My 77 Mustang based kit car is dying after running about 15 minutes, especially upon acceleration from a red light. Runs great for the first 10-15 mins. A huge bog kicks in, feels like all spark is being lost, will sometimes sputter awhile and take off again and be fine, but sometimes just dies. New ICM, starter, solenoid, coil installed, new carburetor......dont't believe its fuel. Battery is charging at 14.25, voltage at rest is 12.5. Everything seems grounded properly including solenoid, ICM. Trigger wire from starter only showing 1.2 volts though and I understand this should be more like 9V? What might the cause be? What are the symptoms of a lot voltage going to solenoid? Can this be causing my running/stall problems? Thanks!
What Should The Voltage Be From The Ignition Trigger Wire To "i" On Solenoid?
- Thread starter hunt_bird
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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!!
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!!
Thank you! Very helpful! My firewall is fiberglass so I'm wondering how I do this:
"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."
"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."
jozsefsz
15 Year Member
You'd need to run a ground wire to the solenoid's mounting bolt(s) from battery ground. A ring-terminal will do the trick.
There's a small mistake in the posting that was copied over. If you disconnect the wire from the "I" terminal you won't have any voltage reading on the terminal. The poster is correct when he says the voltage is back-fed from the ignition wire on the coil. The voltage on the wire you've disconnected will be ~8V, but the voltage at the "I" terminal will be 0. That's not all that critical (the rest of the info is good), but if you follow his steps and find "0" at the terminal, nothing is wrong.
Your car will start and run fine without anything connected to "I." Just a little crappy on a cold day. In your case I suspect you're missing a ground due to the fact that your solenoid is mounted to an insulator (fiberglass) when its body needs to be grounded.
There's a small mistake in the posting that was copied over. If you disconnect the wire from the "I" terminal you won't have any voltage reading on the terminal. The poster is correct when he says the voltage is back-fed from the ignition wire on the coil. The voltage on the wire you've disconnected will be ~8V, but the voltage at the "I" terminal will be 0. That's not all that critical (the rest of the info is good), but if you follow his steps and find "0" at the terminal, nothing is wrong.
Your car will start and run fine without anything connected to "I." Just a little crappy on a cold day. In your case I suspect you're missing a ground due to the fact that your solenoid is mounted to an insulator (fiberglass) when its body needs to be grounded.
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