[31 Ford A]

I have a 1931 Ford "A" with a 350/350 and TCI frame. The master cylinder is located under the floor. The stop lites all of a sudden went out. I thought fuse and checked-OK. Tail lites are ok. Directionals are ok. So then I thought stop lite switch and bought another. Still no lites. (the switch is located in a "T" which is screwed "in line" on the break line about 4" from the master cylinder and it contains two lugs for the wires to plug onto). If I connect just the two wires together, bypassing the switch, and with the break peddle pressed, I have break lites. This tells me that the wire coming down from the fuse block is good and the wire going back to the tail lights is also good. I checked for low fluid in the master cylinder and it was ok. I disconnected the break lines from both sides of the "T" fitting and blew it out thinking maybe there was something in it but it was clear. It is my understanding that the break lite wire going to the switch is always hot. When the breaks are applied the pressure from the hydraulic fluid closes the switch contacts, making the connection, and your breaks come on. What am I forgetting?

[steve t]

you may have a bad new switch, any way to bench test the switch

[docvette]

Doc here,

Your assessment is correct..

Your switch is INTERNALLY mechanically bad..If you can bypass it and get lights..Could be the Hydraulic piston/Diaphragm is shot .. OR the Contacts internal the switch are burned up, Stuck or pitted..

In any case, you have proved the switch WILL not pass Electricity..so it must be replaced .

Doc

[31 Ford A]

Thanks for reply. I have replaced with two new switches and all act the same.
When I connect the hot wire to one of the terminals the other should regester hot, but it does not?

[docvette]

Doc here,

No, the only time the other terminal will register hot is when the switch senses Hydraulic pressure at the switch orifice..(pedal pushed) Otherwise, It is Normally Open.

When pressure is present at the orifice, it moves a "Shuttle" or shaft internal to the switch, that Closes mechanically, the Electrical contacts..thus completing the lamp circuit..

When pressure is released, The shuttle returns to the rear, and the contacts open..Opening the contacts across the switch.

If Two didn't do the job, sounds like you might have a master cylinder problem, or Air stuck in the switch orifice circuit..that needs bleeding.

Doc

[steve t]

brake pedal would not have to be pressed,for lights to shine when tie to hot wire, not switch

[31 Ford A]

Doc, the problem you suggested, would that be internal to the mastercylinder? Would just needing to bleed the breaks cause this problem?

[gdubstub]

When you order from places like Ron Francis, specifically, they provide the disclaimer that you can't directly power brake lights through the switch. Rather, you need to power a relay with the brake switch.

George

[docvette]

Originally Posted by 31 Ford A

Doc, the problem you suggested, would that be internal to the mastercylinder? Would just needing to bleed the breaks cause this problem?

Doc here,

It might...

Don't specifically know what cylinder you have, but if it works like a "Low Brake Warning" air (or lack of fluid pressure) could cause it to malfunction..(or in the case of a low pressure warning, Function)

Are the brakes spongy, and was the master cylinder bench bleed when installed? Or do you have that info?

At any rate , If they are I'd do it anyway..you need to have them right and it removes another possibility from your list.

As far as relay's on a brake lamp system, most STOCK vehicles don't use them, I think , aftermarket manufactures put out that disclaimer for two reasons..Better / Brighter lamps ..and CYA..In case the switch does fail..

Admittedly, It is not a bad idea, and a sound electrical practice..

Doc

[66ImpSS396]

I am assuming that you actually have brakes that are stopping the car OK you are just not getting brake lights.

Quote : (the switch is located in a "T" which is screwed "in line" on the break line about 4" from the master cylinder and it contains two lugs for the wires to plug onto):

When you say you replaced the switch are you replacing this entire Assembly (T and all) or is the switch a separate piece that connects to it. If the switch is separate insure that the internal plunger/piston in the T is moving when you apply hydraulic pressure if you can.

[31 Ford A]

The brass "T" fitting that the switch is screwed into is inline in the break system. There is no moving parts in it that I know of. Only in the switch. I have breaks and a full master cylinder. As far as the breaks needing bleeding, maybe now that I have fooled with them, however, the lights went out prior to my fooling with the lines or switch. Does anyone know if the mastercylinder could be the cause and still have the breaks work? Since the break lights work by bypassing the switch I can only believe it has to be in the mastercylinder. Any ideas?????

[docvette]

Doc here,

You are not forming enough pressure from the master cylinder (or TEE) to Mechanically close the switch contacts..be it caused by air or other problems .. that's your trouble area..

To Verify, hook up the (or a Switch from the past two) to the wires, and hit it with compressed air while someone watches..I just bet they light and go out when air is removed..That will pretty much tell the story.

You are sure the Brass TEE is a TEE , and not a Differential or proportioning Valve? those DO have a shuttle inside them. (although the brakes would be "Kind of out of Sync" Front to rear when applied)

Doc

[31 Ford A]

Actually the air pressure test to the switch may show it is the lack of hydraulic pressure to close the switch. Leave the break lights leading to the rear attached. Disconnect the line from the opposite side of the "T" and insert an air nozzel. How much air pressure would be required to close the switch points-? and, if air is in the lines, how would it be that compressed air in the lines, activated by the pressure from the mastercylinder, would not do the same thing??

[docvette]

Originally Posted by 31 Ford A

Actually the air pressure test to the switch may show it is the lack of hydraulic pressure to close the switch. Leave the break lights leading to the rear attached. Disconnect the line from the opposite side of the "T" and insert an air nozzel. How much air pressure would be required to close the switch points-? and, if air is in the lines, how would it be that compressed air in the lines, activated by the pressure from the mastercylinder, would not do the same thing??

Doc here,

not sure what your saying here..BUT

DO NOT put compressed air into the brake system!!! You Will/Can Damage it!!!

REMOVE the switch, and where the orifice is on the bottom of the SWITCH is where you will insert compressed air.

Read the answer on your other post.

Doc