Just joined you guys. You are my last hope of getting to bottom of this annoying problem.
1979 Ford F-150 4 x 4 with 300 6 cly. 4 speed. Up until 2 years ago, no starting issues with truck. then last year wouldn't start once it started snowing. found dist cap ajar with broken clip. Replaced it and rotor, still no start. Here comes spring, starts right up! Have gas flow, changed voltage regulator, starter relay, then noticed resin leaking out of ignition module, replaced that, then low and behold truck started right up. Thought problem solved, wrong! As soon as it started to snow it would not start again?
Could it be the coil, or ignition pick up coil in distributor. I did check for spark when cold out, not a very strong spark. Battery is good.
Live in southwest mi. This was my plow truck! Can someone please help?

"Frustrated"

My thought would be spark plug wires or ignition coil?

Welcome to FTF, I second the ignition coil.

Check every connector I had a 85 Ranger that as soon as it hit 20 ° outside no start. Had a conntor that was dirty and as soon as the temp dropped it lost connection. Cleaned it up applied dielectric grease and it started right up.

I'm assuming that the truck has outside storage. As others have said it is likely ignition related and the problem is likely temperature/moisture doing the deed. Though uncomfortable, you'll probably have to wait until it fails to start and then troubleshoot the primary side of the ignition first. Any connection could be the culprit. Check them for continuity. If the coil is actually generating secondary voltage your plight should be easier. When I was young I knew a fellow who had a '50s something Chrysler product that didn't like to start in high humid conditions (fog or rain). That was due to too much moisture in/on the distributor cap. Best wishes.

When it won't start, warm up the coil with a hair dryer then see if it starts.

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Thanks for all the input! Could someone please tell me, or show me a pic of how to remove wiring harness off coil. I can't seem to figure out where to insert screw driver or what to remove harness. It's awkward, cause top of harness faces front windshield!
Thanks again and Happy Holidays

If it is an OEM unit, the wiring connector should slide off of the coil from right to left.