From: Wally Plumley [wplumley@bellsouth.net]
Sent: Saturday, September 06, 2003 7:03 PM
To: 928
Subject: [928] Re: Dual brake pressure sensors - need a hint

At 12:45 PM 9/6/2003, Glen Larson wrote:

>I've been having some issues with the brake lights and the warning light 
>on my '80 Euro. Sometimes the pressure warning light comes on indicating 
>an imbalance between circuits. Sometimes the brake light will not come on.
>
>At no time have the brakes felt soft or has the car pulled to the side. 
>I've looked for leaks and found none. It looks like the brake hydraulics 
>are fine, just the electrics are acting up.
>
>I have found some relief by playing with the wires and connectors. Not 
>discouting that I may be just entertaining myself before something decides 
>to work again.
>
>I've looked at the diagrams and through the manual, to no avail. It's not 
>clear what the operation of the two sensors is supposed to be. Simple 
>switches? Piezo resistive?
>
>I'd like to diagnose this before tossing parts at it. New sensors are $81 
>a piece.



Well, no - I don't think so. The last time that I checked, the early brake 
light switches were less than $30 each.

Up thru 1983, the brake lights are operated by twin pressure switches on 
the bottom of the brake master cylinder. From 1984 - up, the brake lights 
are operated by a simple plunger switch mounted on the brake pedal under 
the dash.

The earlier brake light switches do double-duty, operating both the brake 
lights and the brake pressure warning signal. These switches can cause some 
problems.

The switches are identical - one in each hydraulic circuit. They are 
single-pole, double-throw switches. This means that you have only one 
circuit (single pole) thru each switch, and that the switch chooses one of 
two outputs (double throw).

Each switch has three connections:power, central warning computer, and 
brake light.

Not Activated
There is a power feed into each switch. When the switch is not activated 
(no pressure in the brake circuit) the power isn't hooked to anything.
When the switch is not activated, the central warning computer and the 
brake lights are connected inside the switch. Nothing happens, since there 
is no power applied on either circuit.

Activated
When the switch is activated by brake hydraulic pressure in one of the 
circuits, the central warning computer connection is broken, and the brake 
lights are hooked to the power feed. This removes the central warning 
computer from the circuit, and puts power on the brake lights.
(This might be easier to visualize if you think of three wires: one for 12 
vdc, one to the brake lights and one to the warning computer. The lead that 
goes to the brake lights is connected to the one to the warning computer 
until brake pressure pushes it off of that wire and over to the one going 
to power.)

All three leads go to both switches.

When you apply the brakes, both switches should activate. Both switches 
disconnect the warning computer and connect the power to the brake lights. 
The brake lights come on, and no signal is sent to the warning computer, so 
it is happy.

If only one switch activates (either no pressure on one side of the brakes, 
or a bad switch), then things change.

On the switch that is not activated, the brake light circuit is still 
hooked to the warning computer.
On the switch that activates, the brake light circuit is hooked to 12 vdc.
The brake lights come on.
In addition, power flows on the brake light circuit back to the 
non-activated switch. Since in this switch the brake light circuit and 
warning computer are hooked together, 12 vdc is applied to the warning 
computer circuit. This set the Brake Pressure Warning light.

If you have a bad brake light switch, I strongly suggest that you replace 
both switches.
(P/N 113 945 515 G). Replacement of the switches requires bleeding the 
brake system unless you are very quick and pretty lucky, so this is a good 
time to flush the brake and clutch systems.

Wally Plumley
928 Specialists