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It's not hard to get to, it sits attached to the steering column, you just need a trim/clip tool to pull it down if memory serves correctly. Why do you need one? I work at a dealership and have never seen one of those fail yet.
 

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Discussion Starter · #3 ·
It's not hard to get to, it sits attached to the steering column, you just need a trim/clip tool to pull it down if memory serves correctly. Why do you need one? I work at a dealership and have never seen one of those fail yet.
Thanks for answering.

It's my daughter's car, she's in Ohio, I'm Arizona, so I am doing "remote diagnostics" for her.

I think the Stop Light Relay is going bad for the following reasons.
  • Stop lights (in the back of the car) stay continuously on or flicker
  • Disconnecting the plug from the brake light switch (the one on the brake pedal) has no effect - stop lights stay on
  • Similarly, removing the "BRAKE SWITCH" fuse (green oval in the pic) has no effect - stop lights are still on
  • Removing the "STOP LAMP" fuse (orange oval in the pic) turns stop lights off
  • Cruise control works normally. Pressing the brake pedal disconnects cruise control (as it should)

Based in the symptoms, it doesn't look like the brake switch at the pedal is the culprit (which would the most obvious cause). If the brake switch was "stuck" ON, cruise control would not engage. Also, disconnecting the plug from the switch would kill the stop lights.

So the next item in the electric circuit is the Stop Light Relay. It looks like a multi-contact solid state relay (right?) which not only powers the stop lights, but also connects to the ABS/ESC module. So it's either the Stop Light Relay is going bad, or something (but not the brake switch at the pedal) is telling it to turn the stop lights on.

Two additional pieces of information:
  • Her car had the ECU software update to detect rod bearing knock done recently. Looks like a coincidence, because updating engine ECU should have no impact on brake electric/electronics, but universe is a mysterious place.
  • She disconnected the battery overnight to prevent it from draining due to the stop lights on. Next morning, the problem went away - but we don't know if (or when) it will come back.

137181
 

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Well that makes it tough to diagnose from 2k miles away but let's see if I can shed a little light on this. The stop lamps switch or brake switch is actually 2 circuits, one normally open and one normally closed. Both need to function correctly as one side feeds your relay and the other signals to the ECM and ABS etc. but can also control the relay (for example you can ground a wire at the abs module and make lights stay on whole time as well). You can have your daughter physically pull up on the pedal and see if it goes away, perhaps someone can be looking at the back of the car or she can park against a wall or garage door at night and see the reflection. I have seen this a few times and it has always been the switch which fortunately doesn't even require any tools to replace as it twists out and back in. She can pull it out and physically press the little plunger in all the way, or just press the brake pedal down and push in the plunger on the switch super far up and see if it goes away. This should give you an idea if the switch is the culprit based on her results.
 

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Discussion Starter · #5 ·
Well that makes it tough to diagnose from 2k miles away but let's see if I can shed a little light on this. The stop lamps switch or brake switch is actually 2 circuits, one normally open and one normally closed. Both need to function correctly as one side feeds your relay and the other signals to the ECM and ABS etc. but can also control the relay (for example you can ground a wire at the abs module and make lights stay on whole time as well). You can have your daughter physically pull up on the pedal and see if it goes away, perhaps someone can be looking at the back of the car or she can park against a wall or garage door at night and see the reflection. I have seen this a few times and it has always been the switch which fortunately doesn't even require any tools to replace as it twists out and back in. She can pull it out and physically press the little plunger in all the way, or just press the brake pedal down and push in the plunger on the switch super far up and see if it goes away. This should give you an idea if the switch is the culprit based on her results.
Thank you for the additional information. Learning that the pedal switch has two sets of contacts, one NC and one NO, helps a lot.
We did check the switch adjustment (gap) vs. the pedal, and it is OK. But it makes sense that if one of the contacts in the switch is misbehaving, it can "confuse" the ABS / ECM into "telling" the stop light relay to turn the lights on. It would be nice to have the circuit / logic diagram :) . I wish I was there to check the switch with a DMM.

Given that it is an intermittent problem, I will start by telling my daughter to swap the switch. I'm sure she can do it, she's pretty handy with tools and such.

Thank you again for your help. I owe you one.
 
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