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Discussion Starter #1
@Midlife
I purchased the color wiring diagrams for my 67, and I have found a few odd inconsistencies.

Below i will try to isolate one issue I have found as I am trying to sort through the wiring on my gauge cluster. (standard non tach).

Looking at the Car (under-dash) side of the harness.

One wire (shown here as blue/orange) goes to the ignition switch.

The other wire (shown here as Red/Yellow) is supposed to go to the Brake Warning switch. (has to be the one by the master cyl, ans this car does not have the convenience lights)

(Question): Why am I getting switched 12v power on that top-left (Shown here as orange/yellow) pin? ... And not any power at the pin below (blue/orange) as this diagram would suggest??

Is this diagram wrong? Car is wired with original 67 harness, so I don't see how the car could be wrong..???

763342
 

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@Midlife
I purchased the color wiring diagrams for my 67, and I have found a few odd inconsistencies.

Below i will try to isolate one issue I have found as I am trying to sort through the wiring on my gauge cluster. (standard non tach).

Looking at the Car (under-dash) side of the harness.

One wire (shown here as blue/orange) goes to the ignition switch.

The other wire (shown here as Red/Yellow) is supposed to go to the Brake Warning switch. (has to be the one by the master cyl, ans this car does not have the convenience lights)

(Question): Why am I getting switched 12v power on that top-left (Shown here as orange/yellow) pin? ... And not any power at the pin below (blue/orange) as this diagram would suggest??

Is this diagram wrong? Car is wired with original 67 harness, so I don't see how the car could be wrong..???

View attachment 763342
The brake system warning lamp is switched by grounding the circuit. With the key in the "ON" position, power is provided to the warning lamp, then out to the switch. When the switch is in the "neutral" position, the circuit is open so the light does not come on. When the "shuttle" moves in either direction, power is switched to ground to turn on the lamp. A second "grounding" is spliced in to the "switch side" of the lamp going to the "prove out" terminal on the ignition switch which tests the integrity of the bulb by providing a ground when the key is turned to the "START" position.
 

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Discussion Starter #3
Thanks.. that confirms how I thought it was supposed to work.

What has me confused, is the two wires on my harness/cluster seem to be reversed from what I see on my diagram. That top, Orange/Yellow wire should trace to the switch, that will ground it when activated. **and I would not expect to see 12v from that harness pin unless the circuit was closed by the little shuttle tripping the sensor.

And I would expect to see 12volts at the pin below that the diagram shows being connected to the key/switch.

I am trying to figure out, if my car/harness is wrong, or if my diagram is wrong.

I have a Haynes manual out in the barn somwhere. I will take a look there and see if that agrees with the color diagram



The brake system warning lamp is switched by grounding the circuit. With the key in the "ON" position, power is provided to the warning lamp, then out to the switch. When the switch is in the "neutral" position, the circuit is open so the light does not come on. When the "shuttle" moves in either direction, power is switched to ground to turn on the lamp. A second "grounding" is spliced in to the "switch side" of the lamp going to the "prove out" terminal on the ignition switch which tests the integrity of the bulb by providing a ground when the key is turned to the "START" position.
 

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It is a pretty simple design actually. If the brake switch is normal (open to ground) you will have 12V on both sides of the bulb. If either brake fluid reservoir is drained, the shuttle valve moves fore or aft, and the fluid level switch goes to ground. This will cause the lamp to light as long as the key is in the run position.

The ignition switch provides an alternate ground while in the crank position, in order to "prove" that the light is working. This is commonly referred to as a "proof circuit". Note that it does not prove that the fluid level switch works.

This is classified as a "neutral switching" type circuit, which can confuse some.

The picture below is simplified and should get the point across. You can see the two parallel methods of causing the lamp to turn on - on the left is the the brake fluid level switch and on the right is the proof circuit switch.

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Hmmm… There is power to the warning switch on the distribution block at all times when the key is on. When an imbalance pushes the internal pin off-center, the switch touches the pin, it grounds the circuit and illuminates the warning lamp.

Just a note, it is not a low brake fluid warning.
 

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The imbalance would be caused by an empty bowl, so I tend to refer to the switch with a functional description. It is designed to warn the driver of a partial loss of braking capacity. This was introduced when the dual reservoir was introduced. The purpose of that was to...avoid a complete loss of braking due to a loss of fluid.

Another way to look at it is this - if you don't consider the function to be to warn of a low brake fluid situation, what do you consider it to be?

And yes, there is power at that switch at all times under normal running conditions. It is current limited by the bulb.

For switches, whether switching the high side or the low side, there is always power to one side of the switch, otherwise it would not work. Switching on the low side can be confusing. This is why I made the drawing as clear as possible, to convey the concept.
 

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Discussion Starter #7 (Edited)
Hmmm… There is power to the warning switch on the distribution block at all times when the key is on. When an imbalance pushes the internal pin off-center, the switch touches the pin, it grounds the circuit and illuminates the warning lamp.

Just a note, it is not a low brake fluid warning.
That seems to be consistent with what i am testing on my car. I am just confused with trying to make sense of what i am seeing on the wiring diagram I have. (the $40 color one I bought).

*With my gauge cluster out of the car (not plugged in)...

I am trying to compare my diagram to the pins to ensure connectivity and function.

I get that pins 640 (red/yellow) & 977 (violet) go to the Brake warning light on the dash.

According to my diagram. The harness side that connect so [640] should to straight to the brake pressure sensor switch.
And the harness side connecting to pin (977) connects to (A) on the ignition switch.

pic below Is a simplification of my color wiring diagram.

So, if my diagram is correct (?), then how am I getting 12v (switched) on that top (640) pin when I don't even have the cluster plugged in?? Could be diagram be wrong?
***This wiring below is NOT actually correct. It was based on a wiring diagram I purchased
763483
 

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Discussion Starter #8
John, your description makes sense, and does seem to be consistent with my testing. It just frustrates me, because it does NOT seem to align with the schematic.

It would not surprise me if this wiring schematic is wrong. I have already identified several things on it that are wrong.

it calls the brake warning switch a "Parking brake switch"
Wires for the half of the brake warning circuit are wrong. Harness is Violet on both sides of the plug. Diagram shows it as Red on one side and Blue on the other.. ??
Wire colors are also wrong for the AMP gauge wires. they are actually yellow and Red. Red has been swapped for Violet for some reason??

It is a pretty simple design actually. If the brake switch is normal (open to ground) you will have 12V on both sides of the bulb. If either brake fluid reservoir is drained, the shuttle valve moves fore or aft, and the fluid level switch goes to ground. This will cause the lamp to light as long as the key is in the run position.

The ignition switch provides an alternate ground while in the crank position, in order to "prove" that the light is working. This is commonly referred to as a "proof circuit". Note that it does not prove that the fluid level switch works.

This is classified as a "neutral switching" type circuit, which can confuse some.

The picture below is simplified and should get the point across. You can see the two parallel methods of causing the lamp to turn on - on the left is the the brake fluid level switch and on the right is the proof circuit switch.

View attachment 763472
 

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I can't attest to the accuracy of the schematic you bought. But facts from real cars should help right?

The lamp has a power source, which is simple, and then wiring to the switches. That is a little more involved and what I'll focus on here.

Following the purple wire from the instrument cluster connector:
1. From instrument cluster to the ignition switch
2. From the ignition switch to the connector that mates the under-dash wiring harness to the headlight harness
3. Headlight harness through the firewall
4. Leaves the headlight harness near the master cylinder. From the one I just looked at it has a white stripe, and looks more like a brown or beige color. But that might be discolored purple.

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763516
 

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Discussion Starter #10
Thanks! that helps a lot!

My conclusion is that the wiring schematic is wrong.

Thank you for confirming!!
 
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