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Troubleshooting Ignition and Starter Solenoid

4K views 9 replies 8 participants last post by  awhtx 
#1 ·
'66 Mustang Coupe, 302 V8, C4 Auto
New battery
Neutral Safety Switch - functioning properly
Mini starter - works

I have searched these forums and read many threads on the subject, so please forgive me - I'm just not quite sure I understand everything. I'm using a set of colorized wiring diagrams and looking for a little verification.

I don't think I'm getting the right voltage at the coil. 12.3V on the big left post of the solenoid. When the key is turned to "Start", I see 12.2V on the red/blue wire to the "S" terminal, 12.2V to the "I" terminal, 12.2V to the far right terminal for the starter, and the starter motor operates. So far so good, right?

If I remove the brown wire from "I" and turn the key to "On", I get low voltage (0-300mV) on "I" and 12.1V on the brown wire. Turn the key to start, and I get 12.2V on "I" and the brown wire. Therefore, I also see 12.1V on the red/green wire to the coil when the key is in "On" and 12.2V when the key is in "Start".

I believe my starter solenoid is operating properly, but I think I'm seeing too high of a voltage to the coil when "On". The big pink wire is supposed to drop it to 7-9V, right? I should see 7-9V on the red/green coil wire, and 7-9V on the brown wire, right? Perhaps the pink wire has been bypassed? Best I can tell, the pink wire is connected to the brown and red/green wires at the main disconnect at the firewall. It disappears into a wiring loom between the main disconnect and the starter switch.

Thanks in advance,
Brian
 
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#2 ·
From what I understand, when the ignition is in the "Start" position, 12 volts goes to the coil from the solenoid, through the brown wire, and then into the red/green wire to the coil. This is to help the engine start. Once the engine is started, and the key is released to allow the ignition switch to go back to the "On" position, then the pink resistance wire comes into play and lowers the voltage to the coil.
 
#4 ·
I you're using a multimeter on an open circuit you'll never see the resistance in the circuit reflected by your reading. You need to connect a jumper between the coil - and ground to test, but make sure you don't leave it like that for more than a few seconds to avoid getting the coil too hot.
 
#5 ·
Exactly. What bartl is saying is Ohm' series law. In a series circuit you add up total resistance and divide that into the source voltage. That gives you the amperage flowing through the circuit. No matter the resistance, the amperage is the same through everything. Now you multiply the amperage to the resistance of each device and that gives you the voltage drop on that device. The voltage added up of each device will equal the source voltage. The volt meter has such an extremely high internal resistance so it won't influence the circuit. Compared to other resistances such as the coil resistor wire, the other resistance in negligible compared to the meter resistance. It's like a piece of copper wire in comparison. What you're actually reading is the voltage drop of the meter itself which is basically the source voltage.

In a parallel circuit the voltage stays the same and amperage adds up to the source amperage.
 
#6 ·
So, are you making theses comments because you have an ignition firing problem or to learn how the craning and ignition circuits are supposed to work?
 
#7 ·
Well, both I suppose. My son and I were troubleshooting an ignition problem, so we were checking the circuit from the battery to the coil via the ignition switch and starter solenoid. I was looking for verification that my expectations of voltages at the different locations were correct. Bartl straightened me out on why I was seeing 12v at the coil in "Run". Our problem was a bad distributor cap. Put on a new coil (with no need for an external ballast resistor), cap, and rotor, and was able to start the car today (for the 1st time since we've had it).

Thanks for all of the responses.
 
#8 ·
Resurrecting this thread.

A Google search led me to this thread. Hot start problem. A jumper from the battery straight to the + side of the coil and it started immediately. What would be an effective testing sequence. I have a digital meter and a test light.
 
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