XJ40 ( XJ81 ) 1986 - 1994

Intermittent high mount "third" brake light

 
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Old 06-20-2018, 08:45 PM
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Question Intermittent high mount "third" brake light

Perhaps related to the following thread?

https://www.jaguarforums.com/forum/x...-works-203253/

.... no idea, yet.

But I've also noticed that when I apply the brakes, the high mount "third" brake light intermittently lights. It mostly does not light, perhaps every 10-15 cycles it will light, but my observation is not very scientific at this point.

I can and will check the basics such as the connector at the light, the bulbs, etc, but wondered if this is a known issue and if so where I might look to save some time.

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Old 06-22-2018, 07:32 AM
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After doing some initial testing, it looks like the problem might be with the brake/interlock switch that is triggered by the brake pedal. It seems like if I apply the brake with a certain pace/force and even wiggle the pedal a bit I can get the high mount light to work more consistently. Although this also could just be coincidence.

As far as I can tell, the interlock and main brake lights work fine every time, even if the high mount light fails to light.

I don't yet know how that brake light and shift interlock switch is wired on the XJ40, but as I recall on the X300 the switch only has two circuits: 1) shift interlock 2) brake lights, not a separate circuit for the high mount light. Does anyone have a diagram of the brake light switch circuits, or just know if there is a unique circuit from that switch to the high mount light?

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Old 06-22-2018, 10:14 PM
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Does anyone know the circuits that run through the XJ40 brake light switch and if there is anything else in-line for the high mount/third light to check?

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Last edited by al_roethlisberger; 06-22-2018 at 10:21 PM.
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Old 06-23-2018, 09:53 PM
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Hope this helps. Sure sounds like the CHMSL relay is on its way out...

Maybe take it out and give it a firm rap a few times?

And so you know, that diagram suggests the brake input for the Anti-Lock Brake system is fed from the output of that relay... If your CHMSL is not working, neither is your ABS... Go for a little drive on a gravel surface and test my theory
 
Attached Files
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exterior-lights-2-of-4.pdf (42.4 KB, 9 views)
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exterior-lights-3-of-4.pdf (60.2 KB, 8 views)
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exterior-lights-4-of-4.pdf (58.6 KB, 8 views)

Last edited by NTL1991; 06-23-2018 at 10:15 PM.
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Old 06-24-2018, 11:36 AM
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Question Where is the CHMSL relay?


Thanks, I can't seem to find the location of a single relay that controls the high mount stop light as well as ABS.

The diagram below shows the ABS modules under the left hand dash and the high mount stop light relay in the trunk.

That being said, the two relay modules in the trunk are:

DBC10010 which controls:
1) Heated rear screen(window)
2) Aerial(antenna) up
3) Aerial(antenna) down
4) Ride leveling

DBC10422 which controls:
1) High mounted stop lamp <--
2) Remote boot release
3) LH dip flash
4) Fuel pump

Then there's one standalone relay for the fuel pump apparently.



 

Last edited by al_roethlisberger; 06-24-2018 at 11:44 AM.
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Old 06-24-2018, 11:51 AM
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If that is the problem, good news is that it is still available new for not terribly unreasonable cost:

https://www.sngbarratt.com/us/#!/Eng...4-1558bb1ab082

https://www.sngbarratt.com/us/#!/Eng...3-6785d024b2d5

Heck, one can replace all three trunk relays/modules as preventative maintenance for not too much cost. ....after all, they are all almost exactly 25 years old. So it may not be a bad idea.


https://www.sngbarratt.com/us/#!/Eng...a-7fd75e3d2680

https://www.sngbarratt.com/us/#!/Eng...d-98ef4aaa9369

I'm somewhat surprised at their reasonable cost to be honest, especially those custom integrated relay modules.


.
 

Last edited by al_roethlisberger; 06-24-2018 at 11:55 AM.
  #7  
Old 05-08-2019, 06:15 AM
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Default Intermittent High Mounted Stop Lamp

Did you resolve this issue? I have the same problem. While having my dad press the brake pedal, the light did not come on until I smacked the module DBC 10422. So, I guess I will open it up and reflow some solder.
 
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