Figured out why I'm being tailgated so often...

So, this morning I was gently decelerating toward a stop sign with a Ford Ranger right behind me when I noticed something odd: no brake lights reflecting in his chrome. Not good! I had to brake fairly hard for them to show up. After searching the forums it appears that this is not uncommon, and in my opinion needs to be fixed ASAP. Rather than going to the dealer I'm going to see if I can adjust it myself this afternoon.

From the online service manual it looks pretty simple. The switch is way up toward the top of the pedal assembly and is depressed only when the pedal is at the top of its travel. Once that switch is released the brake lights activate. In my case it would appear that the switch body is screwed down too far. The adjustment simply involves loosening the lock nut and backing the switch out of its threaded hole until a more acceptable activation point is achieved. Torque the lock nut back down and drive safely.

The interesting part will be seeing if I can reach the thing. If I can't get in there without major disassembly it will be off to the dealer, afterall.

 

According to the service manual there are two switch assemblies. The big one is the stroke sensor which I presume is responsible for telling the car how hard to brake. (I would be very hesitant to mess with that one.) The smaller one is called the "stop light switch." It is only supposed to contact the pedal when the brakes are fully released which makes sense for something that just tells the computers if the brake pedal has been touched.

I'm trying to think of reasons that the current adjustment would be intentional and nothing convincing comes to mind. And given how little spacing people leave around here I want to provide them with as much warning as possible, like my last car that activated the brake lights just about immediately.

I'll be headed out to look at it shortly.

 

Done! This is an easy fix for anybody comfortable around wrenches and interior trim panels. It took me the entire evening to figure it out and procure the right tool (I'll explain) but it would probably be a 15 minute job tops if I had to go in there again.

The first thing I attempted was simply climbing under the dash and looking for a way at the part. It became apparent that this wasn't going to work because there's just waaaaaaay too much stuff under there. Then, per the service manual, I pulled off the lower dash trim (everything from the button blanks down to the hood release...I'll explain that, too). That gave me much better access to the switch. Here's what she looks like (and sorry if that's way too big):

[attachmentid=7959]

That barrel-shaped object in the top-center of the image with the white connector is what we're after. As you can probably see it threads onto its bracket and secures via that lock nut between the barrel and the bracket. Now what you can't quite tell from the picture is that this is a really tight workspace. There is absolutely no way as far as I can tell to get at that lock nut with a regular open-end wrench. What does work is a stubby wrench (I think it was a 14mm but it might have been a 13...I was upside down for the wrench selection and wasn't paying close enough attention.) So get that tool first!

The other end of that switch is the plunger, which rests against a flat portion of the brake pedal arm. The pedal holds the plunger in when your foot is off the brake and releases it when you press the pedal, activating the brake lights and canceling cruise control. The service manual does give a specification for how close the threaded part of the plunger should be to the pedal when it is at the top of its travel: 0.5mm to 2.5mm. My switch, however, was actually serving as the top of the pedal's travel. There was zero clearance, which is in clear violation of Toyota's specs. You can test it out fairly easy while you are under there because you will hear relays clicking if you press the pedal with your hands.

So now I've told the story of how I spent my first two hours crawling around and buying a set of stubby wrenches, here's how it's done in 15 minutes:

1)
Gain access to the switch by removing some dash trim. Remove the air vent trim by the window first by pulling outward and slightly upward on the lower part to release the lower clips. Once those clips are free it's a simple matter to pull it straight out of the dash. Be firm but don't do too much bending! Bending is what breaks clips.

The main trim panel is removed by first removing the two phillips-head screws that secure the lower edge. One used to be hidden by the air vent trim and the other is visible if you get down there and look for the depression in the general viscinity of the hood release. Next, use your fingers to pull the panel away from the dash and release the clips. They only take a few pounds of force to pop free, so take it easy. You will have several electrical connectors to disconnect: the dash lighting dimmer switch, smart key switch, tire pressure reset switch, and two connectors to the key slot. Finally you have to deal with the hood release. Looking down onto the inside of that panel you can see that the release handle is its own assembly and can be slid out of its slot toward what was the inside of the dash. You just have to press down FIRMLY on the white strip of plastic on top that holds it into its slot. I hurt my fingers doing that one but it worked. But you have to release the cable from the handle in order to free the panel because the handle only goes out one way. The end of the cable is just a little barrel that can be slid out of its mount...you should be able to see that fairly easily. There may even be a way to get the cable out of the handle without removing the handle from the panel but it's harder to see that way and I wasn't about to figure it out never having done this before.

2)
Now that you have access to the switch, get under there and have a look at where the end of that threaded barrel is relative to the part of the brake pedal that it buts up against. You may find that the threaded connector touches the pedal itself like mine did, which is incorrect according to Toyota. Press the pedal with your hands and listen for the clicks as the switch does its job. At this point you can pretty much see what's going on.

Adjustment simply consists of releasing that lock nut, threading the switch upward by rotating it as a whole, and finding the point at which the switch trips its circuits after the pedal has only moved a few millimeters. Again, the end of the threaded part of the switch should be 0.5 to 2.5mm away from the brake pedal when it is at the top of its travel. And be sure to adjust it so that the brake lights not only activate when you press the pedal, but deactivate when you release it! If you go overboard you may wind up with brake lights that are permanently on, which is just as bad or maybe worse than the original problem. Once you get it where you want it, go back in there with your stubby wrench and tighten that lock nut down to the metal bracket to keep the switch from going anywhere. It should be about as snug as you can get it with that short little wrench.

SIDE NOTE: I did this whole operation without unplugging the electrical connector from the brake light switch. I was able to do this because the wires had a fair amount of free play and I didn't go more than about a full turn. But pay attention, because you don't want to wind those wires up so tight that stress is placed on the connector. Over time the connections could weaken and break. I think the right thing to do would be to disconnect the connector, unwind it, and re-connect it like it was before. But I didn't do that so I don't know what will happen. Procede with caution.

3)
Verify your work. You want to see the brake lights activate near the top of the pedal travel and deactivate when the pedal is fully released. My method was to tape a piece of scrap paper to the spoiler so as to reflect the center brake light enough for me to see it from the driver's seat. Press and release the pedal a few times to make sure that you are satisfied with the results.

4)
Button her up! Re-attach the hood release cable, re-attach all of the electrical connectors, carefully snap the large trim piece back in place, and don't forget about those two screws on the lower edge. Snap the vent trim back on and you're good to go!

Or just go to the dealer and make them fix it for you under warranty. Either way I don't recommend driving around with your brake lights out of adjustment. I do recommend that all Prius owners test the lights to see if you need to adjust yours. It's starting to sound like this may be a widespread problem.