Introduction: Prius Brake Light Repair

I recently purchased a 2005 Prius. It was purchased “as is”, knowing that pitfalls awaited me.

After a few days, a friend mentioned the “RIGHT” brake light was not working. Once home I tried the brake lights. The “LEFT” brake light wasn't working. I called my friend and he insisted the “RIGHT” brake light had been out.

The brake lights were intermediate on both sides.

This instructables is about how I fixed my Prius brake lights. It is not about the best way, or the correct way.

It also is not about how the system works. It is about how "I" think it works.

I first tried to repair the Toyota brake light system. (Plan A)

Then I replaced the Toyota system with Radio Shack LEDs. (Plan B)

Finally I replaced the whole Toyota system with super bright LEDs from Amazon. This worked.

Step 1: Tool and Supplies Needed to Repair the Brake Light System.

1. A 2005 Prius with bad brake lights.

2. Hand tools...Hacksaw, pliers, screwdrivers etc..

3. Soldering iron and necessary soldering tools.

4. Multi meter and some electrical knowledge.

5. A tough, plastic, kitchen cutting board.

Step 2: Remove the Tail Light Assembly.

I removed the tail light assembly, according to many good online tutorials.

I downloaded a wiring diagram for the Prius.

I removed the trunk light assembly so I had a ready source of 12V for testing. Using jumper wires, I confirmed the LEFT brake lights did not light. The left turn signal, reverse light, and tail light all were working fine.

I soon figured out that the brake lights are activated by the brake pedal anytime the car is unlocked. The car does not need to be on, or running. Using a cane jammed between the drivers seat, and the brake pedal made an easy way to do a brake light test.

Since the right brake light was working, I removed it, and plugged it into the left side to check the left side wiring. It worked perfectly.

I then plugged the left tail light assembly into the right side plug, and it worked perfectly.

Now both brake lights were functioning correctly, but were plugged into the wrong side. I then moved the assembly units back to their correct sides. Now neither brake light was working. I swapped them back and forth many times, but neither, ever, came on again

This was good news, because I can fix things that are broken, but intermittent troubleshooting is much more difficult..

Step 3: Gain Access to the Brake Light Portion of the Tail Light Assembly.

The tail light, reverse light, and turn signals are normal 12V bulbs, but the brake system is a set of 6 very bright LEDs.

The brake light system has the LEDs sealed into the assembly. They are not accessible or replaceable. This is an evil design for a DIYer. I decided to repair the right hand assembly first.

I took a dremmel tool and "made" access to the LEDs. I used a cutting disc, and cut between the clear lens and the black plastic of the assembly. It turned out to be more of a heating and melting operation, than a cutting operation.

Step 4: Removing the Resister Board.

On removing the front clear lens, I could remove the brake LED section, only after removing the center turn signal assembly. The LED section has two wires going to it from a resister board. A yellow wire (+12V) and a black wire (Ground).

I unplugged connector CN2 from the resister board so I could completely remove the LED sub-assembly from the tail light assembly.

Step 5: Follow Wires and Figure Out How the Brake Light System Works.

I got online and found MANY people with brake light problems on the Prius. To me, the Prius has a poorly designed, and poorly manufactured brake light system. It is way to complex and fragile. I would have preferred a simple light bulb.

Toyota wants over $300 to replace the tail light assembly.

Online were many people claiming to know all about the Prius brake light system. Some say the LEDs were in series,so when one goes out, they all go out, and some say they are in parallel. Some say the resister boards were the problem, and some say the LEDs themselves were the problem. Some gave very detailed analysis of the electronic components and their values, and much theory. Some people had only one or two LEDs out. The highly technical explanations of the brake light system could not be true if all the above was also true..

I decided that the most reliable information must come from myself.

Since the failures swap sides, I looked for the closest common connector. It is J31 which is a large multi pinned plug, located behind the trim on the left side of the trunk area, just about above the gas filler door. I opened it up and cleaned the connector. No change, everything was still the same, neither brake light was working.

With a volt meter reading the voltage to pin 2 (+12V) and pin 5 (Ground) on both connectors at the tail light assembly R9 (left tail light assembly) and R10 (right tail light assembly), the voltage supplied to both connectors was 12V and never wavered as I played with all the connectors.

Next was to put 12 volts straight to the assemblies. +12V to pin 2 and ground to pin 5. The LEFT assembly brake light came on, but flickered, and 2 LEDs were out. The RIGHT assembly brake light would not come on.

Putting a meter on the incoming wires on connector CN1 on the resister board, I confirmed that 12V was arriving at the resister board. Re-connecting CN1, applying 12V, and measuring the voltage out of the resister board at CN2 revealed that about 7.06 volts (with no load) goes out of the resister board, to the LED assembly.

I made a little voltage divider circuit to give me 7V from 12V with the LEDs attached, and used an old 12V battery for testing at the dinning room table. Without working LEDs I used values of the resisters to “guess” at the voltage drop across the LED assembly.

Step 6: Testing the Circuit.

I hooked up the LED assembly to my test circuit, and all LEDs were dead. I pushed and poked all the connections , and suddenly all the LEDs lit up. I removed all the spade connectors and replaced them. Now I had only one LED out.

Step 7: Plates and Crimped LEDs

Toyota had decided to use pin mounted LEDs, so therefore they needed to mount them differently than with just wires. They use steel plates as buses between the LEDs. Each LED is crimped/pressed into the metal plate. Wiggling these plates produced on,off flashing of the LEDs. I tried to re-crimp the LEDs to the metal plates. This resulted in the LEDs falling off the plates. Bad Idea.

Step 8: "Plan A" to Fix the System.

Soldering the LEDs to the plates was difficult, as each metal plate has a large surface area, and heating them would require lots of heat.

The whole brake light system is complex and delicate. The design process must have been determined by cost only, not reliability or robustness.

I borrowed a large soldering iron and soldered each LED to the respective plate.

Now all LEDs light up, and I could not make any of them fail. I assembled everything back together, and (resealed with silicone glue) put the big clear lens back on, then used gorilla tape to also hold the lens on. The right brake light now seemed to work, but was very dim.

I should have done a few things differently. I should have scratch and score the plates so solder would stick better. I should also have soldered the spade connectors onto the plates.

Step 9: Making the Wiring Diagram.

I labelled the LEDS 1, 2, 3, on the bottom row, 4 and 5 in the middle, and 6 at the top. I drew out a wiring diagram, and found that LED 1, 2, 3 are in series between the yellow wire and the black wire. LED 4 is in parallel with LED 1. LED 5 is in parallel with LED 2, and LED 6 is in parallel with LED 3.

LED 4 was not lighting up. A few minutes later, and after much man handling, LED 4 was now working and LED 6 was out. Since some LEDs are in series, and some in parallel, means that if just one LED is out, the other 5 will still work. BUT if 1 and 4, or 2 and 5, or 3 and 6 both fail, then all LEDs will be out.

Step 10: Re-build the System According to "Plan B".

I did things differently while repairing the LEFT brake light assembly.

The dremmel process to remove the clear lens, went about like the last time. I was much neater this time. With the lens off, the brake light sub assembly came out easy. Once again, the center reflector cluster had to come out first, then the brake assembly can come out. Both are held in with plastic clips, that are not possible to get to. This whole tail light unit is not made to come apart. Of course with a dremmel tool, most things will come apart.

The whole system made me feel like I was working on junk. (I don't do "junk" very well.)

I bought 6, 12V LEDs from Radio Shack. The LEDs are 5mm Red, 12VDC with integrated Resister. Part number 276-0209. I wish they were much brighter.

I removed the Toyota LED assembly, and the resister board. The slide in plastic parts that the metal plates, screw onto looked useful, so using the same plastic parts, and the same reflectors, I began to install the Radio Shack resisters.

After removing the Toyota LEDs and the metal plates they were crimp/pressed onto, there was no place to hold the new Radio Shack LEDs.

Step 11: Making the LED Holders.

I took an old kitchen cutting board that is made out of a tough clear plastic, and cut small insert plates to fit inside the holes where the Toyota LEDs came out of.

Step 12: Small LED Holders.

Then I drilled a 7/32 in hole in each small insert plates to stick the new Radio Shack LEDs through. I glued the small insert plates onto the plastic trays, and then glued the Radio Shack LEDs into the small insert plates.

Step 13: Wiring in the New LEDs

Next was to wire the 6 new LEDs in parallel. I elected to solder everything. No crimping. I cut the yellow and black wires off the old Toyota LEDs so I would have the plug onto the resister board before I realized that I didn't need the resister board any more. I cut the wires going to the CN1 connector on the Resister board and soldered them direct to the yellow and black wires. The yellow and black wires then got soldered to the new Radio Shack LED brake light harness. After testing everything, I put the brake light subassembly into the left tail light unit. Then silicone sealed all around the clear plastic lens, and mounted it. To hold it while the silicone cured, I used gorilla tape to hold the whole tail light assembly together.

Step 14: Realize That "Plan B" Wasn't Good Enough.

I tested the brake lights in the dark.

I was happy with the way the left side worked. It worked like a brake light, but it was not bright enough. The right side now had 2 LEDs out.

The tail light is much brighter than the brake lights, and the center high brake light is what I think the brake lights intensity should look like.

Step 15: Re-Re-build the System With Brighter LEDs.

I ordered super bright 12V LEDs from Amazon.

Here is a link to the bright LEDs I bought.

The Radio Shack ones I had used on the left brake light, were 1.5 MCD

The ones I received from Amazon were 8000MCD. I don't understand the MCD rating. The 1.5MCD LEDs draw 20ma and the 8000MCD ones draw 200 ma. That sounds like 10 times the power, not 7000 times the power.

Step 16: Installing the New LEDs.

I re-soldered all the connections on the right brake subassembly. The left side had been redone using cheap Radio Shack LEDs that were not bright enough. I now was going to replace the LEDs on the left side with the brighter ones, and remove the Toyota system from the right side and install new bright Amazon 12V LEDs.

Step 17: Put the Assemblies Back Together.

I used lots of tape and silicone glue to to try to keep moisture out of the assembly.

What it needs is a small heater to remove the moisture. Maybe the resister board's purpose was to act like a heater.

Step 18: Testing in the Dark.

Now the brake lights are bright. :-)

I never figured out what the resister board was for. There are many many theories online for their purpose, but none make sense to me. I took them out.

Now the whole brake light system is simple, robust, AND cheap.

PS: One year, and 10,000 miles later, and the brake lights are still working perfectly.