Left Brake Light out. Inspection due today - Urgent!

Hi. I'm hoping I can get some quick help here as I've got my Prius booked in for roadworthy inspection in about an hour and I've just noticed the left brake light is out. I'm pretty sure it's gonna fail as it is now.

The right assembly and the center ones (on the spoiler) are nice and bright, but the left assembly is really dim with some of the leds flickering off completly. I thought maybe a bad connection, but I unplugged and replugged the little square connector in the back of the taillight assembly and it made no difference.

Maybe I'm going to have to replace the whole assembly. Can anyone give any insight here. Are there any other obvious places that I should look for a bad connection first?

Thanks.

 

No longer urgent. It passed inspection, ye-ha!

My wife took it in and they *passed* it.

I've got no idea how she did it, she must have smiled nicely at the guy or something. I'm dammed sure I wouldn't have got it passed roadworthy with a blatantly faulty brake light.

 

OK, that's the annual roadworthy is out of the way, but obviously I still want to get it fixed as soon as possible.

First I think Ill try removing the left hand assembly and temporarily plugging it into the right hand unit's little square wiring connector, just to make sure it's not a wiring issue. Can anyone see any problems in doing that?

Then I'll see if I can locate the LED driver board in the assembly and maybe just check for a bad power connection onto the board. At this point I'm not even sure if the board will be accessible though. Any thoughts on this?

Failing that I'll order a whole new left hand assembly.

 

Ok I removed the fitting today. It was a pretty easy job, just held in by two threaded studs (and nuts). It was pretty easy to find a few "how to videos" for that part of the job.

Here are the photos I took while I had it apart.

First up is the fitting removed from the car.


Viewed from the other side shows the two studs that hold it to the car. There are also two plastic locating pins that simply snap in and out.



The LEDs are located in the top (narrower) part of the fitting and are not accessible without some kind of surgery to open up the sealed unit. The LED driver board however is located down in the bottom (wide) section of the fitting, and is easily accessible as shown in the following photos.

Carefully peel back the foam seal near the bottom of the fitting to expose the LED driver board.


A closer view showing the two screws holding the board in place


Remove the screws and the board lifts out without any resistance.


Slightly blurry photo of the same, viewed from the other side.



Finally, here's the LED driver board fully removed.


As you can see, the driver board is extremely simple, with only passive components. Connector CN1 (top right) carries the ground (0V) and the 12V signal from the brake switch.

The 12V signal simply passes through the forward biased diode (D1) and then through four 5.1 ohm series resistors (left hand column in above image) and onto the output connector CN2. The ground connection is a straight pass through from CN1 to CN2.

Curiously the five series 43 ohm resistors (right hand column in above image) don't do anything at all, except to act as a burden to the input 12V brake signal (that is, they *only* connect between the 12V signal and the ground of the CN1 input). Presumably this is just to provide some minimal loading on that signal, so that it's current can be sensed regardless of what the LEDs are doing.

The two output wires (black yellow) on CN2 simply run back to the top of the fitting and disappear into LED part of the fitting. The LEDs themselves don't seem to be accessible.

 

Good point Jeff.

BTW. Here's what I did to test it.

1. I measured the 12V signal into the board and determined that it was getting the correct signal when the brake pedal was pressed.

2. I tested the voltage on the other side of the diode to make sure that wasn't open circuit.

3. I measured the voltage drop on the output (5.1 ohm) resistors and determined that no virtually no current was flowing into the LEDs, so I assumed an open circuit somewhere in the LEDs themselves.

4. I didn't want to try cracking open the sealed LED section, so I just bared back the wires (black and yellow) at the last point before they went to the LEDs, to confirm that the bad connection wasn't in the accessible part of the wiring. The continuity was good so I just taped those wires back up.

5. At this point I was going to just put it all back together and order a whole new fitting. Surprisingly however when I put everything back together it started working again. I assumed at the time that where I taped the bared wires that it had simply pulled the strain on the wires to a slightly different position and somehow caused the open circuit (in the sealed LED part of the fitting that I couldn't reach) to regain connection. It's working now but I assume it will fail again soon if the above is truly the case.

Jeff's comment above however, has made me realize that I may have overlooked checking for ground continuity of the 0V signal running to the input (CN1) on the board. I definitely tested for continuity of the ground signal from CN1 to the output CN2, and then from CN2 to as far as it was possible to trace the wiring up to the LEDs. But I don't think I tested for continuity from the external ground (car body) to the ground input on CN1.

So thanks Jeff, if it fails again I'll definitely test for that before ordering a new unit. A simple test for this will be to measure the voltage on the black wire coming from the LEDs back to the board. If that goes high when the brake pedal is pressed then I'll know that the real problem is not in the LEDs, but somewhere accessible in the ground wiring. That definitely is a viable alternative to explain the presence of 12V signal but lack of LED current that I observed. Dam, I'm kicking myself for not thinking to measure that while I had it all apart.

Anyway it's working again for now. If it stops working I'll take another look and update this thread.

 

Yes I was very tempted to try and open it up, but I'm a bit worried that if I make too much of a mess of it that it will no longer seal out the weather, and we're getting a fair bit of rain here at the moment.

The local Toyota dealers here charge ridiculous prices (like $25 for an oil filter and $80+ for the air filter), so the thing I want to avoid is the need to replace it urgently, as then I'll have to get it local. If it does go bad again and I trace it definitively to be in the LEDs, then I think I'll order a new one online first up, and then once I get it replaced go crazy on the old one just to satisfy my curiosity.

There didn't seem to be an problems with the plastic, none that I noticed anyway.