Was looking at things under the hood of my '73 F100 before it got too hot outside. Noticed a blue wire, coming off of a connector near the firewall.
Had a Ford part number printed on it. I went inside, to check a diagram in my 1973 Ford truck shop manual, turns out that it is the primary resistor wire, as seen in the attached photo of the diagram. Shows 2 wires coming out of a quick connect, one going to the + side of the coil, and the other wire going to the I terminal of the starter relay.
So, should I hook this wire in, by running an extra wire to the coil and another to the I terminal on the starter relay? Or is it not needed for anything?

 

Is the engine running now? If so, it seems like an additional wire.
Second, and I haven’t looked at the picture yet, I’ve never heard of a blue wire being the resistor wire.
The actual circuit color for the ignition coil is red with a green stripe, while the branch coming from the I-post of the starter relay is solid brown.

I don’t have a 73 diagram in front of me, so you may be perfectly correct and that one section of blue wire could be the resistor. And while normally Ford tended to put the resistor wire section under the dash, I have seen some of the early pick ups, use it in the engine department.
Or I have heard of it here, rather than actually having seen it myself.
The only blue wire I remember was for a carburetor solenoid. Perhaps blue with the black stripe?
Is this a solid blue wire? No stripes? I’ll go look at the picture now, but I’m on my phone and don’t have much luck in that regard.

 

OK, I see now. Sure looks likes you got it nailed. But again, does the engine run right now? Or are you going through it and getting all the wires sorted at this point? Did it run before that you’re aware of? Did you just get the truck?

 

Runs erratically at the moment, can't get it to run long enough to idle. Last time I had it around the block for a test about a month ago, it ran good. I's acting like it is trying to start but won't catch. New fuel pump, which is working as it should. Lines cleaned and brand new tank, Freshly rebuilt carburetor.. All new ignition parts.

According to that diagram from my manual it sure shows what I found as a resistor wire.Not sure if that part number marked on the wire would tell or not what it is. Similar colored wire coming out of the ignition switch plug, but that one is marked "Resistor Wire", but same color as the underhood wire

 

[QUOTE=1TonBasecamp;21263540]OK, I see now. Sure looks likes you got it nailed. But again, does the engine run right now? Or are you going through it and getting all the wires sorted at this point? Did it run before that you’re aware of? Did you just get the truck?[/QUOTE
Barely runs long enough to notice.Just going over everthing, since I'm not sure what all the previous owner did

Bought it 4 months ago towed it home

 

The part number, or “engineering number“ that is visible, should tell you exactly what it is. Lots of members here have extensive Ford part numbering system information. So someone should be able to answer that question.
But it appears to be a duplicate. Perhaps it has failed and there is a replacement already at work under your dash. Either that, or someone replaced it with a jumper wire of non-resistor type.
You can turn the key to the ON position and measure voltage at the positive side of the coil.
If it reads battery voltage, or maybe a volt or two lower, then make sure the points are open and then take a measurement again.
If it reads 9 V or less, there is a resistor in place.

 

I tried it again this morning, it ran for about a minute then quit. It would do this a couple more times, otherwise it would crank and not fire. I pulled the coil wire from the cap, and it has a good fat spark. This is very weird, as when I took it out for a couple runs around the block last month, it ran great. Has sat for the past month while I dealt with sticking front brake issues

 

Still could be something electrical, but let’s try fuel first.
Pull the air cleaner housing off of the carburetor and verify that the choke is closing when it’s cold. Then rock throttle lever back-and-forth a few times to make sure that fuel is actually squirting out of the accelerator pump nozzles.
If not, or frankly, even if it is, pour gasoline into the float bowl of the carburetor. Pouring it down the throat doesn’t do any good at this point, because we already know you at least initially have fuel and spark.
If it’s a fuel delivery issue, the engine should run for several minutes on what’s inside the float bowl. If you just dribble fluid into the carburetor throat, it’ll only run for a few seconds.

Perhaps one electrical test you can perform, would be to remove one plug wire and get it stabilized near a good ground so that you can watch the spark.
Fire up the engine and let it run for that minute or so that you say it has been running today. It will run rough, but should still run on seven cylinders.
When it starts to die, keep your eye on the spark, jumping the gap on the removed plug wire. If there is still spark, right to the end, then it’s very likely a fuel issue.
If the spark disappears about the same time that the engine stumbles and tries to die, you may have an ignition problem after all.
Could be the coil overheating, if the resistor wire was indeed bypassed. So back to what I think I said previously, check for voltage at the coil.
Not just sparks out of the distributor, but drill down into it a little bit and see what’s going on.
I’ll re-read to see if you have electronic ignition, or points still.
The 73 would’ve had points originally, but could’ve been converted.
Anything is suspect at this point still.

 

I will check for voltage at the coil. My truck has original points system, gapped at .017 . Has a manual choke Autolite 2100,was rebuilt.. Has to be something stupid, just gotta figure it out!

 

Is getting good squirts of fuel from accelerator pump. Used spark plug to check for spark, very weak spark. Measured voltage from Dist terminal of coil. to a ground point on the dist, voltage with key on, was 2.7 volts

 

As a test only run a wire from the battery + to the coil + and start the motor and see if it will stay running.
To stop you will need to turn key off and pull the wire off the battery and motor should stop.

BTW the wire from the solenoid "I" to the coil + sends a full 12 volts to the coil for more volts for a hotter spark on start up.
Dave ----

 

Thanks I will try that. Do I disconnect the existing coil + wire first before connecting the jumper wire?

 

You can, but it’s not necessary.
If your overall wiring condition is questionable, it’s certainly not a bad idea to remove the original wire.
but normally, no.

to expand on what was said about disconnecting the jumper wire to turn the engine off. That’s a good thing, because you do not want to leave that wire directly connected to the coil for very long.
If the engine is not running and you leave 12 V connected to the coil, it can overheat the coil.
While you are experimenting with this, just get in the habit of disconnecting it from the battery when you’re messing around with other stuff.
While you’re looking for tools, keys, a cool and refreshing beverage, just make sure the wire is disconnected.
There certainly a little leeway, and many of us have experimented this way. But at some point, that leeway time runs out!

 

Thanks 1 ton for that.
Motor not running remove wire from coil.

What you are doing is "hot wiring" the IGN system.
It is done ONLY as a test to see if it will start and run with full 12 volts as of right now it will not run with less volts on the resistance wire.
Dave ----

 

I looked over the ignition switch, and I was amazed, it was the original Motorcraft, likely the one that's been in the truck since new. So for safety sake I am putting in a new one. But before I do, should I check for power at each terminal at the plug, before I plug it in to the new switch?