Well...
Still no joy.
Brake pads, rotors, boots, seals, sensors wires all OK. The sensors - bottom of master, unknown -
leads to same, ok.
The paint shop has lost my owners manual , ordering a new one.
Before I order new sensors... is there a fuse or relay I need to look at?
If the brake fluid level is to high - how high is it? - will that affect the sensor and cause a master warn?
TIA as always.
John S.
<img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" /> <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" />
P.S. Jay, see response on previous Brake warn post

 

John,
Pull one sensor connection off, disconnect battery to clear master warning, reconnect and check. If you get brake lights, and no pressure warning, the disconnected switch (or the wiring from it) is faulty. If you get NO brake lights, the OTHER switch is faulty, OR its wiring.
It seems that either no activity from a sensor, or even a delayed response, will trigger the alarm.
jp

 

On the earlier cars, there are two switches on the brake master cylinder, one operated by each hydraulic circuit. These switches operate both the brake lights and the brake pressure warning system.

These switches are single-pole double-throw. The brake lights are connected to the common terminal, the warning system is connected to one switched terminal, and the power is connected to the other switched terminal.

This means that:

When there is no pressure, the brake lights are connected to the warning system on both switches, but since no power is connected on either switch, neither the brake lights not the warning system are activated.

When you step on the brakes, and everything works correctly, there is pressure on both switches, and both switch the brake light crcuit from the warning circuit to the power, and the brake lights come on. Since the warning circuit is now disconnected form both switches, it is not activated.

When you step on the brakes, and there is low pressure on one hydraulic circuit (or one switch malfunctions as if there were no pressure), one switch will switch the brake lights from the warning circuit to power, applying the brake lights. The other switch will not switch, so the warning circuit is still connected to the brake light circuit on that switch. The brake lights thus send power to the warning circuit, setting the warning.

If you get a brake pressure warning (which has nothing to do with pads, sensors, etc.), you either have a hydraulic pressure problem with one brake circuit, or you have a bad brake light switch or circuit.

If you change one brake light switch, change them both, so as to keep similar response times on both switches, and to avoid having to do the job twice if you guess wrong on which switch to replace.

 

Thanks John P. and Kudo's as usual to Wally.
We attack within the hour...
John S. <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

John S,
Like Wally said - it was his explanations that helped me get it fixed. I actually used a phone wiring tracer to locate the switch leads at the fuse board, and verify not broken or shorted.
There are 3 connections on each switch
Pin 82 12V with Ign on
Pin 81 0 V with brakes off, 12V with brakes on
Pin 81A 0 V with brakes on and no warning, 12V
if a failure of switch, or shorted wires
Pins 81 and 81A are closed with no brakes on, 82 and 81 are open without brakes. Applying brakes makes 82-&gt;81, brakes 81-&gt;81A. If both switches dont brake 81-&gt;81A, you get the warning light. You can check the switches for make/brake without ign on, with connectors removed.
As Wally says, best to replace both switches at same time, as they will have suffered the same degradation over time.
jp