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Old 11-16-2017, 10:22 AM   #1
BagRidersJohn
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Member#: 372859
Join Date: Nov 2013
Chapter/Region: NESIC
Location: Burlington, VT
Vehicle:
2006 WRX Wagon Ltd.
Sedan Wide Body UGM

Default Testing Brake Fluid Sensor

Hello,

I've had a persistent BRAKE indicator illuminated in my gauge cluster for many years now, and with the interior of the car completely removed I thought it to be a good time to try to diagnose and fix the issue.

After some thorough testing I believe the issue to be with the brake fluid level sensor so I'm wondering if anyone could describe how I can test that. Below I will describe how I ruled out the hand brake circuit as a culprit. The brake fluid level is not low and therefore I think something must be up with the level sensing mechanism.

Back story / My understanding
My understanding is that the only circumstances in which the BRAKE indicator light will be illuminated is when the hand brake is pulled (therefore the plunger is lifted, grounding the circuit) or if the brake fluid level is low and the fluid sensor triggers the indicator. I think it will also illuminate when there is an issue with the charging system, but in that scenario the battery/charge system fault light will also illuminate which is not the case for me-- just the BRAKE light indicator is on.

Hand Brake Circuit Testing
I expected the issue to be related to the hand brake, so I began by testing that system. I first performed a ground continuity test on the contact below the hand brake. The circuit is grounded while the hand brake is pulled/engaged, and the circuit is broken (i.e., the contact is not grounded) when the plunger is depressed (hand brake is released/down). This confirms that the "plunger" is functioning as it should and interrupting the circuit when the hand brake is not engaged.

After confirming the plunger was functioning, I proceeded to test the continuity of the signal wire connected to the contact below the hand brake. The wire runs below the passenger seat to a larger ~20 pin connector, from there it runs up and across the dash to a small brown plastic piece that houses two 1.5a diodes, located roughly behind where the gauge cluster would be. Continuity was confirmed from the plunger contact to the terminal on the plug, then from the other side of the plug to the terminal on the diode, and finally from the plunger contact to the terminal on the diode. In my opinion, this rules out the hand brake circuit as the culprit therefore leaving the brake fluid sensor as the only remaining possibility.

If anyone could shine some light on this system I'd really appreciate it. I'll be digging through wiring diagrams in hopes to figure it out myself but I figured I'd get a post up here as it seems to be a really oddball issue.
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Old 11-16-2017, 12:53 PM   #2
BagRidersJohn
Scooby Newbie
 
Member#: 372859
Join Date: Nov 2013
Chapter/Region: NESIC
Location: Burlington, VT
Vehicle:
2006 WRX Wagon Ltd.
Sedan Wide Body UGM

Default

Well I cracked open the wiring manual and its very simple.

Here's a diagram of the parking brake / brake fluid level sensor wiring.

I removed sections of the diagram that are specific to the STI or manual climate models.



So, I've tested the continuity from connection "R4" up to B112 Pole #4, confirming there are no breaks in that circuit. As expected, B112 #4 is grounded when the hand brake is engaged.

Next test for me is going to be confirming that the RY wire on the i10 instrument cluster connector (center one) is grounded, which turns on the brake light, which it surely is. Next, I'll test the Lg wire on the BMC plug which I also expect to be grounded. Assuming that is true, that will indicate that the plunger on the BMC is stuck, grounding the circuit.

I know my way around electronics pretty well but I'm not an engineer by any means. My understanding of this circuit, specifically the usage of the diode in the B112 connector, is so that the plunger in the BMC can ground the circuit for the indicator light without also grounding the handbrake circuit which is shared by the DCCD circuit. And that makes sense to me. The DCCD controller is aware of when the handbrake is pulled, but does not need to be aware of when brake fluid is low. Any electrical gurus here can confirm?
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Old 11-16-2017, 07:10 PM   #3
BagRidersJohn
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Member#: 372859
Join Date: Nov 2013
Chapter/Region: NESIC
Location: Burlington, VT
Vehicle:
2006 WRX Wagon Ltd.
Sedan Wide Body UGM

Default

I decided a better place to post this would be in the Electrical forum, so here's a link to the thread: https://forums.nasioc.com/forums/sho...8#post45423858

Mods, please close this thread!
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