Usually when that happens, it is caused by the internal contact in the solenoid that feeds 12V on start being hung up.  The next time it happens, pull the two small wires off the thermostat and see if it dies.  If it does, you need a new solenoid.

You can also do a percussion engineering exercise on the top of the solenoid with a small hammer to try and jar the contacts loose.

I went through solenoid hell last year with my 66 at the bash.  I went to the third one before I got a good one.

If you are continually welding solenoid contacts together, you may want to check your starter cable.  If it is loose or corroded. it will have higher resistance and can cause solenoid issues.

I tried OReilly and Autozone and NAPA for solenoids.  They may be different color plastic but they are all the exact same part, made in China, of course.  I used to say get only a FORD solenoid.  I am not sure that is any better, but given the choice, that is what I would buy.

There is a chance you could have a switch failure, but they usually produce a NO START condition instead of a NO STOP.

If you have converted the car to an aftermarket ignition system, some or all the above info may be void!  If so, let us know exactly what you have.

When I converted my car over to MSD ignition system I remember reading something in the instructions about that very thing happening and what to do to prevent it.  Problem is I dont remember the specifics!  If you do have afermarket ignition check their instructions if Its MSD call them their support was really good for me to deal with!!

If you decide that the solenoid is the culprit,you might consider getting a solenoid that is for "continuous" duty.....The contact surface area is larger the magnetics are stronger and the return spring is much stronger....They are more dependable and longer lasting because they are designed for harsh use...They are found primarily on electric powered equipment that is self propelled and or runs a hydraulic pump or other pump,motor or high amp drawing light assemblies....NAPA has them.....just a thought

Solenoid 101     Basic operation of a solenoid. Sometimes a pic is worth a 1000 words




Battery to #1       Starter to #2

#3 is where you give it 12 V to pull in the solenoid(it goes to ground thru the metal case, where it's attached to the metal on the car.

When #3 gets 12 V  it connects #1 to #2,  it also connects #1 to #4 so that you get 12V to the coil while the starter is engaged. 

What everyone is telling you is to remove #4 the next time the car keeps running. That should stop the car(assuming the solenoid is the problem)  If it keeps running then something is wrong in the keyswitch(wired wrong, or PO wired something up screwy)   Good luck and let us know what you find, cause I'm sure others have had the same problem and are curious.

MustangSteve wrote:

Yeah, those THERMOSTAT wires are real tiny and hard to find!  My solenoid went bad at the bash last year.  Changed it out and the replacement made it so the engine would not shut off.  The replacement for the replacement is still doing fine.  If you can sitting in the garage not running doing fine...

I plan to fire up the new 427 this weekend, so that will be the test!

 
Can someone who speaks texan interpret this. I've read it a dozen times and got nothing.
 

Dang, MS can't get a break.   I just assumed he hit the "n"  instead of the "ll" butteye(TS) did have to look at it for a minute to figure it out.

Thank ya'll for the clarification. I wasn't slamming anyone, I just didn't get it.

66gt have you pulled the wires off of the solenoid when car won't shut off?   If so what were the results? Or is this a problem that might not happen again for 6 mos.?

The same thing happened to my '69 F-100 Ranger about a year ago. Well, it had two problems...

From time to time, the engine would continue to run even after I turned the ignition off and the key was removed from the ignition switch.

The more prevalent problem was, fairly often, with the starter staying engaged even with the key turned off. If I got out, raised the hood and popped the top of the solenoid with my hand, the starter bendix would finally drop out.

I replaced the solenoid and all was fine ...for a little while before the grinding starter problem intermittantly came back. I had finally found the source of both problems but, initially, I didn't consider the battery as the source of the problem because (at the time) it was 'new.' When the grinding bendix problem came back (after I had replaced the solenoid), I did a voltage check on the 'new' battery to discover it was undercharged (had low voltage).

As voltage goes down, current goes up and vise-vera (they are inversely proportional). It was easy then for me to see my 'new' (undercharged) battery's low voltage state had driven the current up and burned the contacts of the first solenoid and then done the same thing to the 2nd solenoid. This was causing them to stick and not release.

My truck doesn't get driven but a few times a month so, the charge can dwindle on the battery between uses (as with any vehicle that sits with infrequent use). I replaced the solenoid for the 3rd time and put a full charge on the battery. Periodically, I put the charger on the battery to maintain its charge. Since changing the solenoid the last time, fully charging the battery and maintaining its charge for a year now, I haven't had any more problems of the engine continuing to run or with the starter bendix staying engaged.

In order to produce the same HP on a motor, if the voltage went down, it would have to use alot more current to turn it over, to get the same equal end result, would it not?.  This is looking at it from equal working load perspective.

wsinsle wrote:

I have to apologize in my answer to Ultrastang and all. After thinking about this some, I did a little reading. When a DC series motor (this is what a starter uses) first engages and first starts to spin the current draw is very high. As the starter motor picks up speed current draw drops off. I see how if voltage is low enough and the starter motor is not picking up speed the current draw would higher. Sorry for any confusion.

 
No problem.  In a simple resistive circuit, like you were intially talking about, voltage & current are directly proportional. If one goes up (or down), so does the other.

While a DC starter motor is a motor used to crank an engine, it, like all DC motors, is also a generator. It creates EMF (Electromotive Force) as it spins or, in this case, it generates "back EMF." The initial in-rush current on a DC starter motor is very high under normal circumstances. If the starter motor is already being dragged down from a low voltage supply to it to begin with, then this is going to further increase its resistance to spinning over. Additional resistance to allow current flow through it means the current will have to go higher than normal to try and get the armature of the starter up to its normal rotational speed.

This current increase above normal is going to damage or burn items up if it isn't stopped pretty quickly; starter solenoid contacts, battery cables, ignition switch, control wiring or the starter itself are just some of the primary items that can be damaged due to excessive current loads.

If the static charge on the battery is low, the starter is not the only thing the battery is having to contend with (resistive-wise) in its struggle to get the starter up to proper cranking speed. There is also the issue of trying to spin the engine over, itself, that's also dragging the starter speed down thus, adding more resistance to its already limited ability to spin --more resistance is even more (higher) current load added to the circuit. As the engines pistons are nearing the top of their stroke in each bore, the compression is steadily building. This (mechanical) resistive load is transfered from the piston, down through the connecting rod, into the crank, through the flywheel/flexplate and then to the bendix drive on the starter. This mechanical resitive load becomes an electrical resistance load on the starter that increases the current load to the starting circuit.

I didn't find the following article until just prior to making this post but, it confirms the 'epiphany' I had of what was going on with my truck, a year or so ago, when it was acting up. It would have been nice to have seen this prior to when my F-100 was acting crazy. ...as I said before, since I've kept a good charge on the battery, the engine running after the key is turned off or the starter bendix grinding with the ignition off hasn't happened again since then.

http://mustangmagazineonline.com/article/sad-story-suddenly-sticking-solenoid

Last edited by ultrastang (9/10/2013 6:22 PM)

I've been trying to make the ohm's law thing work too and can't,but what about the fact that the solenoid coil is also dependent on battery voltage and probably requires full voltage for a maximum pull on the contacts....A weak coil (low voltage) can't pull and hold the contacts together tight enough and they arch when current begins to flow...They could even be "chattering" to some degree...just thinking with my 2 fingers