Brake Pressure Differential Valve Switch

[ejgator]

Okay, I've done the search and read all the applicable posts. My question is: How does the switch (not the valve) turn the light on and off. I have four of them now, my original and three from the salvage yard. All of them exhibit continuity with the plunger out and the plunger in. Looking at the terminals on the switch, it appears that they are bridged together.

Can anyone offer an explanation of how this circuit was designed to work?

[FORDification]

According to the factory shop manual:

"Should a failure of the rear brake system occur, hydraulic fluid pressure in the rear brake system would drop. During brake pedal operation, the fluid pressure buildup of the front brake system would force the differential valve toward the low pressure area (secondary system outlet port). Movement of the differential valve forces the switchy plunger upward over the taper shoulder to close the switch electrical contacts and lights the warning lamp, signaling a brake system failure."

In laymen's terms, the switch inside is being pushed on from both ends (front and rear brake circuits). If one side loses pressure, then the switch gets pushed off-center and activates the warning light.

...and here's what they say about re-centering it:

"Turn the ignition switch to the ACC or ON position. Loosen the pressure differential valve inlet tube nut of the unfailed system, or teh side opposite the system that was bled last. Operate the brake pedal carefully and gradually until the pressure differential valve is returned to a centralized position and the brake warning light goes out. Tighten the tube nut."

[ejgator]

Right. Thanks for the effort. I clearly understand how the hydraulic part of the system works. My question has to do with the electrical circuit.

I have four (4) of the electrical switches with the plunger. All four of them have continuity whether the plunger is depressed or not. There are two electrical terminals on each switch. All the switches appear to have a metal strip connecting the two terminals. All the switches have continuity from terminal to terminal: a) when installed in the assembly; b) when they are removed from the assembly with the plunger not depressed (referred to as "off" in the schematic), and; c) when they are removed from the assembly and the plunger is depressed (referred to as "on" in the schematic).

My contention is that no electrical continuity should exist when the plunger is depressed (switch out of valve assembly). It seems beyond the realm of possibility that all four are defective.

If no one can explain this, I have ordered a new switch from Mac's, and I will check it when it arrives, and post the result. My bet is that it will behave the same way, which will put me right where I am now.

Thanks.

[marz68]

Hey Keith I just finished most of the plumbing on my brake lines and just noticed on the picture you posted that there is a high pressure side and a low pressure side, now my question is if I am not going to use the warning light switch dose it mater witch side gose to the front or to the back? I didn't think it maters dose it.

[ejgator]

The mystery is solved. Even though there are two terminals, they function as one since they are tied together. The continuity of this switch is tested from the terminal(s) to the plunger. When the plunger is extended, no continuity exists. When the plunger is depressed, continuity exists. In the valve assembly, when the valve is off center, the plunger is depressed and also grounded, completing the warning light electrical circuit.

My local NAPA dealer in Shallotte, NC suggested that this was the case.

I hope this information is some help to others.

Gator

[marz68]

That's good news that you got it figured out, I was thinking that it was impossible for 4 switches to be bad.

[FORDification]

Hey Keith I just finished most of the plumbing on my brake lines and just noticed on the picture you posted that there is a high pressure side and a low pressure side, now my question is if I am not going to use the warning light switch dose it mater witch side gose to the front or to the back? I didn't think it maters dose it.

The 'high-pressure' and 'low-pressure' sides in the above graphic are when one side goes bad. If you spring a leak in the front brake circuit, that front circuit is referred to as the 'low pressure' side, and vice versa. When your brakes are functioning properly, there is equal pressure on each side.

[marz68]

Thank's Keith for the info, now I can finish the front brake lines.