What Should The Voltage Be From The Ignition Trigger Wire To "i" On Solenoid?

LILCBRA said:

Copied from allfordmustangs.com forums, author was "The Greek"

You should have the following voltages coming from these terminals with the key in the "on" position. Zero volts at "S" and about 8 volts at "I". The starter solenoid "S" terminal is nothing more than the positive side of a wire wound electro magnet. The negative side of the winding is internally attached to the mounting bracket of the starter solenoid and becomes a ground when the bracket makes contact with chassis ground by the bracket's contact with the inner fender when it's bolted down, completing the circuit.

When terminal "S" gets energized, (12 volts, key in "start" position), it creates a magnetic field that draws the contacts of the large terminals together and the "I" terminal also gets energized and delivers a full 12 volts at the same time. You will only have the 12 volts there when the solenoid is energized and voltage to the starter side, (large terminal), is present.

Again, the only time there should be voltage at the "S" terminal is when the key is in the "start" position. The "I" terminal wire will have 12 volts at the same time. This wire is connected to the ignition coil circuit and provides a full 12 volts to the coil to make it easier to start the engine. It by-passes the coil's ballast resistor wire, which when in the "on" position, delivers a lower 8 volts to the coil (the voltage is now routed through the ballast resistor) to prevent the coil from overheating and to help the ignition points to live longer.

If you were to disconnect the wire to the "S" terminal with key in the "on" position and take a voltage reading at the wire itself, it should show zero volts. At the "I" terminal, again disconnected and checked the same way, there should be approximately 8 volts present. (the voltage is backfed through the wire from the coil)

The 8 volts at "I" will have no effect on the solenoid itself in the "on" position because it is now an open circuit, at this point, unless the solenoid is energized and the key is in the "start" position.

I have had something similar happen to one of my cars years ago. For me, it turned out to be junk spark plugs. I have also had something similar happen with a bad ground, although it was a little more erratic. It sounds like it's heat related to me. Are you running a stock setup and do you have the ballast resistor installed? And you say you have a new carb installed, is it somewhat tuned to your setup? It could be fuel related if not. Too much/not enough accelerator pump, running too rich, bad float setting, choke not disengaging, etc. But, assuming that everything else is fine and it is something in the ignition system, it looks like you've replaced everything but your plugs - what brand are you running? Like I said, this sounds almost exactly like what I went through when I found I had bad plugs. This was the case that steered me away from Champions! I had just installed the Champions, then went with Autolites and the problem disappeared! That was the only thing that had changed before the problem started, so it was fairly easy for me to figure out. Just food for thought - good luck and let us know what happens!!

Click to expand...

LILCBRA said:

Copied from allfordmustangs.com forums, author was "The Greek"

You should have the following voltages coming from these terminals with the key in the "on" position. Zero volts at "S" and about 8 volts at "I". The starter solenoid "S" terminal is nothing more than the positive side of a wire wound electro magnet. The negative side of the winding is internally attached to the mounting bracket of the starter solenoid and becomes a ground when the bracket makes contact with chassis ground by the bracket's contact with the inner fender when it's bolted down, completing the circuit.

When terminal "S" gets energized, (12 volts, key in "start" position), it creates a magnetic field that draws the contacts of the large terminals together and the "I" terminal also gets energized and delivers a full 12 volts at the same time. You will only have the 12 volts there when the solenoid is energized and voltage to the starter side, (large terminal), is present.

Again, the only time there should be voltage at the "S" terminal is when the key is in the "start" position. The "I" terminal wire will have 12 volts at the same time. This wire is connected to the ignition coil circuit and provides a full 12 volts to the coil to make it easier to start the engine. It by-passes the coil's ballast resistor wire, which when in the "on" position, delivers a lower 8 volts to the coil (the voltage is now routed through the ballast resistor) to prevent the coil from overheating and to help the ignition points to live longer.

If you were to disconnect the wire to the "S" terminal with key in the "on" position and take a voltage reading at the wire itself, it should show zero volts. At the "I" terminal, again disconnected and checked the same way, there should be approximately 8 volts present. (the voltage is backfed through the wire from the coil)

The 8 volts at "I" will have no effect on the solenoid itself in the "on" position because it is now an open circuit, at this point, unless the solenoid is energized and the key is in the "start" position.

I have had something similar happen to one of my cars years ago. For me, it turned out to be junk spark plugs. I have also had something similar happen with a bad ground, although it was a little more erratic. It sounds like it's heat related to me. Are you running a stock setup and do you have the ballast resistor installed? And you say you have a new carb installed, is it somewhat tuned to your setup? It could be fuel related if not. Too much/not enough accelerator pump, running too rich, bad float setting, choke not disengaging, etc. But, assuming that everything else is fine and it is something in the ignition system, it looks like you've replaced everything but your plugs - what brand are you running? Like I said, this sounds almost exactly like what I went through when I found I had bad plugs. This was the case that steered me away from Champions! I had just installed the Champions, then went with Autolites and the problem disappeared! That was the only thing that had changed before the problem started, so it was fairly easy for me to figure out. Just food for thought - good luck and let us know what happens!!

Click to expand...

LILCBRA said:

Copied from allfordmustangs.com forums, author was "The Greek"

You should have the following voltages coming from these terminals with the key in the "on" position. Zero volts at "S" and about 8 volts at "I". The starter solenoid "S" terminal is nothing more than the positive side of a wire wound electro magnet. The negative side of the winding is internally attached to the mounting bracket of the starter solenoid and becomes a ground when the bracket makes contact with chassis ground by the bracket's contact with the inner fender when it's bolted down, completing the circuit.

When terminal "S" gets energized, (12 volts, key in "start" position), it creates a magnetic field that draws the contacts of the large terminals together and the "I" terminal also gets energized and delivers a full 12 volts at the same time. You will only have the 12 volts there when the solenoid is energized and voltage to the starter side, (large terminal), is present.

Again, the only time there should be voltage at the "S" terminal is when the key is in the "start" position. The "I" terminal wire will have 12 volts at the same time. This wire is connected to the ignition coil circuit and provides a full 12 volts to the coil to make it easier to start the engine. It by-passes the coil's ballast resistor wire, which when in the "on" position, delivers a lower 8 volts to the coil (the voltage is now routed through the ballast resistor) to prevent the coil from overheating and to help the ignition points to live longer.

If you were to disconnect the wire to the "S" terminal with key in the "on" position and take a voltage reading at the wire itself, it should show zero volts. At the "I" terminal, again disconnected and checked the same way, there should be approximately 8 volts present. (the voltage is backfed through the wire from the coil)

The 8 volts at "I" will have no effect on the solenoid itself in the "on" position because it is now an open circuit, at this point, unless the solenoid is energized and the key is in the "start" position.

I have had something similar happen to one of my cars years ago. For me, it turned out to be junk spark plugs. I have also had something similar happen with a bad ground, although it was a little more erratic. It sounds like it's heat related to me. Are you running a stock setup and do you have the ballast resistor installed? And you say you have a new carb installed, is it somewhat tuned to your setup? It could be fuel related if not. Too much/not enough accelerator pump, running too rich, bad float setting, choke not disengaging, etc. But, assuming that everything else is fine and it is something in the ignition system, it looks like you've replaced everything but your plugs - what brand are you running? Like I said, this sounds almost exactly like what I went through when I found I had bad plugs. This was the case that steered me away from Champions! I had just installed the Champions, then went with Autolites and the problem disappeared! That was the only thing that had changed before the problem started, so it was fairly easy for me to figure out. Just food for thought - good luck and let us know what happens!!

Click to expand...