starting and stay running issue

[green88gt]

i got my 68 f100 a few weeks ago and it done the same thing in the driveway.the guy i got it from told me that it was the coil getting to hot,but i dont know whats doing it.

[Thunderfoot]

The ignition switch is a common problem that does these exact symptoms; the other could be the starter solenoid (which you replaced already). There could also be a short in ignition wire from the coil to the switch or the coil to the starter solenoid… or the wire from the coil to the distributor could be bad as well; the way to test it is to put a test light on the (-) side of the coil when cranking the engine over the test light should pulse, as it is getting the signal from the points. If the light stays on steady then it is not getting the signal from the points. If the light is not coming on at all or goes out when cranking then there is a problem with the ignition switch or ignition wire, as the power is not getting to the coil.

[Wes]

I'm with Thunderfoot on this it sounds like the starter solenoid is providing the ignition coil w/ 12v during cranking, then the ignition switch is supposed to provide the voltage to the coil.
Disconnect the ignition wire from the coil(non distributor side) turn the key to run and check for voltage on the wire. You should have 7-9 volts. Wiggle the wire while watching the volt meter then jiggle the key & ignition switch. If the voltage drops replace the item you were moving when it did. I just bought an ignition switch from Parts America, was an exact replacement. was under $30.00 I think even under $20.00 but don't remember.

[Banjo]

when you mention new points and coil, did you install new condensor also?

[fordman]

behind the ignition switch is a pink wire. its not very long. that is the resistor wire. if it burns in two then the truck wont start. the pink wire ties into the wire that goes to the coil. i think the wire is red orange or maybe green. i can't remember. but i am in the same boat with my new parts truck. i have no fire out of the coil. so i have to check that out in the morning too.

[Wes]

Here is a link to wiring dia. http://www.fordification.com/images/sch ... rging2.jpg. To replace your coil voltage supply circuit, you need to either use a rated resistance wire, or a ballast resistor and regular wire. Ideally you want 7-9 volts at the coil when running. In the diagram you can see where the start supply circuit and the run supply circuit come together at plug D. The resistance portion of the circuit is under the dash if you check for voltage at plug d w/ key in run, you can figure which part of the wire is open.

[fordman]

i didn't look to hard at mine. but i don't have any power to the coil so it must be in the resistor wire or maybe something to do with the ignition switch or the plug fo r it.

[Thunderfoot]

Looking at the wire diagram it seams to me that constant voltage to the coil will come out of the S on the starter relay. I have no power coming from the relay with key on, should there be?

No, that will only have voltage when the starter solenoid is activated (key in start, turning over the motor) this is a temporary full (12V) voltage supply to the coil to help the engine with cold starting while cranking the engine over. After it starts (key in run) this disengages and the voltage to the coil drops to the 7-9V via the resistive ignition wire.

One thing to check though is that this (S) terminal is not grounded, (have seen bad solenoids do this) if it grounds it will ground the voltage to the coil from the ignition wire and the motor wont run...

[Thunderfoot]

How could I check the relay to see of S was grounded?

Hook a test light to it (I terminal) and the (+) battery terminal, if the light comes on it is grounded

NOTE: I made one mistake here on this (S) terminal in this discussion... The (S) terminal should be the one with the Red/Blue striped wire on it and it is the one that activates the starter. The (I) terminal should have a Brown wire on it and is the one that goes to the coil for cold starts and is the one you want to check for a short to ground.

(S) will be the small terminal same side as the battery cable connection.
(I) will be the small terminal same side as the starter cable connection.