If you're getting it at the I terminal you should be getting it at the coil - the brown wire that attaches to the I terminal splices into the red/green to the coil at that end of the resistor wire. Its purpose is to provide full voltage to the coil during cranking.

The wire to the ignition module hooks into the ignition switch at the same point as the resistor wire - that, or they splice off from a common wire within a few inches of the switch, I can't remember which.


To verify: with the ignition switch in the run position, do you have 12V to the terminal on the switch that the resistor wire and ignition system power attach to?

 

I will put the new ignition switch back in tonight after work and retest the red wire at the plug. Then I'll retest the voltage at the switch and report back with the results.

Here's the ignition switch terminals and the wires that go to them.

BATT = Fat Yellow Wire—has constant power with key off
BATT = Fat Yellow Wire—has constant power with key off
ACC 1 = Fat Black Wire—power with switch on
ACC 2 = Fat Orange Wire—power with switch on
ST (2 wires) = Red/Blue, Red/White—power only when cranking engine. To “S” terminal on solenoid.
P1 = Purple/White Wire—no power with switch or crank. Believe this is the brake switch.
P2 = No wire to this terminal
I1 = White Wire—power with switch on
I2 (2 wires) = Red/White, Orange/Green—power with switch on

 

The wiring schematics here might be helpful:

1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net

They don't have all of 1978 posted, but 1979 is, which should be a good reference.

 

I had a little time tonight to do some continuity and voltage testing on the old and new switches and here's what I found....

Both the old and new switch had continuity at all terminals (with the switch on) except the P1 and P2 terminals. However, the P1 terminal is the brake switch and the P2 terminal has no wire going to it. So this test sounds normal and both switches tested good.

I also did a voltage test on the wires in the plug on the back of the ignition switch with the key on. I tested all the terminals, but I focused mostly on the two terminals that had the parallel wires coming out of the switch; ST terminal (Red w/blue, Red w/white) and I2 terminal (Red w/white, Orange w/green)....

Excluding P1 & P2 terminals, all of the terminals had 12v to it with the key on except the ST terminal. The ST terminal had 9.83 volts when cranking the motor. The cranking voltage seems a little low, but it's enough to power up the coil and start the engine.

The red wire in the ignition module plug still didn't have any voltage and this really has me confused as to why.

The color of the wires in the two plugs going into the ignition module are: 4 wire plug = green, black, orange & purple. 2 wire plug = red & white

I haven't had a chance to open up the harness all the way to the pink wire splice on both ends, but plan to do that in the coming week or so.

Upon further inspection, I was wrong about the coil wire going into the distributor plug & then to the coil I was going off of bad memory when I stated that. Actually, the green wire goes into a rectangle shaped plug that goes through the firewall on the driver's side. I assume this wire splices into the pink wire. The plug has one green/red coil wire going into the plug and 2 red wires coming out of the plug and going through the firewall.

Any thoughts???

 

The green wire from the coil (-/Tach) should go to the green wire from the ICM.

The red/green wire from the coil (+/Batt) should attach to the resistor wire from the ignition switch.

Have you somehow wired your coil backwards?

 

Actually I don't have the green/red coil wire hooked up to the coil because I'm using a MSD 6A ignition box instead of the factory ignition box, but the green/red coil wire is being used to provide the 6A box switched power. The purple and orange wires from the 6A box is hooked up to the coil per the MSD instructions. However, because the green/red coil wire isn't getting any power, its not turning on the 6A box when I turn on the ignition switch.

 

Convention is to name the main color of the insulation first and the tracer second. Thus, the positive wire to the coil is red/green, not green/red. The trigger wire from the ICM is green as a main color, and I can't recall what tracer it may have.


The coil wire needs to be attached as designed, and definitely shouldn't be used to power anything but the coil, with the resistor wire in place. The MSD box should be powered from a switched 12V source (e.g. the DuraSpark wiring) that does not go through the resistor wire.

 

You're absolutely right about naming the main color first and the striped color second. I've got so many irons in the fire right now and my brain is fried and I simply got the color sequence backwards and didn't catch it. My apologies. For the most part I have been naming the main color first follow by the striped color.

As far as the MSD 6A wiring, the instructions say to connect the red switched wire from the the 6A box to the red w/green stripe coil wire, so that's what I did. It's been working fine up until a few weeks ago when the truck just shut off for some reason.

Here's the 6A ignition box wiring diagram. This is how I have everything wired:

https://www.google.com/search?q=msd+...FzwgoP4G4QM%3A

However now that I'm not getting power at the red/green coil wire, my plan was to bypass the factory resistor wire and use a ballast resistor by connecting the red switched 6A ignition box wire to the red wire in the factory Duraspark ignition box plug and connect it inline with a ballast resistor. But the problem is I am not getting any voltage at this red wire in the plug for some reason. The white wire in the same plug has 12v, but not the red. So I can't do this setup until I can figure out where the problem is.

I won't get a chance to work on this again until next week. We're moving this Saturday, but as soon as we get moved, I'll get back to it. I think I need to finish opening up the wiring harness to the pink wire splices and do some continuity and voltage testing. That's the plan anyway, unless someone has any ideas...

 

Not trying to nitpick on the colors, just avoid confusion.


They did change the colors around a bit from year to year, but for the ones I'm familiar with have 12V to the red wire with ignition switch in Run, and 12V to white wire with ignition switch in Start (red wire should also get a slightly lower voltage back-fed through the resistor wire from the coil I terminal, even in Start). Is it possible these are switched in your year? Is the red wire going hot in Start? If so, maybe the white wire is what you need rather than the red?

 

The red/green wire is what I have my aftermarket spark box tied to for 12v switched power. I have it tied in directly behind the ignition switch bypassing the resistor wire altogether. Might try running a wire temporarily to that point to see if that cures you. Good luck. Might still be able to use the truck to move.

 

blue04.5--Thanks for the input. That's a good idea and I might have to try that if I can't figure this out...

Bkaul--I have a question for you....Are you positive the red wire at the duraspark plug coming through the firewall is supposed to have 12v??? Let me explain why I ask you this question...

I have two 1979 Broncos that are 20 miles away at my mom's house and I was out there today working on one of them (had to install a new fuel pump). After I got done and got the Bronco fired up and running, I decided to test the voltage at this plug since it's the exact same setup as my 78 F150 that I'm having problems with. So I tested the red wire in the plug with the key on and there's absolutely NO voltage at this wire. However, there is 12v at the white wire. This confuses me since the diagram seems to be backwards from what's actually happening. I get the exact same results on my Bronco that I do my truck. The white wire in the diagram goes to the "Start" position on the ignition switch, which I would assume wouldn't get any voltage until it is being started. However, that's not the case. It's getting 12v with just the key on....Weird!!!!

So can I just run my ballast resistor off this white wire instead of the red wire at this plug? I'm assuming I can, but I thought I'd verify this with you guys. Hopefully my only problem might be the resistor wire. But in the mean time I want to bypass the resistor wire and see if I can get it started.....

Thanks for the advice...

 

The wiring varied a bit between different DuraSpark revisions over the years. The wiring diagrams here 1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net show a red/blue going to start and a white going to run. Most general DS diagrams I can find online show plain red going to run and plain white going to start. If one has +12V in run on your truck, that should be the right one to grab for the hot-in-run wire. The DS systems used that for power, and the extra start signal wire was there to tell it to retard timing during cranking for easier starting.

 

Thanks for that clarification....Ok, I think I'll use the white wire in the plug and hook up the ballast resistor. But first I need to finish unwrapping the harness to find the splice of the pink wire. I'm sooooo hoping this works. Hopefully I'll get to it early next week.

 

Well after a weekend spent moving, I do have an update on the truck's electrical problem. Last Friday night; the night I brought the uhaul home to start loading it, I took 20 minutes and gave it one last shot to get the truck running. I hooked up a wire to the white wire in the factory ignition module plug (plug with the red & white wire) and connected it to a ballast resistor & from there I ran another wire from the other end of the ballast resistor to the MSD 6A box. Then I clipped the factory resistor wire. When I turned the switch on, my tach responded and the light on the 6A box lit up, which gave me a ton of hope. Then I cranked the motor and it fired up!!!!!!!!! I have never been so relieved and happy is such a long time. Talk about coming through in the 11th hour...

So now I will be reinstalling the throttle body for the EFI (I put the carb back on to rule out the EFI as the problem) and replacing the resistor wire once I get one ordered. I've driven the truck the last two days finishing up our move and so far no problems.

This has been a very frustrating and stressful month since my truck went down, but I would like to thank all of you for your input, advice and help. Without your help, I might not have figured this out. So thank you very much everyone! You guys rock!!!! I hope I can return the favor to someone.....

 

Glad you got it going!