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This is coiling me

Dear All,

The coil on my 93 Jeep Cherokee has died 3 times within the last month. It’s very frustrating seeing that my car breaks down on the highway going around 60 to 70 mph. The first time it happened my mechanic figured it was the coil but couldn’t figure out why it was consistently dying. So far I’ve changed the spark plug wires, distributor, and the rotor. I’m about to take another long trip so I’m a little pensive about traveling not knowing what to expect. Do you have any suggestions?

Are we talking three consecutive coil packs here, or is this the same coil that keeps breakong down?

If it’s the same one, change it. Coil windings are coated with a varnish like catting that can develop microscopic cracks over time. Then as the coil heats up and the copper expands the cracks open, the windings short, and the coil fails.

If it’s three consecutive coils, you may want to look for another cause, such as an exhaust leak cooking the coils (heat shield missing?) or even a crank angle sensor. Remember that without signals telling the ECU to open the primary circuit the field will never collapse and you won’t get an ignition pulse out of the coil.

Some some reason the coil is getting the wrong amount of current or voltage. This is an electrical problem. Changing spark plugs, plug wires, rotor etc. isn’t addressing the problem. It is a problem with the current going into the coil not out of it.

Yeah, but I’m wondering if they’re saying it’s a bad coil because there’s no spark pulse off the coil without ever checking for the squarewave on the primary. A coil with constant DC voltage applied is useless. It’s the field collapse that does magic.

I also can’t tell from the post if he’s talking about three coils failing or one coil failing three times.

I hope he comes back.

The coil is being damaged because too much current is running through it for some reason. I don’t know if this system used a ballast resistor or not but if so that needs to be checked. The resistor reduces the voltage to the coil in the RUN position which also reduces the current to safe level.

Thanks for your reply. In answer to your question it’s three coils failing. The last two within a week of each other after a two hour highway drive.

The coil is frying because the ignition module is failing to go into the run mode once the engine starts.

When starting the engine, the ignition module goes into the start mode which allows full battery voltage to the coil. Once the engine starts the ignition module goes into the run mode. The ignition module then drops the voltage to the coil. If the ignition module fails to go into the run mode while the engine is running, it allows the full voltage from the battery to the coil which can be up to 14.5 volts because of the charging system. And if the coil see’s that much voltage it won’t last long.

The ignition module is located below the coil.


Dear all thanks so much for your responses,
Hmmm? All this makes so much sense except that the picture above doesn’t look like the coil I have. It’s just a wire coming from the distributor cap and one going out, I assume to the battery? My earlier research mentioned the heat issue but there’s no heat shield around the coil at all. Note, when I looked inside the old distributor there was a lot of build up on the points where it sparks. This all has been really helpful and I plan to discuss with my mechanic soon.

Jeeps have a long history of using whatever ignition system was available at any given moment…Motorola, Motorcraft, Prestolite, Mopar, every model year, something new, something strange…

Odds are the coils are not failing and the problem is elsewhere; more than likely a loss of power to that coil. A look at the schematic shows the coil is not only powered by a resistor but also a resistor relay that operates through the ECM and that’s speaking about it in simple terms. Talk about overkill.

Even the ignition switch could be at fault.