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Brake warning light
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=84833		 Page 1 of 1
Author:  lacabrera [ Thu Jun 23, 2016 9:41 am ]
Post subject:  Brake warning light
Recently replaced brake master cylinder complete with reservoir and brake level warning light reed switch. Problem I now have is the brake warning light will not go out. The switch also has a resistor built into it. The BCM checks for a this resistance and if not detected will give a warning light. The old switch has a higher resistance than the new one x10. My question is, is it possible to recalibrate the BCM to suit the lower resistance? The switch is glued into the plastic reservoir, so reluctant to reuse the old switch. Any one had this problem? Thanks for any advice.
Author:  Billwill [ Sat Jun 25, 2016 3:48 am ]
Post subject:  Re: Brake warning light
lacabrera wrote:
Recently replaced brake master cylinder complete with reservoir and brake level warning light reed switch. Problem I now have is the brake warning light will not go out. The switch also has a resistor built into it. The BCM checks for a this resistance and if not detected will give a warning light. The old switch has a higher resistance than the new one x10. My question is, is it possible to recalibrate the BCM to suit the lower resistance? The switch is glued into the plastic reservoir, so reluctant to reuse the old switch. Any one had this problem? Thanks for any advice.


You can connect some resistors in series with the new resistor to get back to its original resistance....cut one of the wires going to the switch and put an extra resistor in series there. :wink:
Author:  joe_ [ Sat Jun 25, 2016 2:47 pm ]
Post subject:  Re: Brake warning light
Service manual says it should be a 1k ohm resistor. Out of curiousity, what are you measuring on the old and new one?
Author:  lacabrera [ Mon Jun 27, 2016 8:39 am ]
Post subject:  Re: Brake warning light
joe_ wrote:
Service manual says it should be a 1k ohm resistor. Out of curiousity, what are you measuring on the old and new one?


The new switch/resistor is a 1k ohm? but the old one is 10k ohm, That's if I am reading it correctly? the old switch/resistor has 10x the resistance of the new one.
I have tried a third switch/resistor and that also has 1k ohm.
As soon as I plug the old, higher resistance switch the warning light goes out.
Author:  lacabrera [ Mon Jun 27, 2016 8:46 am ]
Post subject:  Re: Brake warning light
Billwill wrote:
lacabrera wrote:
Recently replaced brake master cylinder complete with reservoir and brake level warning light reed switch. Problem I now have is the brake warning light will not go out. The switch also has a resistor built into it. The BCM checks for a this resistance and if not detected will give a warning light. The old switch has a higher resistance than the new one x10. My question is, is it possible to recalibrate the BCM to suit the lower resistance? The switch is glued into the plastic reservoir, so reluctant to reuse the old switch. Any one had this problem? Thanks for any advice.


You can connect some resistors in series with the new resistor to get back to its original resistance....cut one of the wires going to the switch and put an extra resistor in series there. :wink:


Thanks for the info, I agree bring up the resistance to 10k ohm would cure the problem, but i would guess it would need to be wired in parallel with the switch to obtain resistance.
Author:  Billwill [ Wed Jun 29, 2016 4:14 am ]
Post subject:  Re: Brake warning light
lacabrera wrote:
Billwill wrote:
lacabrera wrote:
Recently replaced brake master cylinder complete with reservoir and brake level warning light reed switch. Problem I now have is the brake warning light will not go out. The switch also has a resistor built into it. The BCM checks for a this resistance and if not detected will give a warning light. The old switch has a higher resistance than the new one x10. My question is, is it possible to recalibrate the BCM to suit the lower resistance? The switch is glued into the plastic reservoir, so reluctant to reuse the old switch. Any one had this problem? Thanks for any advice.


You can connect some resistors in series with the new resistor to get back to its original resistance....cut one of the wires going to the switch and put an extra resistor in series there. :wink:


Thanks for the info, I agree bring up the resistance to 10k ohm would cure the problem, but i would guess it would need to be wired in parallel with the switch to obtain resistance.


You need to either remove the faulty resistor and replace it with a 10 k resistor which works or add 9 K Ohms of resistors to your 1 k resistor in series to bring it up to 10 K.

Is the existing resistor in series or parrallel to the reed switch?

The basic rule of resistance in series R = r1+r2+r3 etc. so to bring your failing 1 k resistor up to 10 k you have to go in series.
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