A fully charged battery at rest should be 12.6-12.8 volts. What does it measure while cranking?

I am still reproducing the 0.1 - 0.8 ohm variation in primary coil resistance reading, even when clamping it on and putting it down. :worried: Is this, indeed, a problem as I suspect or normal conditions?

Readings were taken this morning in cooler South Carolina temperatures (if there is such a thing) and just now, around 5:00 PM; there was about a fifteen degree variation between outdoor temps this morning and this evening. Ambient and operating temperature are appearing to have no effect on this issue, as far as I can tell.

Thank you for your admonition.