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Discussion Starter #1
On '68s there are two terminals on the ignition switch labeled (in the shop manual) as "prove out". As best I can tell, they merely provide a path to ground while the switch is in crank position. These terminals don't provide power - just a path to ground.

On my car, only one of these is used, and it's only used on the brake warning light. So, the brake warning light only illuminates (if you have a system failure) when you're cranking the engine - that makes no sense.

What also makes no sense is why there are two PO terminals. Heavy users like "accessory" only get one.

Wazzup w'dis PO terminal thing? What's it for?
 

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The PO feature is there to test the brake system warning bulb to make sure that it is not burned out.
 

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It provides a temporary ground to all warning lamps to test them. After the ignition is released back to the run circuit from start the warning lamps revert back to their own ground paths (switches, sensors,etc.). Remember, path of least resistance. When the ignition switch provides the temporary ground, the warning lamps take it.
 

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Discussion Starter #4
StrokerDude said:
It provides a temporary ground to all warning lamps to test them. After the ignition is released back to the run circuit from start the warning lamps revert back to their own ground paths (switches, sensors,etc.). Remember, path of least resistance. When the ignition switch provides the temporary ground, the warning lamps take it.
Then, there must be another path to ground after the brake switch. I haven't found it yet, but this explanation makes perfect sense.

My wiring diagram manual is a bit weak on the details. The brake warning switch and it's circuit aren't even shown. My harnesses are out and nekkid, so I can trace it down visually.
 

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On a '67 the switch itself is the ground. (68 might be different) When there is a pressure differential between front and rear brake circuits large enough to move the shuttle in the distribution block to one side or the other a pin slides down and completes the circuit and the light comes on.
 

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Discussion Starter #7
AzPete said:
Here is another wiring diagram that may help you.
http://hammar.dyndns.org/~djhamma/wiring/1968/E7.jpg
That's the diagram I've been working from - it's in my "Wiring Diagram Manual". But, the connector, switch, and circuit are absent in that diagram. The wires are purple (violet) with white stripe.

This might be shown in another diagram not included in that manual. There's a big, impossible-to-read entire-car schematic that might show it, but...
 

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Discussion Starter #8
JeffTepper said:
On a '67 the switch itself is the ground. (68 might be different) When there is a pressure differential between front and rear brake circuits large enough to move the shuttle in the distribution block to one side or the other a pin slides down and completes the circuit and the light comes on.
I think '67s have one wire whereas '68s have two wires. I have disassembled my block and see no path to ground there or in the switch. One of those purple wires probably goes to ground somewhere.
 

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Discussion Starter #10
OK, I figured it out.

12v is applied to the distribution block (actually just a switch) and the circuit is grounded when a big pressure differential occurs. The piston inside moves one way or the other and somehow creates a path to ground. I haven't figured exactly how this switch works, yet, but I will.

The reason for the second wire is to provide a second path to ground - the prove-out terminal. So, even if the switch is in "neutral", the prove-out terminal will provide ground during cranking so you can see if the light works.

I've heard that '67s don't have two wires. If that's so, they also probably don't have the bulb test feature, either.

I had to reference the big, all-together wiring schematic in the shop manual to find this out. Only there is this brake warning circuit shown.
 
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