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A recent post and others relating to advice not to leave the ignition switch hot for extended time or risk "frying" the points (if they happen to land closed when the motor stopped turning) got me thinking about installing a "dead crank" switch to allow the engine to crank without actually starting it. Useful troubleshooting feature as well as part of a "security-in-depth" measure to maybe hinder somebody from driving the truck away. I know, I know, another unauthorized modification. Why don't I just put the rotor in my pocket?

My M38 converted to 12V, using a CJ distributor. The ignition switch looks just like a regular M38 switch where it should be on the dash, but is "Off-On-Momentary (start)" type. Starter is remote solenoid activated.

I'm thinking of using a mil-spec locking toggle switch for a dead-crank: part number MS24658-22D. Single pole, single throw, straight "on-off" function.


My question is --- would this switch be better installed in line with the wire feeding the primary side of the coil (from the ignition switch), or in line on the coil primary wire that goes to the distributor?

Seems to me that interrupting the feed to the coil primary would prevent the coil from generating anything (crank but no start), whereas interrupting the distributor wire is just preventing a ground from triggering the secondary (still crank but no start). I'm guessing this might be harder on a coil? Don't want to do anything to shorten life expectancy of anything.

The switch is rated 20 amps for up to 28VDC so I'm pretty sure it can handle either location --- 14 gauge wire. Haven't decided on a mounting location yet, under hood or under dash. Under hood seems to be favored (shorter wire runs).

Is it 6 of one and half-dozen of the other? I feel like I'm over-looking something important. What am I missing? Your advice appreciated as always.

One would need to see exactly how your 12V ignition system is wired to offer any intelligent suggestion to you. What you are trying describe definitely sounds like you have two feeds to your coil unless you are confusing an extra connection to an external coil ballast resistor with a during start feed from the starter solenoid. The plan with an ignition interrupting switch is to remove all and any power to the distributor primary side.

The M38 was simple, you just interrupt wire #12 anywhere along it's route from the switch to the coil.

Hi Wes. Yes, I do have 2 wires feeding the coil primary --- one from the ballast resistor, and one from the solenoid to furnish the full 12v during cranking only.

If I understand you correctly, if I install the switch to interrupt both of those wires, then I have the equivalent of interrupting wire #12. I believe the switch will allow 2 wires on one terminal with no problem.

You said: "The plan with an ignition interrupting switch is to remove all and any power to the distributor primary side."

I cringe asking, but did you mean to say coil primary side, vice the distributor primary side? Wire #12 is coil primary side isn't it?

Either way, it seems clear to me that interrupting the primary wire between the coil and the distributor isn't the way to go.

Thank you!

Technicalities! Where does the #12 wire enter the distributor? That is the primary input to the distributor and the distributor's coil. Depends on your perspective but not an issue in this discussion. We are only discussing interrupting #12 wire power to the distributor. After it gets to that little threaded gadget on the side of the distributor no one here is going to play with that wire anymore.

The Ignition Disabling Switch Or in your terms "Dead Crank Switch" is most easily installed between the ignition switch and the firewall and hidden some where there-in. Most civvy ballast resistor set-ups power the coil direct from the starter solenoid's main power out. So a bypass of the ballast resistor only occurs while the starter solenoid is energized. If you are running two wires all the way from the coil to the key switch with only one passing thru the ballast resistor you are doing it the hard way.

Thanks Wes. Yes, I understand the technicality (that's why I cringed when asking ). I believe we're saying the same thing. It's awfully hard to communicate clearly this way. Very easy for words to be mis-understood, in both directions. I could be wrong, but I believe a lot of technical threads end in frustration because of this. The best I can do is be patient and not make it worse.

One wire goes from my ignition switch, through the ballast resistor, and on to the primary input side of the ignition coil, which puts about 8 volts into the coil only when the ignition switch is "on" (and the motor has started and is running). This being the same voltage known to "fry" points if the motor isn't running and the ignition switch is left "on" and the points happened to land closed when the motor stopped.

The other wire connected to the primary input side of the ignition coil comes directly from the starter solenoid primary coil, which is active only when the starter is cranking (ignition switch in the "momentary" (start) position), and puts the full 12v into the ignition coil only when the starter is cranking.

The starter solenoid I have has two primary coil terminals ("Ford style"?), one is connected to the "start" position of the ignition switch, and the other connected as described above to relay 12v into the coil primary.

I'm pretty sure I'm wired correctly. I built this harness myself after a lot of study. Everything checked and double checked, then checked again with a meter. Everything is working good, and there's no smoke, sparks, or flames.

By the way, thanks again! I really appreciate your patience!

Oh, and I didn't invent the name "Dead Crank Switch". Every military diesel generator I ever saw had one on it, labeled "Dead Crank" on one side and "Normal" on the other side -- if I recall correctly. It allows the mech to troubleshoot and crank the engine without starting it, bleed fuel lines, and such. Good stuff......and yes, I know my jeep isn't a generator.

Except for that suction sound out of my wallet that it generates.........

Now that was quite simple Ron. You described your circuit exactly like I described mine above it just using slight different wording.

But we got the primary side of the distributor and coil squared away and now we seem to be stumbling a bit on the description of the starter solenoid terminals.

First they fall into certain classes.

Continuous duty and intermittent duty.

You should be using an intermediate duty solenoid?

Now these solenoids are really just heavy duty relays. The BIG posts on each side are for the heavy juice from the battery and the heavy load to whatever is using the load.

These terminals are called BAT and Load.

Now here is where things can get a bit odd. There are two types on intermittent duty solenoids. Those with coil power terminals grounded internally (Single switch terminal) and those with coil terminals grounded externally (two small terminals) one marked Sw (Switch) and one marked Grd (Ground).

The Internally grounded, single small terminal solenoid operation. The ignition switch 12V output is delivered to the single terminal marked Switch. Then the 12V flows thru the small magnet coil and to an internal ground and the magnet is energized and closes the large contacts.

The Externally grounded, Dual small Terminal solenoid operation. The ignition switch 12V output is delivered to the small terminal marked Switch (SW). Then the 12V flows thru the magnet coil and out thru the other small terminal marked Ground (GRD) and to a vehicle ground thus energizing the coil magnet and closing the large contacts.

As you see they function the same but the grounding of the 12V in to the magnet coil happens two different places.

Now I think the solenoid you are describing is the Two small terminal externally grounded type.

What becomes very odd is how you wired your solenoid and how you named those terminals?

The starter solenoid I have has two primary coil terminals ("Ford style"?), one is connected to the "start" position of the ignition switch, and the other connected as described above to relay 12v into the coil primary.

Since the small magnet coil in the solenoid is the only coil in there most of us just call it the magnet coil. The terminals aren't primary?? but as I said above one is Switch and one is Ground. What you have done is route your start solenoid's magnet coil ground thru your ignition coil first then to ground. Yes your solenoid will still function but the loads imparted on it by the detour you took in it's ground wire will have some effect on the quality of the ground.

The industry standard with either type solenoid is to attach the hot 12V when cranking wire from the Switch terminal to the input side of the ignition coil. Remember the only time this solenoid Switch terminal is hot is when the ignition/start switch is in the start position.


In this illustration no matter which of these two solenoids you have the ballast resister bypass wire to feed 12V direct to ignition coil while cranking would be attached to terminal #2.

I would advise you to transfer your straight 12V juice wire for your ignition coil to the switch terminal side (#2) of your solenoid and connect a dedicated single ground wire from the solenoid's magnet coil Ground terminal (#3) to a chassis ground.

Hi Wes,
Thanks! And yes, my terminology can certainly stand some work.

This is the solenoid I'm using, Ford part number B6AZ-11450-A. It's not quite the same as either of the examples you show, yet it has characteristics of both of them (combined). Ford had a "better idea"?

Post 2 in the photo is marked "S" (Start?) on the solenoid body, and Post 4 is marked "I" (Ignition?).

Glad you Accurately ID'd your solenoid. Yes that is a unique variation of the internal grounded magnet coil relay that adds the 2nd terminal for shorting the ballast resistor. Note I said shorting the ballast resistor. Ford used that relay for their 60's vintage transistorized ignition systems as did many other manufacturers. This is an accurate illustration of that relay's internal wiring;



A word of advice here is if that relay ever sticks on you and the starter keeps turning remember that when you act quickly and turn the start switch off the starter will continue to turn and the points will continue to get overheated until you get out of the jeep and disconnect the battery.

Had you use the recommended starter solenoid/relay simply turning the start switch off would have stopped the 12V to your points while you the disconnected the battery to stop the starter from turning!

My advice is cheap but it is based on 55 years of automotive and aviation wrenching experience!

Thanks again Wes! Very interesting. I'll keep this in mind should that solenoid ever act up I may convert to the recommended set-up. Not that much re-work involved and the coil prices are very similar. My M38 came from the previous owner with it, and it seems quite commonly used over in the CJ-2A and CJ-3A forums for jeeps converted to 12VDC, so I thought it should be a reliable set-up. So far, so good.

I seem to recall back in the day watching somebody rap the solenoid with the handle of a screwdriver to un-"stick" it. Worked sometimes, long enough to heed the warning and change it out anyway.

I do have a mil-spec Battery Master disconnect switch installed, so disconnecting the battery is quick and easy. I know, yet another un-authorized modification.

Yes, I understand my set-up shorts the ballast resistor (effectively, taking it out of the circuit, electrically speaking).

I respectfully disagree that your advice is cheap. Quite the contrary. It's priceless!

Flatery gets you no where Ron!

In your application you aren't shorting the resistor, you are actually bypassing the resistor!

That battery cutoff switch is only as valuable as it is accessible! Where did you hide yours?

I would say "right next to the fire extinguisher" , but I went with the hidden in plain sight option:

You can see the bottom of my solenoid peeking out from under the box the Battery Master switch is mounted on. I fabricated that box to mount and hide some of that mess (after I repaired a bunch of old holes that Bubba drilled into the fender wall........

So on a hard start winter day you crank her for a minute, she doesn't start so you release the switch but the starter keeps turning ( you are at 2 minutes now) so you climb out of the seat, unlatch, open the hood and pull that handle at the 3 minute mark of point overheating! Keeping in mind that the damage to the points can occur in two to three minutes.

I live in South Carolina --- where you can still be sweating on a hard start winter day. On a summer day around here she only needs about 1/3rd choke to light up.

Besides, my SOP is to pre-flight the engine bay before every start (check oil, belts, hoses, etc), and I leave the hood up until I get a good start. I don't close the hood and roll until the coolant temp gauge starts to rise.

I worked aviation too!

We usually did not make routine engine starts with the engine cowl open.

For you, it's not that first start of the day that will get you. It's one of the many starts at car show or parade that will get you.