Intermittent Brake Pressure warning light
09-07-2013, 10:56 AM
#16
Rennlist Member
OK, all wiring, grounds and connectors check out good. Both switches changed ( thanks Roger) computer reset; going to take it on a run and see if the warning is corrected. I' ll report back but so far so good
09-07-2013, 11:37 AM
#17
Rennlist Member
Rennlist Site Sponsor
The oxygen sensor counter is of no use or value whatsoever. Disconnect it completely, and forget it.
When the system was designed, no one knew how long an oxygen sensor would last, so Porsche designed a counter to turn the light on every 30,000 miles - nothing to do with whether the sensor is good or bad, just a 30k mileage switch...
Your car has two brake light switches on the master cylinder. The system uses these to monitor brake pressure, as well as for brake light activation.
Power flows from fuse #7 (8 amp) to terminal F6 on the plugs at the bottom of the main power panel. (Plug A is on the left, F is the sixth, 6 has the black wires in it.) Use your 12 volt test to check for power at both ends of fuse #7, then at terminal F6.
From F6, the power flows over a black wire to terminal 82a on the input side of both brake light switches.
From the brake light switches, power flows from terminal 81a to the warning control computer over a blue/brown wire (this power is normally on, turns off when the brakes are pressed if only one switch operates, this power stays on, triggering a brake pressure warning).
Also from the brake light switches, power flows from terminal 81 to terminal F7 on the bottom of the main power panel on a black/red wire. This terminal and wire should have power when the ignition switch is on and Relay X (not the relay in position X, but the one named Relay X) is powered up, and the brakes are activated.
From terminal F7, the power flows to terminal S6 (in counting plugs, there is no "I" plug), and from there to terminal 1 on the lamp control unit on a black/red wire.
From the lamp control unit, power flows from terminal 12 on a black/red wire to the left brake light, and from terminal 9 on a red/black wire to the right brake light.
Refer to the diagram at zone 7 on page 97-165 in Volume VI of the shop manual.
The brake light switches are single-pole, double-throw, and are mounted in parallel.
If either switch works, you will get brake lights.
If both switches are activated, and both work, the contact to the blue/brown wire to the warning computer is disconnected in both switches, so no signal goes to the warning computer.
If there is brake pressure in only one side of the master cylinder, only one switch will be activated. The other switch will remain connected to the blue/brown wire to the warning computer. Voltage will be fed back from the brake light circuit to the warning computer, setting the warning.
When the system was designed, no one knew how long an oxygen sensor would last, so Porsche designed a counter to turn the light on every 30,000 miles - nothing to do with whether the sensor is good or bad, just a 30k mileage switch...
Your car has two brake light switches on the master cylinder. The system uses these to monitor brake pressure, as well as for brake light activation.
Power flows from fuse #7 (8 amp) to terminal F6 on the plugs at the bottom of the main power panel. (Plug A is on the left, F is the sixth, 6 has the black wires in it.) Use your 12 volt test to check for power at both ends of fuse #7, then at terminal F6.
From F6, the power flows over a black wire to terminal 82a on the input side of both brake light switches.
From the brake light switches, power flows from terminal 81a to the warning control computer over a blue/brown wire (this power is normally on, turns off when the brakes are pressed if only one switch operates, this power stays on, triggering a brake pressure warning).
Also from the brake light switches, power flows from terminal 81 to terminal F7 on the bottom of the main power panel on a black/red wire. This terminal and wire should have power when the ignition switch is on and Relay X (not the relay in position X, but the one named Relay X) is powered up, and the brakes are activated.
From terminal F7, the power flows to terminal S6 (in counting plugs, there is no "I" plug), and from there to terminal 1 on the lamp control unit on a black/red wire.
From the lamp control unit, power flows from terminal 12 on a black/red wire to the left brake light, and from terminal 9 on a red/black wire to the right brake light.
Refer to the diagram at zone 7 on page 97-165 in Volume VI of the shop manual.
The brake light switches are single-pole, double-throw, and are mounted in parallel.
If either switch works, you will get brake lights.
If both switches are activated, and both work, the contact to the blue/brown wire to the warning computer is disconnected in both switches, so no signal goes to the warning computer.
If there is brake pressure in only one side of the master cylinder, only one switch will be activated. The other switch will remain connected to the blue/brown wire to the warning computer. Voltage will be fed back from the brake light circuit to the warning computer, setting the warning.
09-07-2013, 02:37 PM
#18
Rennlist Member
Well I believe it was a bad switch, just got home from running her hard on the back roads and the warning is gone. The switches were only a year old and tested good every time I checked them. Cheap and fast fix. No problems changing them with the master cylinder in place. Thanks for all the help guys... So happy
09-28-2013, 05:37 PM
#19
Rennlist Member
Thread Starter
Mine appears to also have been a bad switch. I ordered 2 from 928RUS and this morning realized the forward switch had been leaking.
The connector was full of brake fluid. The switch appears to have been leaking from the pin end.
I replaced that switch and drove it for 50 miles with no Brake pressure warning.
The connector was full of brake fluid. The switch appears to have been leaking from the pin end.
I replaced that switch and drove it for 50 miles with no Brake pressure warning.