Is it a normally open or normally closed switch?

I have my old one in hand, look at the two-prong male end.... there is a brass tab connecting the two male forks!
How is this supposed to do anything if the two prongs are soldered together?????

 

The prong gets grounded to the valve body, thereby completing the circuit and turning on the light.

The dual components are for redundancy... they ARE your brakes, ya know....

 

then why the two wires?
if the valve body is - and one if the wires is+, that would work, unless the 2 wires are going to two different devices....say 1 to the bulb and one to the buzzer?

 

The only buzzers were for the seat belts and, optionally, for headlights-and/or-parking-lights-on.

Two wires, yes, but they're all part of the same circuit and are both purple/white-stripe.

Here is from my 1981 EVTM (next to the yellow):



The cluster receives incoming power from the red/yellow-hash wire (Circuit 640). In typical Ford fashion, we control the circuit's connection to Ground to make the device (a 194 bulb in this case) operate... grounding the switch in the differential valve will light the lamp, turning the ignition key to START also lights the lamp (as a bulb test).

I think it's 1984+ (or 85+) where that purple/white wire also connects to a switch in the parking brake pedal assembly that lights the bulb when the parking brake is on.

All you need to do is ground that purple/white wire anywhere along its length and the bulb will light.

~~~

As you've already noticed, the two electrical connectors are, in fact, connected together in the switch; the same is true in the wire's connector to that switch (although the diagrams don't depict that (the EVTM shows more of a logical layout than a physical one)).

Why two independent connections at the differential switch... I can't prove any of this (it's not going to be in any shop or owner's manual) but this comes from some college Statistics courses I took... you distribute the percentage risk of failure across multiple connections, thereby drastically lowering the overall risk of failure.

For example, pretend you are an engineer at Ford; you had been doing a single-connector junction at that switch for decades, and you (and your corporate attorneys) have determined that, because an automotive under-hood environment is one of the harshest in existence, corrosion at that connection caused by moisture, dirt, etc. causes a certain percentage of those connections to eventually fail, thereby not lighting the lamp when it should. This gets manifested in lawsuits when some poor sap kills somebody because his brakes didn't work and the car (truck) didn't tell him ahead of time the brakes needed attention. (Ford gets sued just about every day over something, they lose a certain number of those suits and some of them are quite expensive.)

~~~

Let's say we consider the failure of a single connection at that switch to be a random event; let's also say you have determined that one in six single-pole connections statistically will fail due to corrosion over the course of , say, 10 years.

This can be depicted mathematically as :

1/6 = 0.1667 = 16.67% probability of corrosion-caused failure of a single-pole connection in 10 years

To help mitigate that risk, we spread it (the risk) out across multiple connections, and we now have:

1/6 x 1/6 = 1/36 = 0.02778 = 2.78% probability that two connections will fail within 10 years


From 16.67% to 2.78%, that's a pretty decent reduction in probability for very little direct cost... this makes your manager happy because he can now retire on the $1Million bonus he received because he (his department (you)) are saving the company bajillions of dollars per year in lost lawsuits.

You? Well, you might get a certificate at the corporate Christmas party.

 

Wow! That looks like a post from someone else I know.

 

Makes perfect sense too, redundancy department of redundancy pays off here!

 

Yeah, I know.

I took two or three Statistics classes in college and this is one of the concepts I remember from them.

Wish I could find one of those books, it did a really good job of explaining this stuff but Google sufficed.

 

Actually I take some of my statement back......What you said makes sense. What does not make sense is having a redundant system for protection, rely on one single-filament, 9 cent bulb, prone to a high percentage of failure.........

 

Yeah... but Ford is going to say, "when operating in a normal consumer environment where regular maintenance is attended to, this setup will most likely work for at least 10 years."

Do you really think the bulbs are so prone to failure? They're used all over the place in both interior and exterior lighting and easily last 10 years in my experience. And, it's just a warning light, it doesn't affect the operation of the brakes at all.