I have a 1978 F350 with the 400CI in it and am having some serious issues with it, when attempting to start it it will crank over and run for a split second when key is in the start position, yet as soon as it returns to the run position it will cut out and sputter, only way to start it is to run a jumper wire from the battery to the "S" post on the solenoid while it is unplugged and to tap the exposed part of the wire to the actual "S" threaded post on the solenoid and will run until the jumper wire is not contacting the plug anymore, I am at whits end and any help will be greatly appreciated!

 

sounds like the coil isn't getting voltage from the run circuit. you need to check the ignition switch as well as the condition of the resistor wire circuit from the switch to the + terminal of the ignition coil. you can confirm the issue by measuring the voltage on the + terminal with the key in the run position...it should be a few volts under your battery voltage. When the key is in the crank position, you should have close to full battery voltage.

 

So just to be clear here, you’re running a jumper wire from the battery to the coil (you said solenoid) and then using another jumper wire to the S post of the relay/solenoid to crank the starter. Correct?

Since the ignition switch seems to be working fine in START, have you tried running the jumper wire from the battery to the positive side of the coil, and then going into the cab to start it with the key?
Sounds like you’re also jumping the starter to get it to crank. Just curious that the starter still works with the key.

 

What you’re describing could certainly be a bad resistor wire, but more often than not it’s the ignition switch itself.

 

It's got the same voltage in both positions

 

The key ignition does still crank the starter

 

And…?
Did you change anything at the back of the ignition switch?
Since you have tested and found approximately 5V at the coil with the key in the ON position, then connecting the jumper wire to the coil should have proven that, either the voltage is insufficient (if the engine runs with the jumper), or that you have another problem (if the engine doesn’t run).

Try another test.Just to narrow things down a little more.
With everything else connected as it would normally be, disconnect the Brown wire from the “I“ post of the starter relay/solenoid.
Put your jumper wire from the positive battery terminal into the Brown wire.
With the key still in the OFF position, measure voltage at the coil positive. You should see 12v there because the brown wire is normally directly connected to the red with green wire. Since you had a fire, then, of course, this connection could have been broken. If the wiring is intact, or fixed by you, you should see full voltage at the coil.
Turn the key to START and see if it runs.

I can’t scroll back to reread the previous discussion, because I’m on my phone. And I’m guilty of not remembering if you still have the stock 78 Dura Spark distributor and ignition module.
If you do have a stock ignition module, then you will need to test power on those wires as well. the engine won’t run properly, or develop proper spark, if power and signals are not supplied to and from the module.
I’m just going to hit send to this current answer, then go back and reread.
Back later…

 

And instead of creating a new thread (edit: see next post) about wiring problems, you should probably have continued this discussion in the original thread about your fire and questions about which wires did what.
I’m going to have to go back to that other discussion to see what you have. In the meantime, post up what kind of ignition system you have, here in this thread.
Maybe even post a link to your other discussion.

 

Sorry about that. I guess that wasn’t your discussion I was thinking about. We just have far too many discussions about fires right now!

 

It's all good, I'm still too new to this, much appreciated for the help

 

OK, so are you still running the original style Dura Spark distributor, coil, and ignition module on the driver side fender?
If so, you need voltage in the ON position to both positive side of the coil, and the correct wire on the ignition module.
Different things can cause your no-start problem. The most common is probably a failing ignition switch.
Next would be a bad component. Third would be a bad wire or connection.
The switch and the starter relay Brown wire, provide voltage to the ignition system during the START process. But only the ignition switch does when the key is released to the ON position.

So when you try to start it and it’s doing this, measure voltage at the positive side of the coil again and tell us what it is exactly.
Don’t leave the key on for very long though, as this overheats the ignition system.
Next try to carefully (as if!) separate the DSII connectors at the ignition module. Should be a blue grommet model, with two connectors. One connector has two wires, the other has four.
The two wire connector should have power to one of the wires when the key is in the ON position.
I think the two wires are usually red and white, but there might be variations.
Can you post pictures of your distributor and the ignition control module on the fender?

If you do not see any voltage at either the red or the white wire, then that’s your problem. Either the ignition switch, or the wire to the module has gone bad.
Different ways of testing that.
If you do have voltage, perhaps the module itself has gone bad. It’s quite common with modern replacements, and even an original now that it’s almost 50 years old!

A long shot perhaps, but I would also test the magnetic pickup i(stator) n the distributor.
Separate the connection at the distributor with the three wires, and with an ohm meter check the reading between the orange and purple wires.
The black with white striped wire is just a ground.
You should see a reading of between 400 and 700 ohms. Near, or outside of that, means the unit inside the distributor is suspect and should be replaced.