Near Intersection of I-35 & I-90 Southern Mn. | A basic SPDT switch would often only have three wires or terminals. Does it have an indicator light? Possibly the fourth is ground for an indicator light.
There are several variations of SPDT. I visualize a SPDT as an old science lab knife switch where the pivot is in the center (common) and the lever could go to a terminal on the left or a terminal o the right. Sometimes a spring is involved so that when the operator lets go the switch returns to a certain position.
The most simple type is designed so the common terminal makes contact with one or the other of the remaining terminals. These may be described as ON-ON. The common terminal makes contact with one or the other of the outer terminals.
Some have only two positions but are momentary so if you push the spring loaded end, a circuit is made but if you release it it goes back the other way to complete the other circuit. These may be labelled as ON-(ON). The parentheses indicate momentary.
Some have three positions and are labelled as ON-OFF-ON. Some of these are momentary and return to the center. They are labelled as (ON)-OFF-(ON). I suppose it would be possible for an ON-OFF-(ON).
Since your switch has four wires it might be a polarity reversing type. These are sometimes referred to as a DC motor reversing switch. Internally they have six terminals but possibly only four are visible to the outside world. These usually would have a center OFF position. Often this type of switch would have 12V on one input terminal and Ground on another. With the rocker one way A might contact C and B might contact D. With the rocker the other way A would contact D and B would contact C. In other words if 12V and ground were connected to A and B with the switch one way the output would be 12V and ground. With the switch the other way the output would be ground and 12V. If the outputs were connected to a DC motor or linear actuator, it would allow the operator to control the direction.
Internally this type of switch is really just two SPDT switches beside each other with a common rocker. The outer terminals are connected together in a criss-cross fashion.
Knowing if the switch has an indicator light and how the rocker behaved when working would help determine the correct type. From the size of the wires, yours appears to be a fairly heavy duty model.
Edited by tedbear 3/11/2019 09:07
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rocker switch
