Ignition question

[TheRktmn]

I am looking for some input regarding ignition systems. Anyone with a running car, a volt meter and a Pertronix or points ignition can answer this...

I want to know what the voltages are on the + and - posts of the coil when the car is running. That's it. Easy, right?  

If you could post anything about your ignition such as Pertronix, points, coil, ballast (pink) wire or not along with the readings would be helpful.

 

[Don C]

Because the voltage fluctuates as the points open and close (or as a Pertronix provides an intermittent ground) from around 7 volts (points closed) to around 13 volts (points open) it's not that easy to get a voltage reading with a digital volt meter, due to the rapid fluctuations. The reason for the fluctuations is due to the loading, and voltage reduction, across the resistor (ballast) wire. Different meters respond with different rates due to internal circuit design. An analog meter is easier to get an average voltage reading with, but the needle on different meters will bounce more or less, depending on how the meter needle is dampened.

So, I guess what I'm saying is that unless two people use the same meter the results are likely to be different, while both are correct. The idling voltage should average around 10 volts. An oscilloscope is the best way to accurately measure the voltage swings.

 

[TheRktmn]

Actually, I am trying to find out what is being generated for a tach signal. I get that the - side will 'pulse' as the trigger opens and closes, but you can still see an average with a meter.

I have a customer that is only getting 8.2 V signal at idle and 10 V around 2700 RPM.The standard for tachometer signals is 10-12 V. It's a square wave so some meters will show 5.x volts switching rapidly from + to -.

He changed the coil to a Pertronix '12V' coil and bypassed the 'pink' ballast resistor but it made no difference.

Granted, the coil has an internal resistance of 1.5 ohms, but I would still expect to see an increase when roughly 3 more volts are going into the coil.

 

[Don C]

It sounds like something else is loading the resistance in the coil, which would keep the voltage from bouncing back to battery voltage in the absence of ground (points open or Pertronix not supply ground to the coil). If he has a Pertronix the module might be bad, or if points, the condenser may be bad. With the negative wire removed from the coil does the negative terminal show near battery voltage with a digital voltmeter?

Are the voltage readings he is taking with a digital or analog meter? Analog meters will load the ballast resistor and the coil's internal resistance, resulting in a lower than battery voltage reading. Most analog meters have low internal resistance, which causes the load across a circuit that has a resistance. Digital meters have very high resistance and don't load a resistive circuit enough to cause a voltage reduction.

Next I would suspect that the ignition switch is bad, corroded, high resistance contacts.

If he runs a jumper directly from the battery to the positive terminal on the coil what kind of voltage does he get on the negative and positive coil terminals with the engine running?

Finally, if he has Pertronix does he have the breaker plate ground wire connected to the distributor housing?

 

[TheRktmn]

Good points, Don. I was thinking about a bad ground as well.

He is running a Pertronix I module and switched to a Pertronix coil because we thought the aftermarket coil was faulty. He went with the '12V' coil and ran a new power feed to it, verified that he is getting battery voltage to the coil with key on.

I'm not sure what meter he is using, but I can recreate what he is seeing on the tach her on the bench by reducing the signal voltage to what he is showing. At his signal voltage I get the exact same issues with the tach.

Thing is, the car starts and runs fine.

 

[Don C]

One more thing I would try is to ground the negative post on the coil and then check the voltage on the positive terminal, with key on.