My 1979 f150 refuses to start!
A little background...
The problems began when my ignition was left in the "on" position for 15 minutes and I returned to find electrical smoke pouring up through the dash. I found it to be the resistor wire that had melted, which I replaced with new wire and a ballast resistor.
When I tried to crank it, it would work once or twice, then it would continue cranking the starter even after I had turned the key back and completely removed it. I figured this was a problem with the starter solenoid/relay, which I replaced. After replacing it, when I cranked the key, nothing happened. No click, no buzz, nothing. I tested it with a meter and found that no power was getting through to the starter wire when it was being cranked. I reconnected my old solenoid, and it worked, but it had the same continuious cranking problem. I assumed the new one was a bad solenoid, and exchanged it. I put the second new one on and the same thing happened. I'm sure all of the wires connected to the solenoid are correct. I used a meter and got some weird readings. On the "s" terminal I got 7.35V in the on position, and 10.2V in the start position. On the "i" terminal I got 7.8V in the on position, and 8.7V in the start position. The starter terminal still showed 0V. Okay, I thought maybe the new ballast resistor was somehow at fault, so I bypassed it and my readings changed to 9.4V and 10.6V on the "s", and 10.8V and 10.8V on the "i".
Shouldn't there only be power to the "s" terminal when the key is in the start position?
I've spent alot of time reading the posts here and tried a few things. I tried grounding out the + terminal and the "s" terminal and nothing happened! I thought maybe its a bad battery ground, so I used a jumper cable to jump from the block to the - on the battery, it didn't make any difference. I am completely confused and frustrated, and I would appreciate any help!

 

Maybe other ignition wires got hot enough to melt as well?

 

On my 75 f100, 302...about once every week or so the starter would continue cranking even after the motor had started and I had turned the key back to RUN. The only way to stop the starter from turning was to turn the key all the way off or to ACC, then turn it all the way to start again, then back to RUN. And occassionally the truck would not turn off when the key was turned to off. I thought maybe it was the ignition switch, so I replaced it and the lock (that costed less than $15). It did not change a thing. Then one day, after these incidents were becoming more frequent, the truck would not start at all. So I first changed the starter, it still would not start. Then I changed the relay/solenoid, and have not had any problems since then. That was about 6 weeks ago. Since you already changed the relay, I would guess the starter could be the problem.
Starters cab do goofy things...I once had a car (73 Datsun 1200 hey cool car) that was overcharging the battery like mad. Battery would start steaming, headlights were crazy bright, etc. I changed the alternator and voltage regulator to no avail. Someone said to change the starter, didnt make any sense to me, but I did, and problem solved.
Hope this helps, Jay

 

Maybe other ignition wires got hot enough to melt as well?

Actually, the last time the engine died on me, I cranked the engine several times within about 3 minutes, until I noticed smoke coming out from under my hood. I investigated and found that the + cable from the battery was smoking where it connected to the solenoid. It had melted the insulation off. I replaced that wire, and I have checked the insulation on all of the other ignition wires (that I could get to), and they were fine. It hasn't started since (there was a fuel delivery problem, which I have since solved). I am in the process of replacing both of the smaller wires that connect to the solenoid, just so I can be sure.

I really have a hard time believing the starter could be the problem, considering it sounds like its cranking fine when I jump across the solenoid. I've also heard that they are a pain to get to and replace on these trucks.

Thanks, I appreciate the suggestions, and I hope to hear some more...

 

You should have battery voltage to S when the key is in Start. S passes through to I and from there to the + coil connector so you should have 12V+ there as well. You should have 0V at S when you are in run, but 9V (with the resistor) to the + coil connector in run. Your solenoid is ok if you can hear it go 'THUNK' when you jump 12V onto S.

Therefore, either you've got your wires reversed somehow behind the dash or you toasted the ignition switch and Run is bleeding onto Start (or similar fault). You may also have a - bat ground problem. 10V is too low for the straight hotline.

ash
['Recheck wiring patch.']

 

That tells me that there is a problem with the ignition switch or the wiring to it - the "s" terminal should ONLY have power in the START position. Check for more damage under the dash.

Marty

 

Ok, so I've replaced the ignition switch, and still nothing happens when I crank the key. I've replaced both of the smaller wires that run from the solenoid to the ignition switch. I'm still getting 7.4V on the "s" terminal in the on position. I don't understand why. I even tested the connection point at the ignition switch, and its getting 7.4V there. Where is this voltage coming from? I know it should have 0V, so I can't figure out where it is picking up the voltage from. I've double checked as much of the wire insulation as I can get to.

Your solenoid is ok if you can hear it go 'THUNK' when you jump 12V onto S.
I can definetly hear the thunk when I do this, so I guess the solenoid is fine. (although I couldn't get it to make the sound last week when I tried to jump it. ??)

I know the starter cranks when I bypass the solenoid. I have had my ignition module tested and it works fine. Are there any other specific wires I should replace to be sure? I still haven't messed with the ground connection, partly because it is so hard to get to, and cause I can get it to crank by bypassing the solenoid.

Any more help would sure be appreciated...

 

I'm still getting 7.4V on the "s" terminal in the on position. I don't understand why. I even tested the connection point at the ignition switch, and its getting 7.4V there.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The S terminal shouldn't have any voltage with the key in run.
That is what the I terminal is for > Ignition

 

The S terminal shouldn't have any voltage with the key in run.
That is what the I terminal is for > Ignition

Yeah, I know. Thats why this is so frustrating. My "i" terminal is getting 8V in the on position, and I don't understand why I'm getting 7.4V at the "s". Any ideas?

 

Have you peeled back the wiring harness to where the fusible link is ?

Get a test light and pierce the wire staring from the S terminal to the ignition switch.
Process of elimination.

 

Mil1ion, look at my previous posts. I have already ran a brand new wire from the "s" terminal to the ignition switch. Do you see why I am so confused? I can't figure out where the voltage could be coming from. The "s" wire joins another wire at the ignition switch, I believe it runs to the ignition module, and it has 7.4V running through it as well. I had my module tested 3 times and it was fine, so...
I appreciate the help, and I'll keep trying to find a solution.

 

Ok sounds like a good, as in extremely nerve-racking, problem.
First lets try and throw out the "red-herrings" or "wild goose" items your choice as what to call them
Voltage on the "I" terminal with the key in "run" is normal......the wire is connected direct to the coil + so any voltage felt at the coil + is felt at the "I" terminal. It doesn't do anything there or go anywhere because the relay has to be closed to connect the "I" to anything else.
The "S" terminal is not connected to the "I" terminal in any way shape or form. The "S" terminal is the power side of the relay close coil, the relay case in the ground side. The "I" terminal is attached to a tab inside the relay that is powered when the relay closes. A metal, I think it is copper or brass, piece is drawn down by the close coil and connects the relay's two large terminals together to power the starter, at the same time part of this metal piece touches the "I" terminal post powering the wire to the coil +.
Ok on to the problem......quick recap..........it all happened after the behind the dash fire...you completely disconnected the old wire to the "S" terminal.....but you are still getting voltage to the "S" terminal with the key off.......
The wire from the "S" terminal, formerly red/blue stripe, runs from the "S" terminal to the neutral/safety switch, if an automatic thru a jumper if not, to a start terminal of the ignition switch.....all by itself no other wire should be on that contact, of course I am working with stock wiring....so who knows what other people have added
The ignition module has a white wire running from a "run" terminal of ignition switch to it, and this white wire should also be by itself on the switch.
In fact, as far as I know, the only factory doubled wires on the ignition switch are two red/green ones, of which one is the resistor wire...which usually looks pink with a black stripe due to age and heat.
I am kind of leaning towards the ignition switch wiring may be a little mixed up......... either that or there arew some wires "welded" together after the smoke show earlier....that happened to my old 73 when the resistor wire burnt up........all kinds of neat stuff happened as voltage was getting everywhere when it wasn't supposed to....bad news was I had to remove the entire dash and trace wire by burnt up wire to figure it out...that is when I first realized that I hated that resistor wire
My best advice is to check the wiring and recheck your replacement wires for correct routing......drop me an email if you need more help.....that is if this long winded post is any help

https://www.ford-trucks.com/user_gal...=39681&width=0

 

Thanks for the info, Sparky. I appreciate you explaining exactly how the relay works, it helps me understand it better.

Now for some more fun
The wire from the "S" terminal, formerly red/blue stripe, runs from the "S" terminal to the neutral/safety switch, if an automatic thru a jumper if not, to a start terminal of the ignition switch.....all by itself no other wire should be on that contact
I have two red/blue striped wires connected to the same contact, one that runs directly to the "s" terminal, and the other one (as far as I can tell) runs to the ignition module. They both look stock. Other than the resistor wire contact, this is the only other contact that has two wires. I don't see where the red/blue wire from the "s" terminal runs through any neutral/safety switch (the truck is an automatic). From what I can see the original red/blue wire runs from the "s" term, through the firewall on the passenger side and goes directly to the shared "ST" contact on the ignition switch.

Another mystery...
When I try to jump from the + terminal on the relay to the "s" terminal, it will only THUNK when the ignition is off. When I turn it to "on" and try to jump across, nothing happens. I tried it with the "s" wire connected and disconnected, and the same thing happened. I was thinking I could just bypass the ignition switch problem and run a new hot wire through a button switch to the "s" terminal, and use that to start it with the key in "on", but it doesn't seem like that will work.

Also...
The ballast resistor I installed after my resistor wire melted, starts to smoke after being in the "on" position for about 3 minutes or more. I'm not sure if this is a continuation of the original problem that made the resistor wire melt, or if I have the wrong ballast resistor, or what. On one side of the ballast resistor, I connected the wire that goes to the ignition switch, on the other side I connected the red/yellow (I think thats the color) wire and the brown wire that it joined with originally. This is the same brown wire that goes to the "I" terminal, isn't it? I replaced it with a new wire, assuming it was, and ran it to that terminal.

So those are the problems and complications I am dealing with. Any more suggestions will be greatly appreciated, as I am learning more about the electrical on this truck than I ever wanted to

 

 

Sounds do me like something is grounded out somewhere. Have you checked all od the other wired inder the dash for melted bare spots ot 2 that may be melted together that would cause feedback to the start wire?
If you jump the solenoid out, it should crank whether the key is on or off. All you are doing is using the voltage from the battery to solenoid wire to energize the coil in the solenoid that pulls it in. If the starter still doesn't turn, you could have a dead battery. bad chassis/block ground or a bad solenoid.
When junping out the solenoid, the ignition switch has no effect on the starter at all.