Electrical function code

Wise ones,
1968 wiring diagram (sheet 3):
What does the code PO at the ignition switch mean? i didn’t find that code in the legend.

Position Open is my wild guess. Aka unused pin connection. I’d have to see the connector in person to confirm.

PO stands for Prove Out circuit.

The ignition switch Prove Out circuit is a ground circuit that is normally open until the switch is turned to the start position where it closes.

Although the dashes Brake light is also powered in the Start and ON position, the dash Brake light does not illuminate in the ON position because their is no ground circuit (assuming that the dual brake warning switch is centered in the master cylinder). The PROVE OUT circuit on the ignition switch are engaged when the ignition switch is in the START position. It is the PROVE OUT circuit that provides a ground to complete the circuit so that the BRAKE dash light lights up.

Coach Jack


CoachJack is absolutely correct.

Boss answer. Thank you!!

Why is there more than one prove out circuit in the start switch ?

One is for the brake proportioning valve; the other could be for an oil pressure lamp or an alternator indicator lamp.

Is this a different prove out than a low fuel light or a emergency brake light ? How many prove out circuits are possible on a Cougar with max options ?

Low fuel and hand brake lights use different circuits for proof-outs that are only indirectly involve the ignition switch, and not the PO circuit in the ignition switch itself.