Brake Lights

[mmaginniss]

hi. i have a '90 LS400 and i took the car to get inspected today and the brake lights won't come on. i checked the fuse for the "stop lights" and it seems fine. i read in another post about the issue possibly being a brake light switch. how could i tell if this is the problem? would i have to buy a new switch from the dealer and try to replace it? if so, how would i do that?

thanks

[1990LS400]

If your brake light switch has failed, you wouldn't be able to shift out of Park without pressing the override button on the center console. You have checked all four bulbs, haven't you?

[landar]

There is a 'ton' of info in this forum concerning electrical issues. Just try a "brake" search with the

provided search engine and you will see. It just takes time to weed thru the discussions but if

you are determined, you will get to the bottom of your particular problem rather quickly.

Having said all of that, there is one particularly pesky trunk ground wire that tends to get stretched

and broken causing lots of interesting phenomenon to occur. Its worth a check.

This being an electrical problem and all, you didnt let the smoke out of something, did you? :whistles:

[jeffls400]

I had this same problem a couple of weeks ago. There was no issue with the brake switch or grounds in my case. The Lamp Failure module in the trunk had failed and kept the brake lights from working at all. As you may know, these are quite expensive so I soldered jumpers inside the module (it's easy to open up) to bypass the circuitry that let's you know when a bulb is out.

I soldered jumpers between terminals 9 and the brake lights (1,2,6 as labeled on the attached diagram) on the printed circuit board and it works great.

See the attached diagram to see where the respective terminals are. The module is labeled "Light Failure Sensor".

Jeff

Stoplight_Wiring_Diagram.pdf

[landar]

I had this same problem a couple of weeks ago. There was no issue with the brake switch or grounds in my case. The Lamp Failure module in the trunk had failed and kept the brake lights from working at all. As you may know, these are quite expensive so I soldered jumpers inside the module (it's easy to open up) to bypass the circuitry that let's you know when a bulb is out.

I soldered jumpers between terminals 9 and the brake lights (1,2,6 as labeled on the attached diagram) on the printed circuit board and it works great.

See the attached diagram to see where the respective terminals are. The module is labeled "Light Failure Sensor".

Jeff

So let me see if I have this straight.... The lamp failure module is supposed to warn the driver if a bulb fails but if the module fails then no lights or warning?

Isnt that a bit of an ironic (or moronic) design?

[jeffls400]

In my case, that is accurate. The warning bulb failed to light up upon start-up in the gauge cluster and that was the only warning (but I already knew the brake lights were out anyway).

[mmaginniss]

If your brake light switch has failed, you wouldn't be able to shift out of Park without pressing the override button on the center console. You have checked all four bulbs, haven't you?

thanks for the suggestion. all rear lights actually work (like when i have the lights on and the blinker on). yet when i press the brakes, no brake light comes on. that leads me to believe that it's something with the electrical system.

i also have a brake system warning light that is always on for no reason... i rarely use the parking brake and it's on even when the parking brake is clearly released. i don't know if this is related or not. i checked the brake fluid and that's ok.

[mmaginniss]

I had this same problem a couple of weeks ago. There was no issue with the brake switch or grounds in my case. The Lamp Failure module in the trunk had failed and kept the brake lights from working at all. As you may know, these are quite expensive so I soldered jumpers inside the module (it's easy to open up) to bypass the circuitry that let's you know when a bulb is out.

I soldered jumpers between terminals 9 and the brake lights (1,2,6 as labeled on the attached diagram) on the printed circuit board and it works great.

See the attached diagram to see where the respective terminals are. The module is labeled "Light Failure Sensor".

Jeff

this seems very similar to my problem. the light failure sensor is in the trunk... i looked around but i am not sure where to find it in the trunk. any idea where i could find it? thanks.

[jeffls400]

If you open the trunk, the module is on the upper left of the trunk, just to the right of the trunk hinge. You have to remove a couple of screws that hold the carpet in place but it is a little black box the size of a playing card deck, with one electrical connection.

The warning light that you are seeing...is it yellow or red?

[mmaginniss]

If you open the trunk, the module is on the upper left of the trunk, just to the right of the trunk hinge. You have to remove a couple of screws that hold the carpet in place but it is a little black box the size of a playing card deck, with one electrical connection.

The warning light that you are seeing...is it yellow or red?

thanks for letting me know where failure light sensor is. i'll check to see if i can bypass this based on the other post.

it's a red warning light. it used to just show up when the parking brake was on but now is on all the time.

[wwest]

I had this same problem a couple of weeks ago. There was no issue with the brake switch or grounds in my case. The Lamp Failure module in the trunk had failed and kept the brake lights from working at all. As you may know, these are quite expensive so I soldered jumpers inside the module (it's easy to open up) to bypass the circuitry that let's you know when a bulb is out.

I soldered jumpers between terminals 9 and the brake lights (1,2,6 as labeled on the attached diagram) on the printed circuit board and it works great.

See the attached diagram to see where the respective terminals are. The module is labeled "Light Failure Sensor".

Jeff

So let me see if I have this straight.... The lamp failure module is supposed to warn the driver if a bulb fails but if the module fails then no lights or warning?

Isnt that a bit of an ironic (or moronic) design?

Yes, this is rather strange.

Inside the brake sensor module there is three nichrome wire/strip sensors used to sense the level of current flowing to the lamps. Those are fairly serious sized, probably large enough to carry 10 amps each without "blowing" so it seems strnage that the sensor module could have failed. I wanted to use LED lamps, 10% of the current flow of the OEM lamps, so I had to change out the nichrome sensors for 0.5 ohm resistors.

[landar]

Yes, this is rather strange.

Inside the brake sensor module there is three nichrome wire/strip sensors used to sense the level of current flowing to the lamps. Those are fairly serious sized, probably large enough to carry 10 amps each without "blowing" so it seems strnage that the sensor module could have failed. I wanted to use LED lamps, 10% of the current flow of the OEM lamps, so I had to change out the nichrome sensors for 0.5 ohm resistors.

Actually, I was being a bit harsh about the Lex design. You must have the series resistors in order to sense the current and everyone must do

something like this. But for heavens sake, the resistors should really be beefy enough to prevent burnout and total failure of the brake lights.

wwest, did you actually exchange the incandescents for LED's? Are LED brake lights readily available or do you have to "customize"?

Thanks...Lan