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MG MGB Technical - Brake Pressure Warning Light

Got new pressure failure switch and testing. Cannot get it to work with dash switch. Tested electric to dash switch and it is getting juice but light doesnt come on when I press it and press failure switch plunger in and out.

Is the bulb working?
D O'Neill

Yes - the bulb works.

Harry. The pressure switch is of the "normally on" type switch which is held in the off position by the rod in the master cylinder. (Assuming you have the later model set up--you do not say.) When there is a pressure differential between the front and rear brakes, the rod moves off center and allows the stud on the switch to move upwards, locking the rod in place and allowing current to flow through the switch to the bulb/ground. Take a look at the wiring diagram for your year and look at how the current flows. Then, check for current as you go from the power input to the ground and see where the problem is.

Les Bengtson

Did you make sure the ground wire for the bulb is properly connected?

If the light doesn't come on from either the test switch either you have two (or more) faults or there is a disconnection between the brake master and the switch/light panel. The test switch puts out a ground on one of the two black/whites, which goes to the connector on the master and is internally linked to the other black/white inside the master. The ground then comes back on the 2nd black/white to one side of the bulb, the other side of the bulb being connected to the purple fuse (always on).

When the pressure imbalance switch is activated it sends its own ground down both black/whites, the one to the lamp should light it, the one to the switch does nothing. The drawing for the remote switch at least shows the switch is a 'normally off' switch, the later masters with integral switch may be different, you don't give the year.

If connecting a ground to both black/whites at the switch/light panel doesn't light the lamp either the bulb is faulty, it isn't making good connection with the bulb holder, or there is no 12v supply to one side of the bulb.

If that works but not if you ground both black/whites as the master then there is a break in at least one of the black/whites between the master and the switch/lamp panel, this could be at the multi-plug behind the dash (later cars) or a pair of single connectors in the black/whites (earlier cars).

If that works, link the two black/whites in the plug that goes on the master and try the test switch.

If that works the fault is inside the switch, if it doesn't the test swich could be faulty or there could be a break in its black/white between the switch and the master plug.

I suppose a faulty switch could cause both problems, but it would have to be pretty crappy manufacture if even the link between the two connections was faulty.
Paul Hunt 2

Oh, the master switch will ony light the lamp if it is attached to the master, or has an alternative ground provided to its body. It won't light the warning lamp if it is removed from the master, even though it has two wires on it. However when removed the *test* switch should light the lamp as long as the wiring is connected to the master switch.
Paul Hunt 2

The plastic switch on the brake pressure warning valve has a connecting link between the two terminals. If the valve has ever leaked brake fluid will destroy the link and the system will not and the test circuit will not work. I'm responding because I'm not sure which switch you changed.

Clifton Gordon

That's interesting, Clifton, is that a design feature or just how it happened?
Paul Hunt 2

What year did the warning light become standard? My 1972 GT doesn't have one.

As I have rebuilt the whole braking system myself it might be a good idea!

While I'm on - when do I bleed the master cylinder when filling the system from new - first or last?


Liam H

Liam. I do not know if/when it was used on the UK market cars. In the US, it was introduced in the 1968 model year by government requirement and all subsequent models had them.

Les Bengtson

Paul; I suppose it's a design feature. The ground for testing the light goes through the push button switch through one of the BW wires to the failure switch on the brake pressure warning valve, goes through the link to the other BW wire to the light in the brake failure switch on the dash. If the link breaks as it did in my car the test light will not work. Another switch with a good link fixed the problem. As I recall when the plunger in the valve is pushed up by the shuttle the pin contacts the link between the terminals turning on the brake failure light. If the link wasn't there the test light wouldn't check the complete circuit.

Clifton Gordon

This thread was discussed between 26/03/2007 and 02/04/2007

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