biggiefl

I have checked all the vac lines and they are fine. The main to the intake is rough but looks sturdy...replace? Pedal is great, stopping is great....what gives...another glitch? What should I check, light comes on about once a week? I disconect batttery and reset codes but she shows up again about every 100 miles.

Jim bailey - 928 International

The brake pressure is the pressure of fluid in the two chambers of the dual master cylinder when you step on the pedal. The two pressure switcheson the bottom of the master cylinder need to see similar pressure at about the same time or it throws the warning that there may be an internal leak possible failure of half of the brakes.

Randy V

Model year of your car?

*bughouse*

If you are sure you don't have a brake prob. just disconnect the negative side of the battery and reconnect after a couple of minutes (seconds). This worked to reset my brake warning light on my '80.
Ofcourse you will have to re-do your radio stations and it might set off your alarm (if equiped).

WallyP

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, $28.31 each plus shipping) Replacement of the switches usually requires bleeding the brake system, so this is a good time to flush the brake and clutch systems.

biggiefl

Car is a 1982 928 automatic. How do I tell if it is a 928s?

So if brakes seem to work fine but I get an intermittent brake light warning and a very intermittent brake pressure warning, replace switches? I know it can't be bulbs because they have been replaced and work fine(everytime I check) even though brake light warning is on. This warning usally goes out once brake pedal is pressed. The brake pressure light as to be reset though when it doescome on(3 times). However he brake light warning does not always come on. Had it ut wednesday and all day no lights lit up on dash....damn 25 year old cars

Thanks or the help so far.
Nick

Jim bailey - 928 International

Nick for it to be an S it would have to have been a Euro spec car to be a Euro spec the vin number will have ZZZ instead of JAO. It could have the competition package ....spoilers flat S wheels etc. BUT standard engine for the US.