Electrical 78 CJ Oil pressure switch

[78 CJ5]

I have a 78 CJ and I am having issues with the oil pressure switch wiring. The oil pressure switch has three posts one wire goes up to the choke and the other two go to a selonoid. One of these wires are hot as the battery is connected to that side of the selonoid. The other appears to be a ground. The wires keep melting. How is this supposed to be wired correctly? Not sure why this is happening but I have fried three different sets of wires now. Only one wire is hot? Not sure Can someone please help me?

 

[TSB8C]

For the gauge wiring, from the passenger side, left to right in the picture:

* Oil Pressure gauge S terminal (left) – Purple wire from sender/switch
* Oil Pressure gauge middle terminal – Black wire (Ground)
* Oil Pressure gauge right terminal – Red wire (Ignition-on hot 12v - NOT always hot from battery)

Testing:

The oil pressure sending unit (switch) is on the engine block and looks like a small 2 X 3 inch filter with one terminal. There may be another sender plumbed in the same area that has two connectors. It is an oil pressure switch that is supposed to close below 4 psi to activate a dash warning light in some speedometer clusters. If you don't have this option, your sender (switch) should have a single terminal. It is basically a variable resistor between the purple wire going to the gauge and ground (screwed into the engine).

To be sure the problem is not the gauge, you can momentarily short the wire from the output of the Sender to ground. If there is no resistance, your gauge should read 80 psi. DO NOT hold it for long in this position. If the needle does not move from zero psi then, either the wiring (open circuit) or the gauge is bad. If it does move, the sender unit is bad.

It is very common for the sending units used with the AMC 258 i6 / 4.2l and AMC 232 i6 engines to be inaccurate. Make sure you have a good connection to the sending unit.

You can also test the gauge by connecting a variable resistor block or potentiometer to the gauge "S" terminal and ground and varying the resistance in place of the sender unit. You can also use these measurements to test the sender by unplugging the purple wire and connecting an ohmmeter between the terminal and ground with the engine running. Pressure (PSI) Resistance (ohms):

0 psi: 234-246 ohms
20 psi: 149-157 ohms
40 psi: 100.5-105.5 ohms
60 psi: 65-69 ohms
80 psi: 32.5-34.5 ohms