So this is new...my '84 F250 won't crank with the key switch. I turn the switch to the start position and nothing happens. This problem first appeared about two weeks ago. I tested the S terminal at the solenoid and there was no power, so I thought the problem was possibly in the clutch lockout switch. I took the switch apart and cleaned the contacts, put it all back together and everything worked normally again for two weeks so I figured the problem was solved...until today. I drove the truck to work today. It started normally this morning but when I went to start it tonight to go home, same problem. If I jump the S terminal to the battery side of the solenoid it fires right up but again, there's no power to the red/blue wire at the solenoid. After I got home I tested the clutch lockout switch and there is power on both sides of it with the pedal depressed. Just to eliminate the switch as a possibility I hooked a jumper wire between the pins on the harness but still no crank. I'm obviously losing power somewhere between the clutch lockout switch and the solenoid, but where? Is there another connector under the dash somewhere? The harness with the red/blue wire breaks out of the firewall near the evaporator case.

 

There are other connectors, as shown below. That's from pages 30 and 31 here: Start & Ignition - ???Gary's Garagemahal. You can see connectors C121 and C128 in the schematic, and then at the bottom you can see where they are. And, C121 is depicted in Figure 2 on Page 14, while C128 is in Figure 3 on Page 35.

By the way, I've had this happen a few times on Big Blue and still don't know why.

 

I have had it happen several times before, and in each case pulling the elbow connector off the solenoid and then sliding it back on cured it. I think it is subject to moisture and corrosion over there on the fender near the edge of the hood.

Take your tester and check over there, if you have power you may have a grounding problem with the leg of the solenoid also.

 

That connection is clean and tight. Whenever it does this I can hook my test light directly to the red/blue wire at the solenoid but there's no current there. I do have power going through the clutch lockout switch so that verifies the crank circuit is working coming out of the ignition switch, but I'm apparently losing it somewhere between the clutch switch and the solenoid connection. I'll try to take a closer look at the wiring and check those connectors this weekend if I have time.

 

I do know after it comes through the firewall it goes to a plug that plugs into a harness that goes down underneath to the trans. In your case with a manual it still plugs into this harness, but it has a jumper right there at the plug and it leaves and continues on over to the solenoid. If you had a automatic they would have a jumper where you clutch switch is located, and the auto harness would not have that jumper, it would run down to the neutral safety/back-up switch.

This transmission harness plug is right underneath the booster area.

 

P.S. The connector with the jumper for the trans harness must be c128 in Gary's diagram above. You can see in the description list c128 is supposed to be near the proportioning valve.

 

Yup. Here's the illustration from Page 35.

 

Yeah, I had completely forgotten about that jumper located inside the frame. This truck was originally an automatic and when I changed it over to a manual I faintly remember plugging that jumper harness in to the connector where the neutral safety switch originally went. That will be the first thing I'll check this weekend.

 

Mods (that are relevant) are not only the first thing to mention when starting a thread, but the first thing to check when troubleshooting.

 

I converted it to a manual transmission when I restored it, but I used all of the parts from the donor truck. Everything was bolted and plugged in from the other truck so nothing in the wiring has really been modified from stock.

 

The problem was the C128 connector. I tried it again this morning but it still wouldn't crank over, so I crawled under the truck and unplugged that connector, then poked a jumper wire between the two pins with the red/blue wires and it started right up. No signs of corrosion or broken wires so it must have been losing contact between the connector pins. I spliced the wires together on the back side of the connector so it shouldn't happen again.

 

Glad you found it. I'll have to check the one out on Big Blue given that he has the same symptoms.