Brake pedal switch has two circuits?

Does anybody know why and the function of the two circuits on the brake pedal switch? What I have found is one circuit closes a switch when the pedal is depressed (this would be a normal function of most any vehicle I have explored to light the brake lights at the rear of the vehicle) and another circuit opens a switch when the brake pedal is depressed. Two vastly different gauges of wire are used as well. The larger gauge is for the historically normal operation.

My theory is that one or more of the controllers (CAN) takes the opening of the switch as an input to do any of the following: allow starting of the vehicle, turning off cruise control, etc.

I wish to understand just what the designers were/are attempting and possibly why they did not rely upon the closing of a switch only.

 

It could be for brake light modulators (I believe they are PnP and will only flash the CHMSL), the trailer brake controller, or for the two foot driving warning.

 

Redundancy.

The NC wires are to ensure that the wires do not get cut or damaged, and to detect fialures in the system. If the controller ever sees both sets of wires in the same state, then it knows something is wrong.

Relying on the closing of a switch only, can cause the system to think that the brake is not depressed in the case of a wire break or other failure.

 

Need more info... Pictures, schematic, where you are seeing 2 stop light circuits (at the switch or at the relay)?

The 2016 schematics only show a single pole switch for the stop light switch in the cabin. That switch feeds into a spdt relay, which allows either the ECU or the brake pedal to activate the STOP lights. The signal also goes back to the ECUs, in addition to the STOP lights. (so the ECU knows you are pressing the brake, for among other things Gas-pedal+brake pedal failsafe).

(I am used to modern vehicles monitoring brake pressure levels to act as the monitor for failed STOP light mechanical switch. Therefore the STOP switch doesn't need redundancy like a fly-by-wire gas pedal, because it is only the STOP LIGHT, not the actual brakes that are affected by the switch... ie, You dont loose stop function if the switch fails.)

2021 may be different, so no guarantee this is what you have (and I may have I messed up and grabbed 2nd gen schematics is always a possibility). (Note, *1 wires are with towing trailer lights, so I assume that is your configuration).

 

When you go into a skid, the Skid Control ECU with Actuator provides a ground to the stop relay and the stop lights illuminate allowing a prudent driver near to back away. This "action" by-passes the stop switch. There are 2 systems available in the Tacoma depending on model, that provide input to the Skid Control ECU, Vacuum or Hydraulic Booster.
Simplified = stop lights are activated without driver stepping on stop pedal. Also with TSS, when reducing approach speed with cruise control engaged and/or emergency stop. (distracted driver).

 

Your response brought up more issues that I had no idea of. Is there really a two-foot driver warning and a brake light modulator on our vehicles? From the schematics below in Post #4 it appears the right/left and center brake lights are in parallel and activate together, not separately. I can't imagine a Trailer Brake controller taking a modulated input and working correctly, unless it was specifically designed for that operation and the second set of two wires on the brake switch are too small to handle the current. Brake Modulation "light" control I can't find unless it is controlled by the Skid Control ECU (black line/wire STPO out of the Skid Control ECU to the Brake Relay coil) again in Post #4 below. I'm still working on trying to understand those schematics. I will respond to the other posts when I get a chance to comprehend them.

 

Its exactly what you were thinking. One for the brakes, one as an input to the ECU for various functions. Even my 1994 blazer was wired this way.

If you're looking at the schematics, note that both switches might not be drawn in the same box even though they exist in the same physical enclosure.

 

There is only ONE switch. The RELAY routes juice to the lamps, via 85a-30 and when the Skid Control ECU provides ground to the relay through 87-30.

 

One switch, two sets of contacts, two circuits (at least according to 30 seconds of googlin')


http://www.ttguide.net/brake_switch_quot_a_quot_circuit_p0571_-1204.html

 

You're reading and interpretations is impeccable...

 

I have my days

 

I was being sarcastic...

 

I didn't say this was a good day

 

It's a 5 pin relay (change over), 3= 30 , 4=87a These are the NC (normally closed) part of the relay. This routes the juice from the 10A stop fuse when stop switch is actuated. The system also gets this STOP "signal" to activate the Skid ECU in case it needs to respond (ANTI-LOCK).
The you have the TSS linked to the Skid ECU via CAN-BUS, so it can also do its job without driver's input. (read post #5)

 

Simplified edit (auto transmission)

Skid ECU providing ground to relay, stop switch not STOMPED.

 

lol, here's your answer OP

1 and 2 are for ABS ecu. red and blue.

3 and 4 is for ECU signal (cruise) black and purple

Toyota has used 4 wire stop switches for a long ass time.

 

Just put your feet on the gas and brake at the same time to see.

95% of the modulators that I see only flash the CHMSL. The tails don't flash. So, the modulator has to either split the signal and be installed in some easy spot. Dealers are not dropping the headliner to install them.

Sorry, but too lazy to search out instructions at the moment. I am seeing them installed randomly on newer cars.

 

I bow before thine glory.
Got to pull up the cruise control schematic to see it all

 

That's part of the equation...