Running Problem - Possible Electrical

[YukonJack]

I've been chasing down a running problem for a week or so now on my 47 Plymouth. Stock engine , but converted to 12 volt negative ground about 20 years ago. Car runs rough and dies out sometimes and won't restart for about 5 minutes. With a test light it will have power on both sides of the coil, but no spark. I have new points and condensor that I haven't installed yet. Checking the ignition circuit, I have 13.5 volts going to the ignition resistor and about 8.4 coming out and going to the coil. The negative side of the coil going to the distributor has about 5 volts. Shouldn't I have the 8.4 volts on both sides of the coil? I also recently ran a wire from the I terminal on my solenoid to the positive side of the coil for faster starts. I've noticed now I have power at the I terminal when the engine is running, not just on start up. It has become harder to start whether I disconnet the wire from the I terminal or leave it connected. This doesn't seem right and wonder if it's going to cause problems with the solenoid. Thanks in advance.

[Sniper]

11 minutes ago, YukonJack said:

Shouldn't I have the 8.4 volts on both sides of the coil?

 

Only if the points are open, if they are closed you should have 8.4v on the coil + and zero on the coil - (12v negative ground system).

 

Properly setup your 12v conversion should have bypassed the ballast (ignition) resistor providing full system voltage to the coil + in start, only.

 

Sounds like your starter solenoid is bad.  You can run a temporary jumper, for testing purposes only, from the coil + to the battery + and see if it starts up properly, if so it's the solenoid or the wiring for the ignition start circuit.  Make sure you are in neutral because you will not be able to turn the engine off until you pull the jumper.

 

Edited July 10, 2023 by Sniper

[YukonJack]

The ballast resistor was by passed from the I terminal to the + side of the coil for 12 volts at start up but I have power at the I terminal after it's running. As far as my 8.4 volts on the + side of the coil and 5 volts going out to the distributer when the car is running is normal in your opinion? I wouldn't thing I should be losing 3.4 volts in the coil between the + and - sides.

[Merle Coggins]

Your slight voltage drop through the coil is due to the fact that something in your distributor is consuming a small amount of current. What may be happening is that your points are burnt due to the higher voltage while running. This can cause a high resistance so that they won't make a good ground connection. As Sniper stated, if the points are closed they should be providing a ground to the coil to complete the electrical circuit. This would be indicated by a 0 volt reading. The fact that you are getting 5 volts out with 8.4 in tells me that you have a complete circuit, but with another load in the circuit. That load could very well be fouled points contacts. Try cleaning, or replacing, the points and recheck your voltages. 

I also agree that you need to revisit your starter solenoid connection. It should only provide 12 volts during cranking. If it is feeding a continuous 12v while running that can lead to premature points failure, as you may be experiencing. 

Edited July 10, 2023 by Merle Coggins

[Dave72dt]

Check your voltage at the I terminal when it's running.  It should read the same as coil in voltage.  It's still connected to the coil unless you put a diode in so you would be getting a backfeed of your 8.something when running and your 12 only when starter is engaged.

 

check your coil out terminal voltage with the dist wire removed.  Still the 5. , you may have an internally resisted 12v coil .  Goes up to 8, problem within or to the dist.

Edited July 10, 2023 by Dave72dt
added info

[YukonJack]

4 hours ago, Dave72dt said:

Check your voltage at the I terminal when it's running.  It should read the same as coil in voltage.  It's still connected to the coil unless you put a diode in so you would be getting a backfeed of your 8.something when running and your 12 only when starter is engaged.

 

check your coil out terminal voltage with the dist wire removed.  Still the 5. , you may have an internally resisted 12v coil .  Goes up to 8, problem within or to the dist.

8.4 volts going to the I terminal with engine running. Would this damage the solenoid? 

 

[Dave72dt]

No,  the I terminal is just a post when it's not activated by the switch in the start position.  That's the voltage you should be seeing when running.  The back feed goes no further when running.

 

Removing the wire going to the dist and comparing in and out voltage lets you check the coil without any other possible voltage draws on it and will let you determine where the voltage drop is coming from, either from the dist or from the coil itself.

[Sniper]

Okay I didn't realize you measured it with the engine running. What you were seeing at the point side of the coil is an average of the off and on voltage as the points open and close. Your multimeter cannot react fast enough to show zero and 8.4 so you're getting an averaging effect. Kind of like how the instrument voltage regulator Works in later Mopars it's just a bimetallic strip that opens and closes and chops the DC voltage to about a 5 volt average.   The condenser likely is what's determining that average.

Edited July 11, 2023 by Sniper