1969 F-600. 330HD and standard ignition (not transistorized). Drove the truck into my shop in Aug 2024. Pulled the heads and intake, made the needed repairs, and put it all back together (mostly). Life got in the way and the truck has sat until now. Got the rocker assembly put back on, and wires hooked up this week. Engine won't crank. It will crank if I pull the brown wire off of the starter relay and ground that stud ("I" stud). I can also run a jumper from the I stud straight to the coil + stud and it will crank just fine, but, if I reconnect the relay and coil wires (jumper still in place), it won't start. I can hear a slight click, but I'm not sure where it's coming from. I don't think it's the starter relay, but hard to tell from in the cab. I've checked all of the wires and connectors. I cleaned all the connectors. I have good continuity on all of the wires. There is a resistor wire that runs parallel to the coil circuit. It comes off of the brown wire and goes to the ACC terminal of the starter switch. It measured 59.9 ohms, but I don't know what it's supposed to be. Also don't see how that wire would affect cranking. Also checked the coil (1.4 ohms) and the ignition switch. The starter relay is brand new since the old one gave out. Battery is good. Hi-tension wire is off. Not trying to start the engine at the moment. Just trying to get it to crank over as it should. The schematic I drew up below comes from a F-100 thru F-350 Ignition, Starting, Charging & Gauges schematic. My F-600 wiring, at least this part, is the same as the light trucks.

 

Here is a wiring diagram for smaller trucks but I suspect your truck will be the same. The coil circuit has nothing to do with the start circuit. The start relays are energized by the ignition switch. Disconnect the wire on the "S" terminal and connect a voltmeter or test light to it, it should have 12V on it when the ignition switch is in the start position.
wiring-1969charging2.jpg (1780×1265)

 

(On vintage Fords only: see below*) The fender-mounted starter solenoid has two large terminals (BAT and STARTER) for the heavy wires to pass current from the battery to the starter: that's its main switching function.

The small 'S' terminal is the trigger wire from the ignition switch.

But the 'I' terminal goes from the solenoid to the ignition coil '+'. Its purpose? The coil is fed ~8v from the ignition switch via an in-harness dropping resistor, AKA a "ballast resistor". Running the ignition coil at ~8v reduces points wear, but when the engine is being cranked by the starter, that load can drop battery voltage (system voltage) from a nominal 12.6v to under 10v, which would reduce coil voltage from ~8v to under 6v. The 'I' terminal connects battery voltage directly to the coil '+', bypassing that ballast resistor, restoring a higher voltage to the coil only while cranking. It both compensates for sagging cranking voltage and provides a hotter, more stable spark during start-up.

The 'I' terminal does nothing when the truck is running, though it's back-fed from the coil, so you can read 8V on it with the key ON but not cranking.

All this went away with later electronic ignition systems, though some early electronic systems kept it in place.

*There exist starter solenoids that look identical to a Ford version, but for which the second terminal ('I' in Ford-land) does not function as I described above. Instead, it functions as a ground for the solenoid's internal coil. These are typically found on industrial equipment. I've seen people accidentally install one of these in place of a Ford solenoid, and it won't work. All this to say: did the truck work correctly with that replaced solenoid and prior to it not working? If not, you may have an incorrect solenoid installed.

It's easy to push the wires on to the wrong terminals, too, reversing the 'I' and 'S' wires. Nothing much will happen when you do that, but it won't crank. A test lamp that you can see while operating the key is very helpful to check whether the 'S' terminal is receiving juice when the key is in the START position.



To see an animation of this, click here:
https://asavage.dyndns.org/Ford/F600...9_Animated.png
It has three screens that change every four seconds.

 

The old solenoid worked until it didn't. Apparently, I cranked the starter for too long or too many times and it decided to weld the big lugs together. I replaced it with a heavy duty solenoid from NAPA. I didn't know what you explained above about the Ford solenoid. I will be going into town tomorrow morning to get a Ford style solenoid. About the only other option that I had left was to get up under the dash and see if the mice had gotten to the wiring harness. I'll probably still do that anyways since the truck has sat for so long. Appreciate your help.

 

I added a simplified and marked-up version of the 1969 Light Truck wiring diagram that @Crop Duster posted earlier. I removed everything but the ignition and starting wiring, and colorized the circuits. This forum's image handling can't pass through an animated PNG without losing the animation, so I posted a link to an animated version; modern browsers will show the animation (tested with Safari & Firefox).

It's fairly easy to install the two small wires on the solenoid reversed. Make sure your Red w/blue stripe wire is on the terminal closest to the front of the truck:





(that's the NAPA "Select" (cheap) solenoid on my '73 F600. The wiring colors haven't changed. That solenoid came on the truck and I'm leaving it be; when it dies, I'll buy an Echlin to replace it.)

To test the solenoid, remove that wire (Red w/blue stripe: R-B) and jumper 12v to the solenoid's terminal ('S') from the big battery cable. The solenoid should clunk and the starter should run. If not, you have a failed solenoid or the solenoid is not grounded well to the fenderwell -- I have not seen that last problem myself, but heard of someone who'd mounted the solenoid on rubber mounts, so the solenoid's coil had no ground.




Unless you have an automatic transmission or aftermarket equipment, I don't think you'll find a relay in the dash for the starting circuit.

If the test above results in the starter working, then the issue is between the ignition switch and that R-B wire. Usually, it's connectors or the ignition switch. Rodents frequently eat and damage wire insulation, but rarely chew threw the copper wire, though . . . I have seen it (90's T-bird injector wiring).

 

My problem was due to my misunderstanding of how the starter solenoid worked. I understand it now and bought and installed the correct solenoid this morning. The truck started up and "ran ok". I say it like that because I still need to get the carburetor adjusted properly. And now the radiator is leaking, so I'm ordering a new radiator tomorrow and will get back on it as soon as I get that installed. Thank you very much. Your help and your knowledge are greatly appreciated!!

 

If grounding the I terminal on the relays engages the starter you have the wrong relay, the relay was used on 57-64 Ford tractors that used a grounding starter button
This relay has a isolated ground that uses the I terminal to ground the relays coil to engage the relay
You need the automotive relay with the grounded base that uses the I terminal as a feed back to the coil when the relay is engaged
This issue is the opposite for older tractor owners that get sold the grounded base relay and can’t get their starter button to work

 

It doesn't help that they look the same I've run into a couple variants -- though not on Fords -- that look like a Ford solenoid but instead of I and S terminals, the internal coil isn't grounded at all: those two terminals are the source and ground connections. Common on industrial stuff.

 

That’s the isolated ground relays I mentioned that Ford used on ag tractors prior to 1965