My 93 F150 w/6cyl won't start when its wet outside. I purchased it new and it now has 125K miles. It cranks but no spark. On a dry day it starts and runs fine. I first changed the ignition module on the fender. I then sent it to local garage and they changed plugs. I had it back 2 days and it rained and wouldn't start. I changed the distributor cap, rotor, and ignition wiring. Wouldn't start next rainy day. Back to local garage - he changed fuel filter. Next rainy day I had it towed to local Ford garage. They changed a fuel pump relay, EGR valve, and oxygen sensor. Next rainy day - no start. Had it towed back to Ford. They kept it 4 dry days and it started every time. Now its home and the rain has returned. I suspect a moisture problem - but have no idea where to start. I have about $700 invested in the problem and can only drive on dry days. Any and all ideas will be appreciated.

 

Go outside on a dry night start the truck pop the hood and look for arcing.
Whatever arcs fix -seal. (You want it to be dark. you may need to wiggle a couple of wire to find an issue). Reseat the distributor cap.
After that maybe try some dielectric grease on the ignition components connectors.

 

Definitely reseat the cap and use dielectric grease on all connections that should cure it.

 

You may have a weak Hall effect module in the distributor. This provides the PIP signal to the TFI circuitry (and to the computer). If moisture is getting into the module, it may be taking out the ignition trigger signal at its source.

The PIP signal is routed from the distributor over to the TFI module on the fender. It runs through a shielded cable. The shield has been known to cut into the insulation on the wires, causing an intermittant short and thus stalling/no start. There is a Ford TSB on that subject.

Since the PIP signal is relatively weak (maybe 1 volt), it doesn't take much moisture acting through some cut insulation to take it out, killing the spark. Other ignition wiring, like 12 volts going to the coil or TFI, or the negative side of the coil primary has a higher voltage level and lower impedance, and should be much more resilient in the face of a little moisture.

Two things you can try, the next time it quits.

1) Remove the SPOUT jumper and see if the spark comes back. This isolates the TFI from the computer and eliminates the possibility of the computer somehow interfering with spark generation.

2) Pull the high tension lead out of the distributor, and place it next to (but not touching) a good ground. Use a jumper wire to ground the negative lead to the coil primary (the one without the RFI capacitor connected to it). Turn the key on. Each time you momentarily ground and un-ground the coil primary lead, you should create a spark on the secondary side. This would prove that you have power to the coil, and that the coil is capable of generating a spark (not internally shorted). If you aren't sure which coil primary lead is positive and which is negative, don't try this.

 

That ford dealer sounds like it has real part changers at your cost.
Well one thing you know that it's a moisture problem. This is where you really need a wiring book to chase the wires. Check to see if you have a connection plug at the fire wall for all your under hood wires if so, this could be the problem area. clean as needed replace dielectric grease also your ignition module plugs even your key switch plug. I would chase back your hot running wire from key switch going to the ignition module Hot going in to it. Then you have to pin it with a test light then go and do the same under dash from key switch to hot ignition module put in place a small test LED or light. Then the next rainy day check if LED test light is ON going to ignition module wire coming from key switch if not then,I think the problem would be Key switch.Hope this helps in some way....my 2cent
orich