When it comes time to install or replace one of the electrical system solenoids, it is important to know that there are three configurations of the 12 volt devices and three more for the 24 volt units.
Which one will be compatible with your electric system and its master and start switches is important to find out before you go out and buy one and install it.
The device is really simple. There are two large 5/16 inch diameter threaded terminals, one coming out each side of the solenoid. Both are isolated from the case, and you put 12 volts positive in one terminal (usually the left), and when the unit is actuated, you get 12 volts + out of the other terminal.
When actuated the two are connected internally with points on a high current armature. There is a wire wound coil inside that and when actuated, it pulls the armature so as to connect the two high current terminals together. This coil is actuated by connecting 12 volts positive to one end and grounding the other end of the coil. So far so good.
One the most flexible model of the solenoids, both ends of the actuating coil are brought out on insulated terminals as shown. To actuate the coil, you apply 12 volts positive to one of the coil terminals and ground the other. It doesn’t really make any difference which one is grounded.
Now, if the control switch in the cockpit is configured to deliver 12 volts + to the solenoid, then you attach that wire to one of the coil terminals and attach a wire from the 12 volts ground buss to the other. If the control switch applies a ground to the wire leading to the solenoid, then that wire attaches to one of the coil terminals. Then there has to be a wire from the other coil terminal to 12 volts +.
That is all pretty basic, and you are probably wondering why I am taking time and space to tell you about it. The rub comes when you are dealing with a solenoid that has two high current terminals and only ONE small designed to work only with one of the two options of control switches. The one coil terminal is connected to one end of the coil. You have to know or find out where the other end of the coil is attached. It may be connected to the left hand high current terminal which is 12V+. In that case, the switch must deliver a ground to the one exposed coil terminal. Or it may be connected to the metal case of the solenoid. In that event, the solenoid case must be connected to the 12V+ ground buss, and the control switch must deliver 12V+ to the one exposed coil terminal on the solenoid.
Some solenoids are mounted with a rubber grommet through the mounting holes, and is thus isolated from the mounting surface, like the firewall. There must be a separate wire that connects the solenoid case to the ground buss. On a single terminal solenoid, measure with an ohmmeter from the terminal to the case. If it reads about 15 ohms then you need to apply 12V+ to that terminal for actuation. If it reads infinity, then measure to the left hand high current terminal. If you get about 15 ohms there, you have to apply a ground to the exposed terminal for actuation. Which way is your switch configured? Turn the master and start switch off. With the wire from the switch to the solenoid disconnected turn the switch “ON”, and measure from the end of the wire to ground. If you get a low reading, just a few ohms, then the switch delivers a ground and you need a solenoid with the other end of the coil tied to 12V+.
If the end of the wire measures a very high resistance when the switch is turned on, then it delivers 12V+ on actuation and you need a solenoid with the other end of the coil tied to case ground. This is the reason that I always use a solenoid that has terminals for both ends of the actuating coil – maximum flexibility and no confusion.