Failed Lamp Sensor Fix

 

Symptoms:

• The (4) rear brake lights do not work, except for the LED brake light on the hatch.
• The (4) rear taillights (same bulbs, different filaments) work OK
• Fuses are OK and the filaments for the brake lights in the bulbs are OK

Diagnosis:

• Most probably, the Toyota Lamp Failure Sensor (LFS) has malfunctioned.
• The LFS (P/N 89373-14080) is in the brake lamp circuit for the (4) rear lamps. It is NOT in the circuit for the LED lamp which explains why the LED lamp continues to work. Its purpose is to sense when a lamp filament has burned out and notify the driver by a warning lamp on the instrument panel.
• Unfortunately, there is no sensor to detect when the LFS itself malfunctions, and the driver is simply left without brake lamps (except for the LED)
• Often (at least 4 known cases) the LFS has malfunctioned simply due to a cold solder joint, or a cracked trace on its printed circuit board. These can be easily repaired and will save the owner a bundle. (Most Toyota dealers charge over $225 for the LFS part alone.)
• The LFS is in a small light blue plastic case (about 2” x 2” x 1”) and is located in the left rear quarter panel of the Supra, in the well behind the wheel. It’s easily accessible by removing two plastic covers in the rear hatch.

 

Tools Required:

• #1 Phillips head screwdriver
• Low wattage (<25 watts) soldering iron

Time Required to Fix:

• Only about 20-30 minutes

Procedure:

• Get your soldering iron plugged in so it’ll be ready – the rest of this will only take about 10 minutes…
• Open the rear hatch and peel back the carpeting on the left rear side (see photos)

 

 

 

• Pry out the black plastic plugs covering the fasteners for the left side plastic panel, and the small triangular panel covering the left rear shock tower.
• Remove the 5-6 phillips head screws for the side panel, and the shock tower panel.
• Remove the (2) Toyota black plastic rivets – one is on the shock tower panel, and the other is holding the left side panel to the rear panel (covering the taillamp). Look closely for it, it’s there, but it’s buried!
• Pull the shock tower panel off, and the left panel out – careful with the left panel – be sure to disconnect the interior lamp from its harness, and if you didn’t find that 2^nd plastic rivet the panel is not coming out!

• Locate the light blue plastic tab protrudng through the sheetmetal. It will probably have the hatch release cable and another harness running right on top of it. This is the LFS.

• Reach into the well opening and hold the LFS with your left hand, and push on the blue tab with your right – it should pop right out.
• Extract the LFS from the well, and remove the plastic harness connector.

• Turn it over and see how the back fits on – remove the back to expose the printed circuit board.

• If you look carefully at all the traces, you may be able to see a crack or a cold joint. Several list members who have had this problem reported finding the fault this way. With your soldering iron carefully try to bridge the crack with the existing solder on the board (there’s plenty), or touch up the cold joint. Try not to add new solder as it may bridge traces where it’s not supposed to!
• Either my eyes were too old, or my LFS simply had a cold joint which I couldn’t detect. After staring at this board under a magnifying glass for 10 minutes, I took the easy way out and “touched up” every solder joint.
• Don’t reassemble just yet, but reconnect the LFS to its harness and see if you have brake lights now. If you do – congratulations! You’ve saved yourself at least $300-400 in dealer service charges.
• If you don’t have brake lights yet, and you didn’t resolder every joint on the board (as I did) – try this approach now, maybe you have more than one fault. Try not to add any new solder!! On my LFS I noted there was plenty of solder and I had to be careful that the excess didn’t bridge adjacent traces.
• Assuming you’ve had success – reassemble everything! If no success, reach deep into your pocket for that $237 for the Toyota dealer. (If it’s any consolation, Jeff Watson at Jay Marks will sell it to you for $150)

 

Any feedback is welcome!