Do brake lights come on each time you apply the brakes?

is that legal down under?

 

The answers to this question probably need to be broken out by model and generation.

Gen 1 Prius:

The US wiring diagram clearly shows that the 15 amp STOP fuse directly supplies the stop light switch on the brake pedal bracket, which directly supplies the stop lamps at the rear of the car. It also is routed to the STP input (S2 position 6) of the shift lock ECU, so that ECU can know when the brake lights are activated.

Gen 2 Prius:

The US wiring diagram shows that the 7.5 amp STOP fuse directly supplies the stop light switch on the brake pedal bracket, which directly supplies the stop lamps at the rear of the car. It also is routed to the STP inputs of the HV control ECU (H15 position 3), the power source control ECU (P6 position 1), and the skid control ECU (S8 position 14) so those ECUs can know when the stop lights are activated.

Gen 3 Prius:

The US wiring diagram shows the 10 amp STOP fuse supplying the stop light switch on the brake pedal bracket and also the stop light control relay. The open-when-not-braking output of the stop light switch goes to the STP input of the stop light control relay, and the relay output directly supplies the stop lamps at the rear of the car.

The stop light switch output also goes to the STP inputs of the skid control ECU (A58 position 33) and the power management control ECU (A22 position 23). The output of the relay also goes to the skid ECU (STP2 input, A58 position 6), so that ECU can tell when you have pressed the pedal activating the switch, and also can confirm that the relay output has responded.

The "relay" is, in fact, an electronic thing with more than one input, so there can be more ways than just the brake pedal switch to activate it and light the stop lights. Its terminals labeled ACC and ESSI in the wiring diagram are wired to terminals STPO and EXO of the skid ECU, respectively (though, on one of the drawings, those wires are shown as existing only in cars with the Dynamic Radar Cruise Control feature). On cars with DRCC, anyway, the skid ECU is able to trigger the stop light relay when braking because of DRCC.

Those ACC and ESSI connections to the relay seem to be documented a little sparsely in the repair manual; Elektroingenieur looked into what information is available.

Gen 4 Prius:

I haven't got a wiring diagram for one of those.

What car were you experimenting on?

Because the stop light switch is just a simple electrical switch mounted on the brake pedal bracket, I would expect in your graph to see the "Stop Light I/O" closely correlated with pedal stroke sensor values above a certain value, and that does seem to be roughly what I see. By very slowly applying the pedal, it probably would be possible to pin down the approximate stroke sensor reading where the separate and independent stop light switch tends to make/break contact.

I am puzzled by something around second 433, when (unless I am going crosseyed) a brief blip of the stroke sensor seems high enough to have coincided with stop lights (at other moments in the graph) but the graph does not show "stop light I/O" there. Without more information about the data collection and graphing, I couldn't be sure what to think about that.

If the stop switch seems to be making contact late (coinciding with too high a stroke sensor value), the position of the switch on the pedal bracket is adjustable.

Although the switch is shown in the diagrams as a plain mechanical switch (and surely they were, for the longest time), there was a recent thread where somebody took one apart and found it to be a contactless version made with a permanent-magnet plunger and Hall ICs.

 

1999 seems surprisingly early for a RAV4 HV with dynamic radar cruise. That feature didn't appear in the Prius line until Gen 3, did it?

In Gen 3 Prius, the DRCC is indeed able to light the stop lights when the pedal is not pushed. That is because the stop light relay has multiple inputs, as described above in post #3 and seen in the wiring diagram.

I had been wondering how slow the polling rate was; at 1 per second, some of what I'm seeing in the graph might be artifacts of that.

It may not be necessary that every mechanical switch in the car has a PID reporting its status. That said, referring to the Gen 3 wiring diagram and repair manual, the switch is wired to both the skid ECU and the power management control ECU, and the repair manual shows that the data lists for both ECUs do include PIDs reporting it. It's called "Stop Light SW" in the ABS/VSC/TRAC data list, "Stop Light SW M-CPU" in the Cruise Control data list, "Stop Light SW 1 (M CPU)", "Stop Light SW 1 (S CPU)", and "Stop Light SW 2 (M CPU)" in the Radar Cruise data list, and "Stop Light Switch" in the Hybrid Control data list.

The ABS/VSC/TRAC data list also includes a PID called "Stop Light Relay Output". It is not clear to me whether this shows the status of the STP2 input, A58 position 6, that monitors the output of the relay, or of the STPO/EXO outputs that allow the skid ECU to trigger the relay for DRCC purposes. That might not be too hard to pin down.

There is also an ABS/VSC/TRAC active test called "Stop Light Relay" that clearly seems intended to trigger the relay. The diagrams don't all agree on whether those wires exist in the harness in cars without DRCC, so it's possible that in those cars the active test might have no effect.

 

A suction cup on the rear window was good enough for an ICE high-speed train; it should do for your car.

The computers can turn on the stop lights—to warn other drivers when the vehicle is slowing or stopping on its own, without brake pedal input from the driver—but they don’t have a monopoly on doing so.

At least for AXAH54L models built for the U.S. market, the “Stop Light (HV Model)” pages in the Electrical Wiring Diagram (more info) show that the center stop light and rear combination lights are connected to the OUT terminal of the stop light switch assembly, for which the Repair Manual on-vehicle inspection procedure states:

Tester Connection: A40-1 (OUT) - A40-2 (GND)

Condition: Brake pedal not depressed

Specified Condition: 1.5 V or less​

Condition: Brake pedal depressed

Specified Condition: 11 to 14 V​

The stop light switch assembly is connected to the hybrid vehicle control, certification, and skid control ECUs, but all except one of these connected terminals is identified in the Repair Manual as an ECU input. The exception is the STPO terminal of the skid control ECU, described (with a schematic diagram) in the troubleshooting procedure for brake system diagnostic trouble code C1380, in which Toyota writes:

When any of the following conditions are met, the skid control ECU (brake booster with master cylinder assembly) sets the drive output (STPO) ON which operates the stop light control relay (stop light switch assembly) and turns on the stop lights.

Illumination Conditions:

• Pre-collision brake is operating.*
• The dynamic radar cruise control system is operating and is applying the brakes.*
• Secondary collision brake is operating.
• Brake hold is operating.

*: w/ Pre-collision System​

Note that this list doesn’t include “Driver is pressing the brake pedal.” In that case, the stop light switch assembly turns on the brake lights without computerized help, just as on the Prius models @ChapmanF kindly described.

 

Just went out and tested this with the wife in her car behind me. Fits exactly with what Elektroingenieur said.

The brake lights came on every time I pressed the brake pedal, even extremely gently. We tested this at least 20 times in a range of circumstances. As best as we can tell, brake pedal ALWAYS means brake lights, which also makes the most sense.

They also came on, on their own, when the dynamic cruise control was slowing me down.

They kept the brake lights on, after I released the brake pedal, when the hill start assist is working - i.e. when stopped on a hill and you release the brake pedal, the brake lights remain on until you move forward.

We didn't test pre-collision.

 

Interesting experiment.
I will see if I can reproduce this on my Euro spec Corolla (2019).
I am especially interested when the brake lights are on when using adaptive cruise control.

 

Or maybe a couple wires poked into the brake light connection at the back, with a resistor and LED up front where you can see it.

Hmm... "stop light control relay (stop light switch assembly)". Does that mean that where Gen 3 has two separate parts, a "stop light switch" and a "stop light control relay":



this later model has essentially integrated both into one assembly that sits on the pedal bracket and has additional inputs? Or are there still two separate parts, with the lights in back really connected to OUT of the relay, and not directly to the switch? Or is there no relay, and the ECU's STPO output has the current capacity to drive the lights directly? Would you be comfortable posting any sort of minimal diagram excerpt showing what's going on there?

On parts.toyota.com, I can find a couple different part numbers claiming to be the brake light switch, but when I image search for either 84340-49105 or 84340-09291 there are pictures of more different-looking switches than you can shake a stick at; it seems people post images of "brake light switches" without much regard to specifics.

 

That would work, too, but it wouldn’t be nearly as exciting.

That’s right, but there is only one external input. As on fourth-generation Prius cars, the switch and relay have been integrated into the stop light switch assembly. On RAV4 HV models, it’s shown as having a seven-position connector:

1. OUT, connected in parallel to the stop lights and to an input of the skid control ECU (STP)
2. GND, connected to ground point AA
3. L, connected in parallel to inputs of the skid control ECU (STP2), hybrid vehicle control ECU (STP), and certification ECU (STP1)
4. ACC, connected to an output of the skid control ECU (STPO)
5. L, connected to an input of the hybrid vehicle control ECU (ST1-)
6. B, fed from the ECU-IG2 NO.3 fuse, controlled by the IG2 NO.2 relay
7. B, fed from the unswitched STOP fuse

I imagine “ACC” refer to adaptive cruise control, a main reason for having an external input to the assembly, and not to accessory power, which isn’t involved here.

According to the Repair Manual, DTC C1380, Stop Light Control Relay Malfunction, is output by the skid control ECU when IG1 is at least 9.5 V and either “stop light drive output (STPO) is ON and STP is OFF for 2 seconds or more” (INF code 761) or “stop light drive output (STPO) is OFF and the input signals for STP and STP2 differ for 5 seconds or more” (INF code 762).

In New Car Features, the Stop Light Switch topic says that “non-contact type stop light switch assembly with high durability is used,” and “2 magnetic sensors are installed and they output stop light illumination signals and cruise control system cancel signals.” There is also a cross-section view.

 

I'd hope not every deceleration in the normal natural ebb and flow of traffic, as drivers let off the gas, glide, coast, or do mild downshifts, would light up the brake lights. That would be far too many brake lights.

At least, not without some change to show deceleration rate, e.g. slow vs fast blinking brake lights. My first Prius was struck from behind in a traffic jam where everyone, all four freeway lanes, had their brake lights on for a gradual slowdown. Then traffic suddenly went from very light to very hard braking, but without any warning from changing brake lights because all the lights were already on. I was caught slightly unawares, but with sufficient stopping distance. Same for the guy behind me. But not for the novice teenager in the car behind him, with a car full of her volleyball teammates, who created a change reaction collision. Variable rate flashing brakelights would have provided better warning for everyone.

If every deceleration is to trigger brake lights, then such variable flash rates will be even more important.

 

Guessing they're aftermarket, but I have seen brake lights that flash on first application. Current EVs with one pedal driving ability use deceleration rate to determine when they need to turn on the brake lights. Just having the lights flash under when under heavy deceleration should be something easy to implement.

I say should be, because it was possible to bring early i3s to a stop without the brake lights ever coming on under the right conditions.

 

I wonder if some of what's bothering RGeB could be a faulty, sluggish, or sticky brake light switch.

At some point maybe one gets away both from fancy OBD logs and from speculating about what some person wants us to think, and just engages in the kind of real-world test where the brake pedal gets held down with one hand and the brake light switch plunger gets poked in and out with a finger.

 

I would start by approaching it simply, and complicate it only if simple somehow fails.

Up near the top of the brake pedal bracket should be something that looks possibly like this:



The brake pedal arm keeps the plunger pressed in, when you are not pressing the pedal.

With one hand, you can hold the brake pedal down far enough to make room for a finger over that plunger. And holding the pedal there will allow the finger to press the plunger in (as it is when the pedal is released) and let the plunger out (as it is when the pedal is pressed).

And that will keep the various pedal-related variables, like stroke sensor and regulator pressure, from being distractions, and reveal what happens when the switch plunger moves in and out.

When the plunger is pressed in, there should not be brake lights. When the plunger is let out, there should be brake lights. The plunger should not feel sluggish or sticky to move, and the lights should respond to it without delay.

With the car in a dark place with a light-colored wall or something behind it, so it is easy to see when the brake lights are on, this test shouldn't be any more complicated than it was in a 1970s Ford.

 

That is a fact, but in exactly the same way it's been a fact for 50 years of Toyotas.



The friction brakes in a 1970 FJ Cruiser are controlled in a different way than the brake lights. The brakes are actuated by the fluid pressure when the brake pedal moves the master cylinder piston. The brake lights are actuated by the electric switch that closes when the pedal moves away from the switch plunger on the pedal bracket.

The pedal has always needed to move slightly (and therefore generate slight fluid pressure) before that switch closes.

There will generally be some specification in the manual for how that switch position needs to be adjusted. I don't have a RAV4 manual, but here's what's in a Gen 3 Prius manual:



So if the switch doesn't work right, it should be replaced, and if it isn't adjusted properly, it should be adjusted, and if it is adjusted properly, it's just working the way they've been working for half a century or more.

The best way to isolate what that click may or may not be is to hold the brake pedal steadily down with one hand, as suggested, and use another finger to push and release the switch plunger. If the click accompanies the pushing and releasing of the plunger, it is something to do with the switch. Otherwise, the click is something else.