1940 D14 Ignition Coil

[Shaman]

While trying to get my car started, I’m leaning towards some electrical issues.  I believe I have a bad cable that I’m replacing now.  When doing a voltage drop test it shows that the positive cable is bad.  Anyway, while doing some research I ran across how to test the ignition coil.  I figured it was good to test as many things as I can during this process, but after testing the external post to the internal connection, on the ignition coil, my meter didn’t show anything with a resistance test.  So I’m assuming the coil is bad.  I’ve been looking for a replacement, but the ones I’m finding locally don’t have the cable on the back side of the coil.  Instead it has two posts on top, beside where the wire going to the distributor cap goes to.  Has anyone replaced an older coil with one of these without the cable going to the ignition switch?  It seems like I should have to have some type of connection back to the switch, but I’m probably wrong.  This is my first real attempt at any type of rebuild, so I really appreciate all the help.

[TodFitch]

Where are you measuring the resistance to/from? I assume one point would be the stud where the low voltage wire to the points in the distributor connects. But where is the other end?

 

For what it is worth, the other end of the low tension winding in the coil goes through the armored cable to the ignition switch. And the wiring of that switch is such that everything is disconnected when the ignition is off. So a resistance test will show an open circuit.

 

If you remove the power and fuel gauge wires from the switch, then I guess you could measure the resistance (with the switch in the on position) between the power feed to the switch under the dash and the stud on the coil in the engine compartment. But that is a lot of hassle. Much easier to simply crank the engine over with the ignition on and check for a hot spark.

[Shaman]

I was using the post and then where the high tension wire that goes to the distributor cap.  I found a video that showed to do that to test a ignition coil.  It was a 12v system instead of the 6v.  

I did get the ignition coil out of the car and I do have good voltage to the ignition coil.  Before I removed it I did the test with the wire going to the distributor cap and I didn’t get any spark.  I do know the cable from the distributor to the ignition coil is not the exact one.  It was black taped to the end and it wasn’t inserted into the coil very well.  Even after trying to get it in the best I could and re-taping it, no spark.  

 

The coil was very corroded on the end where the wire goes to the ignition switch. 

So now I’m not sure if the coil is bad or if I’ve got some other issue.  I’m just going to work through trying to get spark, but I don’t know how to test the coil anymore than what I’ve done.  Here are pictures of the coil so you can see the post and where I tried to get resistance.

 

I did get resistance when touching the bottom post and the top post.  I’m honestly not sure what I’m supposed to get, but I did get some results there.  I get nothing, no matter how I try, with the connection inside where the wire that goes to the distributor cap connects.   

 

 

[Andydodge]

Those original style coils with the armoured wire going from the back of the original coil to the ignition switch were designed that way as an anti theft device however those coils were from what i understand getting hard to find and were generally replaced by a standard type of coil with both distributor & ignition wires at the front........there was also a small triangular bracket available that clamped around the newer style coil and allowed it to be attached in the same place as the original coil.....the wire from the ignition was either spliced into another wire to the coil or the wire to the coil was attached with a small nut & bolt and taped up with electrical tape......this wire was fed thru a small gap in the triangular clamp then attached to the front of the newer coil..........this coil replacement was quite common as the supply of the old style coils dried up, they are still around but are not that common now...............BTW when I updated my 1940 D15 Oz Dodge I kept the original ignition switch, carefull removed the outer armoured cable and installed a new wire then when I rewired the car, installed the 318 Poly etc it all looks o/k and I welded up the original coil hole etc...........regards from Oz.......... andyd      

[Sniper]

A coil is essentially a step up transformer.  You have two sets of wire wrapped around an iron core.  In the case of a step up transformer (and your coil) one set of wires, called a winding, will have more wraps of wire than the other.  The amount of voltage step up is directly related to the ratio of windings.  For example if you had a 10V input signal you wanted to step up to 1000v your winding ratio would be 10 to 1. 

 

The input side of a transformer is called the primary and the output side the secondary, now a transformer can step up, down or not at all, that's called an isolation transformer.   In all electrical devices you need a supply and a return, for both windings.  In the case of the primary side of YOUR coil the supply is the voltage at the negative terminal and the return is the positive terminal, via the points to ground.  The secondary sided is a bit less obvious though.  It goes from the coil wire terminal, across the gap at the plug, thru the block, to the points and then the positive terminal of the coil.  Internal to the coil both the primary and secondary wondings have one leg of the winding terminated at the positive terminal.

 

How a coil works, when the points are closed the supply voltage at the negative terminal flows thru the primary windings and thru the points to ground, this induces a magnetic field in the core.  When the points open the magnetic field collapses and that induces a voltage in the secondary of the coil.  This induced voltage is what makes the spark at the plug.

 

Now when you use a meter to measure a coil it sees the windings as essentially just a piece of wire and your ohms should read near zero.  If you go from the coil wire terminal to either the positive or the negative terminal you should read near zero, it you go between the positive and the negative terminal you should read near zero ohms.  If you are reading something else, or an open, I would touch the tips of the test leads together and make sure the meter reads zero, or close to it, ohms.  If not you meter is fooey, if it does your coil is bad.

 

Sorry for the novel

[Shaman]

Thanks for both of those replies.  AndyDodge, nice looking engine.  So clean.  

 

These will help.  I’ll do some more testing, after reading Sniper’s comments, I’ll go back through and see what I can test again.  If I still have the same issues, I’ll go pick up and a new coil and see if I can get it to work.  I do wonder if I’m getting a good connection going to the distributor cap because of how the that cable is seated in there.  

[greg g]

Assuming you are still dealing with a six volt system,a normal coil will work.  You will need to run a new wire from the run terminal on the IGN switch through a hole in the fire wall to the negative terminal on the new  coil.  The wire from the IGN switch will need to be made longer than the factory one to reach outside to the terminal.  Any non internally resisted coil will work. , try tractor supply or farm and fleet and the like. Shouldn't be much more than 20 bucks.  I would use 12 gauge wire to make the new lead.  

[Shaman]

Ok, so I put a new coil in.  If I tested the resistance on the old and new coils correctly I did have some issues with the old coil.  So now as I progress through, I’m still not getting a spark past the distributor.  

 

So I cleaned up the points a little and tried to follow the voltage a little at a time. When the coil is disconnected from the distributor, the lead to the coil test 6v.  When I attach it to the distributor it sometimes tests 6v and sometimes tests around 3.  I’m assuming that there is a short causing this drop or inconsistency.  So I pulled the small wire out that is inside the distributor and cleaned where it screws in, that then tests 6v through it.  When I connect that wire back to the distributor, which connects to the point breaker arm I get the drop in voltage again.  Interesting is when I open the points it reads 6 volts, but not when they are closed.  I’m assuming that when they points are closed it should read 6v also, but can anyone tell me for sure?  If it is, then I feel like I’ve got some type of short going on.  It just seems odd that I’d be getting half the voltage when the points are closed, but I’m so new to all of this that I really don’t know.  I even disconnected the condenser in the distributor, just to make sure it wasn’t grounding it, but that didn’t make a difference.  

 

I may be describing this incorrectly, so please let me know if I need to clarify anything or if pictures will help.  I’m kind of at a loss as to how continue testing the distributor.  I just know I’m not getting consistent spark, if I get any at all.  

 

Thanks.

[Sniper]

Depends on where you are testing the 6v at on the coil.  Then the points are closed it applies a ground to the + terminal on the coil, you should lose the 6v at that point, when  the points are open you should read 6v at the coil + terminal.

 

Remember this is a positive ground system, I have seen other brands put out a weak spark if the coil is wired wrong but my 51 Plymouth ran fine when I bought it even though the coil was wired backwards.

 

A simple test to get you looking in the right direction.  Pull the coil, disconnect all wires from it.  Take the high tension coil wire (the one that normally goes to the distributor cap center connection) and plug one end in the coil then put a spark plug in the other end and ground the plug body to the engine, if you can fab a jumper so much the better.  you will be looking for spark at this plug later.

 

Run a jumper from the battery - terminal to the coil - terminal.  Then with another jumper hook up to the battery positive connection, do not hook this to the coil.

 

Now that you are setup just tap the positive jumper on the coil + terminal, tap and release is all you need.  Each time you do that you should see a good spark at the test plug.  If you do then the coil is good.  If there is no spark swap out the coil wire to the plug to make sure that isn't the issue, you can swap out the plug as well, one at a time, to eliminate it.  If it still doesn't spark you have a bad coil.  The problem with a ohmmeter is that it can read an open in a coil, but not a short since a coil is essentially a short as far as the ohmmeter is concerned.  There are meters that can read a good coil but unless you have an inductance meter you won't be able to do that.  I have one, for work.

 

Now, remove the jumper from the battery negative to the coil and put the factory wire back on.  You will now be using the factory supply to the coil in this next step.  Turn the key to run and then take your jumper from the battery +and once again tap the + terminal once coil and observe the spark at the plug.  If the spark looks good then the factory supply to the - terminal of the coil is good.  Turn the key off.

 

Now remove the jumper from the battery + and reattach the factory wires to the + terminal on the coil, remember you still have your test plug hooked to the coil.  Crank the engine over with the key and look at the spark at the test plug, if it's good then the factory setup to the - coil terminal is good.

 

If, at this point everything looks good then the issue is downstream of the coil output.  bad coil wire, bad cap, bad rotor.  You can ohm out the coil wire, depending on what wire you are running you should see 0, or thereabouts for solid wires, or 10k+ ohms for a carbon suppression core wire.  If you see infinity (OL) then it's open and bad.  IF the reading jumper around as you wiggle the wire it's also bad.

 

I have seen the carbon button missing from the cap, the tap on the rotor not touch the carbon button, basically the cap and rotor will be a good visual inspection and if you see something odd, post a picture here.

[Shaman]

I’ll double check and run this test.  I guess I may have a bad coil that I just purchased.  Thanks for the advice on the test.

 

I do have one question.  Should I be concerned that on the positive side of the coil I get 3 or so volts when the points are closed and 6 when they are open?  If I read what you wrote correctly, it should go to 0 instead.  If it’s supposed to go to 0 when the points are closed, I’m guessing that I have a bad ground in the distributor, but I’m not real sure.  

 

Anyway, I’ll do this test and make sure that the coil is in good shape. 

 

Thanks again.

[Sniper]

I don't know for a fact that 3v with the points closed is ok, I suspect it is not since it should be a direct path to ground.  This tells me you probably have a poor or dirty  connection and possibly a bad wire between the points and the coil + terminal.

[Los_Control]

With what little I know ... points are like a switch, on or off ... should be .000 when off.

If you get some sort of reading ... you know that is not correct .... it may start and run but you know is not right. It could be a bad ground or connection, or points need replaced. Could be a wire ... you will have to find what is causing it.

 

Pretty sure you can remove the points from the distributor and check them isolated from the car. If they check out fine then you are 1 step closer to finding issue.

[Plymouthy Adams]

when the points are OFF...that means not conducting in electrical terms....the voltage will be the potential of battery each side of the coil....only when on (conducting) should you not read voltage on the distributor side of the coil as it is taking the path to ground....ground is ground...!

[Shaman]

So, with the answers here I had success.  

 

I actually had several things going against me.  First off, the coil was wired backwards.  I thought I had it right, but when I didn’t get spark early on, I changed it.  The odd thing with the voltage being at 3v.  I’m pretty confident that was the points.  I filed those a little and that voltage ended up dropping below 1v.  With what was said above about it would probably be close to 0v that told me that I needed to find out why when the point was closed I had higher voltage.  So after all of that I tested for spark and had it on the high tension wire from the coil.  Then I tested to the spark plug and I had it on the wire, but not on the plug.  The plugs were a little old so after getting new plugs I had good spark. Then I was able to fire her up and she ran.  

 

Now I just have to get the fuel pump working.  I ran it by pouring fuel into the carburetor.

 

Thanks for all the help.

[Shaman]

So I have a new question that goes along with the coil.  

 

Quick update first, she’s running pretty well now.  Got the carburetor cleaned up, flushed the lines, and a new fuel filter and running well.  In fact, I’ve been able to drive around the block a few times and only had to pull her back once, which was some issue with the fuel.  I’m pretty sure it was just some old trash I didn’t get flushed good.  

 

Ok, so now for the question.  Does anyone know what this piece is that goes into the top of the firewall coil?  I can’t find it anywhere, except when I find a new firewall coil.  I think my old coil is good and I’d like to use that one, but I don’t like the fact of the tension wire just sitting in there and not seated into anything.  The picture I’m attaching is from a site that is selling a new coil.  

 

Thanks for the help.

 

 

 

[TodFitch]

I have never seen that part except with an unmolested original coil.

 

I am a little surprised to see a vendor image that actually shows a coil that mounts through the firewall. I'll have to look at their website.

[greg g]

On my coil 46, the coil to dist wire screws into a similar piece that actually screws into the coil for a very secure attachment.