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Brake Light Voltage ?


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#1 Doctor Ed

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Posted 06 July 2019 - 12:02 PM

was working through some wiring today, and came across something weird...

 

now bearing in mind I've got NO light assemblies fitted to the harness anywhere (not sure that should me a difference) when checking voltages at the rear light plugs, with the ignition on and all lights off, I'm getting 6V at the brake lights (both left and right side). all other lights are reading 0V at the plug. apply the brakes, and everything works, voltage jumps to 12V, everyones happy. same goes with tail lights on, indicators going, whatever, resting voltage of the brake light is 6V, it never goes to 0V. works fine going to 12V when the pedal is applied.

 

i check the brake light switch (removed the plug) and still get 6V 

 

i know the VX has some "interesting" wiring, and wondering if this is the case here, and when light assemblies are installed (with resistive load / filament bulbs) this will all normalise? 

 

or is something off?

 

cheers

ed

 

 



#2 Zoobeef

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Posted 06 July 2019 - 10:53 PM

Comes from the ABS module. Not sure what it's for but it meant when I used LED lights they were constantly on. So I cut the wire. I don't run ABS anyway so it's all fine now.



#3 siztenboots

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Posted 07 July 2019 - 02:49 PM

6v measured between what? , either 12v is dropping to 6v via some load , or the ground wire is floating up.



#4 Zoobeef

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Posted 07 July 2019 - 03:25 PM

It comes from the green/violet wire between the brake switch and the ABS. When you press the pedal the switch will send 12v down that wire but for some reason when you aren't then 5-6v is coming back up it from the ABS.

I imagine they are all like that.



#5 Doctor Ed

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Posted 07 July 2019 - 04:06 PM

ABS? Bizarre, I wouldn’t have thought to start looking there.

6V measured between brake signal pin and ground. Other lamp pins measured to same ground measure 0V

Edit- with the lights assembled I can’t measure the pin voltage anymore, but the bulbs (21W) certainly don’t illuminate

Edited by Doctor Ed, 07 July 2019 - 04:08 PM.


#6 AlfaAversion

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Posted 07 July 2019 - 05:25 PM

Is this bulb check function in ABS ?

#7 Arno

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Posted 08 July 2019 - 08:33 AM

AFAIK the Speedster doesn't use dual brake pedal switches? 

 

So they are likely piggybacking the ABS brake pedal input signal (so it knows you are indeed pressing the pedal  :D  ) onto the normal brake switch. And the ABS input is likely some open-collector thing that 'floats' and puts a small current on the system if not used/triggered.

 

https://www.speedste...rake System.pdf

 

https://www.speedste...lamp switch.pdf

 

On other cars of the era you often see 2 or more brake pedal switches with 1 dedicated for the rear lights and another for the ABS unit.

 

These days often back to 1 with everything broadacst on CAN and all units wanting the signal just leeching the requied data off of the bus  :P

 

Bye, Arno.



#8 siztenboots

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Posted 08 July 2019 - 12:25 PM

on the turbo ( failsafe for the cruise control) S43 ( NC and NO ) ,

 

on the 2.2 , S29 ( NO )






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