1991 GW Ignition Failure

[FDSJr]

My 1991 Grand Wagoneer has an ignition failure -- no spark. Before I dig too deeply into it, I thought I would get a few opinions. What is the most common culprit in this situation -- ignition coil, ignition module, or....? My service manual says that the primary resistance on the ignition coil should be around 1.11 to 1.25 ohms. Mine is measuring 2.1 ohms. Is this high enough that I should suspect the coil?

[jpcoutts]

The testing you've done may indicate otherwise, but theses rigs are prone to failure of the ignition module, particularly if it is not a factory part. Aftermarket for these parts is really iffy.

[tgreese]

Ignition coil is pretty easy to test. Power to the coil, ground and lift the ground to the primary. This should make a spark at the coil wire. Or swap to another car and see if it still runs.

Testing low resistances with a multimeter is difficult. I would use a functional test instead. Most cheap multimeters are not very accurate near zero ohms. If you measure, connect the leads together and subtract that measurement from the measurement of the coil.

The TSM should have a chart to test for no start. Did you look there? '89 TSM here should be pretty close. https://oljeep.com/edge_parts_man.html Without a dedicated tester for the module (no one would have that), you eliminate everything else and that points to the module.

[FDSJr]

Both the coil and the ignition module were replaced around May-June 2005, so they have been in there for over 15 years. Considering that they are both relatively inexpensive parts, I might just replace both of them, hope that the problem is solved, and be done with it.

[FDSJr]

But, I will test the coil first. The fix will be a lot cheaper if that is the culprit and I only have to replace the coil. Thanks for the link to the online manuals. The diagnosis chart for the '89 is very similar to the one in my '91 manual.

[tgreese]

A coil is a good spare to carry in a Jeep. Could save you from being stranded a long way from service. BTDT. No great loss if you buy a replacement coil and that is not the problem.

[61Hawk]

Pick up an ignition module, if you haven't replaced it you will soon enough. I used to carry a spare because they last about 12-18 months. You learn early on that you can swap these out on the side of the road in about 5 minutes until you can get to a place where you can bolt it down. I got tired of swapping them out and having the truck die for no apparent reason whenever it felt like that I put an MSD 6AL in it and haven't had any issues since.

[FDSJr]

I tested the ignition coil and it appeared to be bad. I replaced it with a new one and there is still no spark (although I did not test the new coil -- just assumed that it is good).
I am moving on to the ignition control module. I picked up a new one, but I can't remove the fasteners that hold the old one down on the fender well. The thing has been in the GW for about 15 years so the fasteners might be frozen in place. Do I need a special tool for this? It appears that the posts on the fasteners are square (somewhat).
Based on the comment from 61Hawk, it sounds like I could just plug in the new one to see if it will solve the problem and then work on the fasteners later. Correct?

[FSJunkie]

Yes you can just plug in the ignition module without mounting it. It will function just fine without being mounted.

The module is easily the most common cause for no spark or weak spark. Typically the modules wok when they are cold but stop working when they are hot. Rarely do they fail 100% but it happens. Sometimes they just get weak and will misfire under heavy throttle or high RPM or will simply cause a loss of engine power. They can be sneaky and slowly get worse so you don't notice the loss of power until you replace it. New modules by any manufacturer tend to be crap. They work fine for a fear or so then die, thankfully they are usually under warranty still. I prefer to keep original Motorcraft modules. The bad original modules are long gone by now, so the survivors are the good ones. If they've lasted this lon, they probably will keep going. You always carry a tested and verified to be good spare module in your vehicle. Always, always, always. Or two.

Ignition coils fail less often. Coils tend to either be 100% or 0%. They either work or they don't...usually.

Distributor pickup coils rarely fail.

Wiring or connector problems are always possible, but they are usually caused by humans.

[Towtruck]

I'd stick an HEI distributer in it and forget about it for twenty years. Look on Amazon. (Remember to swap in the old drive gear...) Send me a PM if you go this route.

[tgreese]

...
Wiring or connector problems are always possible, but they are usually caused by humans.

My experience with this indicates the wiring is a frequent problem. The connectors of the Motorcraft ignition could require attention. Corrosion seems common, and failure due to corrosion happens - BTDT. Suggest you unplug and clean the connectors between the distributor and the ignition module. The connector to the distributor is of known poor quality and fails by corrosion and by crumbling into pieces - BTDT. The harness side is probably ok, and you get a replacement for the distributor side with a new magnetic pickup coil.

[babywag]

Most of the issues I have seen/found/fixed were wiring/connector related.
Yes, aftermarket modules aren't great.
If connectors or wiring is bad it can kill modules.

The stock duraspark connectors are poorly designed.
Add decades of exposure to underhood environment and temps and they along with wiring degrade/fail.

Coil connector is also a problem child along with wiring of pickup coil.

[Bob Barry]

Most of the issues I have seen/found/fixed were wiring/connector related.
Yes, aftermarket modules aren't great.
If connectors or wiring is bad it can kill modules.

The stock duraspark connectors are poorly designed.
Add decades of exposure to underhood environment and temps and they along with wiring degrade/fail.

Coil connector is also a problem child along with wiring of pickup coil.

A 1991 would use the newer (more expensive) ignition module with the Weatherpack connector, so that fail-point has been remedied by that point. The basic ignition-module remains the same fail-prone unit.

The unit bolts in from underneath the fender; the mounting-bolts are 3/8", and pretty accessible.

[babywag]

True, but the distributor pickup wiring (even in 1991) still used the craptastic duraspark style connector.

[Bob Barry]

True, but the distributor pickup wiring (even in 1991) still used the craptastic duraspark style connector.

That is absolutely true.