T-Series Brake Pressure Warning Switch

[jjbunn]

In the course of doing the brakes, I took off the brake pressure warning switch to check it out. Here are a couple of photos:

I've been trying to understand how this switch works, and if it's broken or not. It sure looks broken!

I can see that the brass end is supposed to move up into the switch if the pressure is unequal between the front/back circuits. But, inside the switch, I don't see how the contact between the incoming wires is supposed to be made when the plunger is fully in the switch.

Is my switch missing one or more parts? Has something broken off inside the switch body?

 

[danmas]

For all you could ever want to know about this switch, see https://www.mgexperience.net/phorum/read.php?1,465646,466196#msg-466196

 

[danmas]

For some reason, the link is split in two, so you'll have to cut and paste the full link into your browser to get to the web page referenced.

 

[jjbunn]

[ QUOTE ]
For some reason, the link is split in two, so you'll have to cut and paste the full link into your browser to get to the web page referenced.

[/ QUOTE ]

Thanks! That looks like a forum I should join: is it better than this one?! (Sacrilege, huh?)

Even after reading that, I'm not sure how this switch works. It appears that I may have a piece missing. The plunger should apparently have a metal piece in the switch body that moves up and touching the wire which connects the two terminals (and which seems to be broken off in my switch).

Looks like I'll need Tony's help again!

 

[DrEntropy]

To my recollection, the back end of the plunger is the piece to complete the circuit. I think the switch's terminals have stayed attached to the plug end, from looking at your photos.

 

[startech47]

The visible copper parts in your rubber plug are part of the contacts and belong in the white switch housing. Phil

 

[rick_ingram]

[quote
Thanks! That looks like a forum I should join: is it better than this one?! (Sacrilege, huh?)



[/ QUOTE ]

Every MG BBS has its own personna. I like this formum...I like the MGBE....I also like the UK-based MG Enthusiasts. By belonging to all three, I get the best (and periodically the worst) of all worlds!!!!!!!

YOMV.

/ubbthreads/images/graemlins/thumbsup.gif

 

[jjbunn]

[ QUOTE ]
To my recollection, the back end of the plunger is the piece to complete the circuit. I think the switch's terminals have stayed attached to the plug end, from looking at your photos.

[/ QUOTE ]

Thanks Doc ... yes! That makes sense now. I don't think I can salvage the switch: it's too far gone.

 

[LLAngus]

It looks like only half the switch is shown. The activator is in the part still on the car. You can see it if you look into the hole where the "switch" goes. It is a metal "pig" looking piece that travels from one side to the other and supplies the contact. I had a devil of a time with mine when I bled my brakes. I thing if there is continuity in the switch you show it is good. And I think there is an o-ring or gasket to seal the switch.

 

[Scott_Hower]

To test the switch, connect the leads to a continuity tester and then press in on the brass part. When it bottoms out, the tester should chirp. If is does not, you have a bad switch.

There's a "shuttle" inside the brass PDWA body. The center of it has a reduced width cut into it with seals on either side. If the pressure is different on either side of the brake circuit, it forces the shuttle off center. The shoulder of the shuttle forces the pin up into the switch body closing the contacts.

At a minimum, replace the PDWA seals and the copper crush washers. Do not overtighten the brake lines or it will leak:

https://www.britishcarforum.com/ubbthread...true#Post215117

 

[jjbunn]

[ QUOTE ]
To test the switch, connect the leads to a continuity tester and then press in on the brass part. When it bottoms out, the tester should chirp. If is does not, you have a bad switch.

There's a "shuttle" inside the brass PDWA body. The center of it has a reduced width cut into it with seals on either side. If the pressure is different on either side of the brake circuit, it forces the shuttle off center. The shoulder of the shuttle forces the pin up into the switch body closing the contacts.

At a minimum, replace the PDWA seals and the copper crush washers. Do not overtighten the brake lines or it will leak:

https://www.britishcarforum.com/ubbthread...true#Post215117

[/ QUOTE ]

Thanks Scott ... I am pretty sure my switch is buggered internally. Until I can find a spare from Tony or someone, I am going to leave it alone.

I'm not sure about the circuit test you describe. It seems to me that the way it works is that the wires coming in to the pigtail are connected together. With the plunger fully extended (i.e. no pressure difference) then no contact is made in the switch body between the plunger and the pigtail wires. When the plunger is depressed (pressure difference) then the pigtail wires are grounded by the brass plunger, and the bulb on the dash will turn on. Another way of turning the dash bulb on is to depresh the dash switch, which grounds one of the pigtail wires (dunno which one). This is used as a test of the bulb, I suppose, but doesn't test the mechanics of the switch at all.

Is that correct?

One more question: if the seals are intact, then I shouldn't see any brake fluid inside the hole into which the switch screws, right? All I should see is the shuttle.