1950 Plymouth P20 6volt Positive Ground - No Spark when Engine Cranks Over

[1950PlymouthFlatty]

Hi All, I have a 1950 Plymouth p20 special deluxe which has a 6 volt positive ground system. I am having trouble with no spark at the plugs. I have power (6volts) going to the negative terminal (-) of the coil from the ignition switch. The positive (+) terminal of the coil goes to the distributor. I have noticed a spark when the key is turned into the "on" position, but no spark when the motor is cranking over. The only spark I see is usually after I stop engaging the key switch and it stops cranking over. I am assuming based off of this that I am only generating a spark when the key in the ignition switch is switched to on and after it returns to on after being engaged to crank over. Any help would be appreciated.. thanks

[Plymouthy Adams]

check to see why you losing power in the start position...may have the wire on the wrong terminal of the ignition switch....OR you flat out pulling all your reserve current with the starter motor due to bad bush or voltage drops.  Do a voltage drop test and current draw on the starter system if you find the wiring is correct...You can verify power to the coil in start by disconnecting the lead to the starter and removing possible high current drain.

[greg g]

Jump the coil negative terminal dire try to the battery is the battery side terminal on the solenoid.  If it has spark then, you need to check out your connections from the ign switch to the coil.  If you do this you will need to pull the jumper wire from one of the connections to shut down the engine.

[Sniper]

2 hours ago, Plymouthy Adams said:

check to see why you losing power in the start position

 

That assumes he is losing power then, the PO made no mention of checking for voltage to the coil.  Had a customer that had this issue with a 78 slant six.  Turns out he adjusted the "points" and couldn't get it running.  There were no points in 78, he adjusted the pickup for the electronic ignition into the reluctor and tore the plastic drive gear off the distributor shaft.

 

His original problem was a bad coil wire, which did not have those symptoms.

 

The equivalent in a points ignition would be the points not opening.  So when the key is turned off the magnetic field in the coil collapses (as it would if the points opened) causing a spark to occur then.  Pull the cap, inspect the points, use a meter to verify voltage to the coil in crank.  Then let us know what you see. 

[Plymouthy Adams]

he makes mention of the voltage to the coil in run...the man states when it does seem to fire it is when he releases the key from start position....we start out with a bit of credit to the PO for setting points even if later we may have been wrong in doing so...

[1950PlymouthFlatty]

1 hour ago, Plymouthy Adams said:

he makes mention of the voltage to the coil in run...the man states when it does seem to fire it is when he releases the key from start position....we start out with a bit of credit to the PO for setting points even if later we may have been wrong in doing so...

Thanks for the responses... The points have been set and there is a new coil and condenser. All the wires on the ignition switch have been hooked up like the manual diagram say's, so I do not believe the wires are on the wrong terminal on the ignition switch. I'm going to go back out in the morning and try what you guys have suggested so far and I'll get back to you guys with the results. I'm all ears for anything

[Go Fleiter]

check voltage while cranking: If it drops under 5V, You will not get a good spark, if ever.

These starter motors on a heavy engine are very hungry.
A new starter motor was not enough to solve the Voltage drop problem on mine.

 

I helped me with 12V while cranking. Needs a second 6V battery in line-

and the result is loss of originality. But this is not a recommandation:

You need to understand the electrical systems.

(eg.: no charging while driving for 2nd battery, charging at home only (1 charge lasts 3-4 weeks of holidays),

no lights on while starting (nobody would do!) - 12v kills the bulbs...)

 

Greetings! Go

 

 

Edited December 7, 2021 by Go Fleiter

[Sniper]

8 hours ago, Plymouthy Adams said:

he makes mention of the voltage to the coil in run...the man states when it does seem to fire it is when he releases the key from start position....we start out with a bit of credit to the PO for setting points even if later we may have been wrong in doing so...

 

He mentions it in the sense of how the coil is wired, not that he put a meter on it to verify things, at least that is how I read it. 

 

He may very well have adjusted the points correctly, however it is not uncommon to have them shorted out due to wiring issues.  In which case the points may appear to open and close, but the coil never sees that.

 

When I got my 51 it has a push button for the start function in place of the cigar lighter.  The ignition switch had the crimps loose and that would not allow the start function to work.  Tightening up the crimps fixed that.  You'd put the key in run, push in the button, the engine cranked over and started.  Now it works using the key to start it as it is supposed to. 

 

[Plymouthy Adams]

guess it is a manner of how posts are read, when he states 'fact' he has power on the coil I would say he did read that voltage so I accredited him with some troubleshooting technique.  As he also stated it wants to fire when key released from start position and returns to run circuit, that for this to happen it is also obvious the points are opening and closing to get that spark.    Based on this the issue is wrong wiring or excessive current drain in start, this is where the man needs to split the issue and proceed with further troubleshooting.   Your 51 as wired (starter switch change mentioned here by you) when you got it being hosed wiring is one of the reasons I often say some folks should not be allowed to own an old car much less work on one due to their inexperience or basic skills are lacking and the car suffers from such poor maintenance.  Often it is a jury rig meant to get the car running and nevers gets revisited as wha-la the car is now running.  Folks just cut way too many corners in the repair of items and it will bite them in the butt later or either they flip the car and move on letting the new owner bare the brunt of proper fixes and clean up their mess.  Our reading the same post in two different perspectives in back and forth comments is not helping this man with his current situation...right now he is armed with a number of valid responses that should help him move forward and therein the ball is in his court...he now must decide if he wants to dribble or run with the ball.

[Sniper]

Problem is that start and run are two different circuits and having power to the coil in run, does not ensure having power to the coil in start.  I believe the difference is inside the ignition switch, if I am reading the schematics correctly.  We can guess all we want but until someone lays a meter on the coil and verifies things in start we are just guessing.

 

[Plymouthy Adams]

 I agree and as we are on the forum and not able to put hands or eyes on the problem best techniques to locate the problem is all we have and I prefer not to guess when a quick proven test can easily be done.  This is the very reason in my first reply for the two test I ask him to make that WILL answer the question of power in both modes...actually in place or in place and being starved...also others did reply similar but used jumper wires and such for their quick test...the objective is to learn what is missing and why....the fix will rely on those finding.  The level of fix will be based on the money he wishes to spend for new parts or go the shortcut jury rig and just get it running for now.  Many folks arrive at the same destination, some go over the mountain, some around the graded passes that switch back and forth and then we have those that immediately start digging a tunnel.

[1950PlymouthFlatty]

FIELD TEST DAY ONE

Okay I have done some tests and here are the results. I will also explain how some of the wiring is set up as of right now and while the tests were done.

 

1) First suggestion was by 'Plymouthy Adams' who started with "Do a voltage drop test and current draw on the starter system."

    - I got the digital multi meter out and tested the voltage at the terminal on the starter. With the key off and also when the key was switched to on there was 0volts, when the key was engaged to start and the engine was cranking there was 5.1v and decreased to 4.8v within 5 seconds as the engine continued to crank. (also just to note, there's a battery charger connected to the battery during all these tests).

    - I did not disconnect the main lead to the starter and do the voltage tests with the starter not connected to the system. If this was a mistake and I should also test this let me know.

 

2) Second suggestion was by 'Sniper' who said "Use a meter to verify voltage to the coil in crank."

    - Took the meter back out and tested voltage at the negative (-) terminal on the coil. The terminal had 0volts when the key was in the off position, when the key was turned to the on position it read 6volts, and when the key was engaged to start and the engine was cranking the (-) negative terminal on the coil died to .1volts.

    - I continued with voltage tests on the coil and tested the positive (+) as well. When the key was in the off position it read 0volts, when the key was switched to the on position it read .1 volts climbed to .2 volts as the key sat for 5 seconds in that position. When the key was engaged to start and the engine was cranking the (+) terminal on the coil read 0volts.

    - I would also like to add that I can confirm the points are opening and closing and at the correct gap. I have also done a light test to see if the light would turn on and off as the points opened and closed and the light did not power on at all unless the engine would stop cranking over and happened to land on a spot when the points were closed, which goes back to my OP about only seeing a spark when the key is switched to the on position (and points closed) and when you let off the key an it goes back to the on position (and the points by chance happen to stop in a closed position).

 

3) There was multiple mentions about possible wiring problems, so I figured I would let you know how it is wired currently and while performing these tests.

     - The wire that is on the ST post of the Ignition Switch leads to the smaller top post of the Starter Solenoid.

     - The wire that is connected to the ACC on the Ignition Switch goes to the (-) negative side of the coil.

     - A Wire which is connected to the (-) Negative side of the Coil is connected to the bottom left (I) post of the Horn Relay.

     - The wire that is on the (X) post on the Flasher under the dash also goes to the (-) negative terminal on the Coil.

     - The (+) positive terminal of the Coil goes to the post on the distributor which I believe internally is wired with the points.

 

4) I have also earlier before I made my OP have tested each post on the Ignition switch for voltage and all posts appear to be functioning as and when intended.

 

- This is a Father/Son project, we are not mechanics, but we are also not hopeless. So with the guidance and knowledge of all of you guys we are confident we can make progress. If any photos or videos are needed for aid in diagnosis let me know and I can include some. Thanks again for your help.

 

    

[Sniper]

47 minutes ago, 1950PlymouthFlatty said:

when the key was engaged to start and the engine was cranking the (-) negative terminal on the coil died to .1volts.

this is your problem.  Looks like the ignition switch is defective.

 

As a testing option, you can jump the - terminal on the coil to the - post on the battery and try to start it.  If is start up and runs you have verified it.

 

In this circuit the voltage to the coil in start and in run comes from two different internal positions in the ignition switch, the connection on the switch and the wiring are the same. 

 

[kencombs]

1 hour ago, Sniper said:

this is your problem.  Looks like the ignition switch is defective.

 

As a testing option, you can jump the - terminal on the coil to the - post on the battery and try to start it.  If is start up and runs you have verified it.

 

In this circuit the voltage to the coil in start and in run comes from two different internal positions in the ignition switch, the connection on the switch and the wiring are the same. 

 

That test result is totally dependent on the meter being used.  I have on 'fully automatic' meter that would show those results on a perfectly operating ignition primary.  It will average the measured pulsing  DC.  Like using it to test a trailer light system, it worked fine on brakes and stop, but showed about 6 volts when turn signal activated.   It detects AC or DC automatically as well as Ohm or volts measurement.  I think it sees pulsing DC as AC and measures a square waveform. 

 

Also some digital meters are just not fast enough to display the voltage on a points system accurately.

 

Now as to the problem, it seem odd to me that the coil is wired to the ACC terminal?

[Sniper]

He read both sides of the coil, the negative side that is showing zero volts or near enough as to not matter.  That side should not be affected by the points operation.

 

 

[1950PlymouthFlatty]

3 hours ago, Sniper said:

this is your problem.  Looks like the ignition switch is defective.

 

As a testing option, you can jump the - terminal on the coil to the - post on the battery and try to start it.  If is start up and runs you have verified it.

 

In this circuit the voltage to the coil in start and in run comes from two different internal positions in the ignition switch, the connection on the switch and the wiring are the same. 

 

I have gone back out to the Plymouth and I have done as you have mentioned. I tested to see if the ignition switch was bad by Jumping a wire from the negative on the battery to the negative on the coil and It is now producing a spark at all plugs. I have used an inline spark test light on all plugs and all had spark now. So now I believe I will pull out the OG Ignition Switch and see if I can repair it, or worst case buy a new one to replace it. Then begins the final timing adjustments. Thanks Sniper for the assist on the solution to the no spark at the plugs problem.

[kencombs]

- The wire that is connected to the ACC on the Ignition Switch goes to the (-) negative side of the coil.

   

 

 

I mentioned earlier , it seems wrong to me that the coil is only powered when the ignition sw is on the ACC position  At least that is what this reads like to me.

Not being familiar with the switch in use, does it have an ignition terminal?   s

The jumper wire test proves that it runs with power supplied, but doesn't ID why it wasn't supplied:  was it miswired or bad sw?

[Sniper]

looking at the drawings, it appears being wired to the ACC terminal is correct, but my drawings are not the best labeled ones they could be.

[Dodgeb4ya]

All 1949-52 Plymouths use the armored cable style ignition switch.

The armored cable with the wire lead coming out of it feeds the "-" negative side of the coil..

This armored cable is the "IGN" terminal on the ignition switch.

Accessory electrical items like the heater/radio are connected to the "ACC" terminal.

[Sniper]

I can state categorically that my 51 Plymouth does not use the armored cable switch

 

It uses one like this

 

https://www.ebay.com/p/1723068383?iid=284057006615

[kencombs]

I can't read the markings, if there are any, on that eBay switch.  But, every4 terminal I've encountered used Battery, Start, Run and Accessory.

 

The issue the OP has seem to clearly indicate that he does not have power to the terminal to which the coil is connected when in Start.  Failed switch?  Miswired?  wrong switch?  If it's wired coil acc, I wouldn't expect the coil to have power when in start as most do not power heaters, radios etc while cranking.

 

But who knows how many times the switch has been changed or how the switch functions internally as compared to original.

[Kilgore47]

You may be able to take your ignition switch apart and repair it.  It could be a dirty contact.  I always try to repair what I have before buying a new one.  What could go wrong - it's already broken.

[greg g]

Other alternative is to put in a secret wire with toggle switch to the coil. This would give you a small degree of anti theft protection also.  We lived in an area for a couple years where auto theft was a high risk. I put in a wire with a hidden switch to kill the circuit to the coil when the car was parked. Bench test your switch, maybe the acc terminal is hot in the run position.  If so you can power the coil from there.  Only problem there is that ign would be hot in the acc position.  This might cause premature point problems if parked with the key in the acc position.

[Dodgeb4ya]

1 hour ago, Sniper said:

I can state categorically that my 51 Plymouth does not use the armored cable switch

 

It uses one like this

 

https://www.ebay.com/p/1723068383?iid=284057006615

Your car has the wrong 1953-54 NON- armored type of ignition switch in it.

[Sniper]

My parts books disagree with you, both of them that I have say the 49-54 ignitions switches are the same part number. .  I have only seen the armored cable setup on firewall mounted coils and I do not have that.