BRAKE Light On - Not sure why?

Hey guys,

Yesterday I popped my hood to add some washer fluid and noticed a sensor or possibly sim that was plugged into the harness right above the ABS module. I took it out, looked at it (looks like three resistors with a cover missing) and plugged it back in.
When I stated my truck, the 'BRAKE' light came on. The brakes felt fine, so I drove it home.
At home, I checked that 1) the fluid was full 2) the parking brake wasn't enables 3) no brakes were overly hot like they were dragging. I also noticed cruise control and TC lockup function normally, as well as my mileage has not changed.

Does anyone know what this sensor is (PN so I can replace it), or what my issue might be?

It has been really cold lately (-32F before windchill last week, -10F before windchill last night) so it's harder to take my time and check everything.

'BRAKE' light on while cruise control is working:


Sim or sensor. Not sure what this is:


Location the Sim/Sensor was plugged into (rearward of drivers battery, just above ABS module):


Thanks in advance!

I went back out at lunch and had a look. It looks like the sensor in the master cyl reservoir just broke off randomly. After looking online, looks like this happens and you can't just get an OEM part... I'm going to check out a scrap yard tomorrow.

If anyone has the OEM part number, I'd appreciate it! Thanks.

Related article:
Brake master cylinder sensor BS - Dodge Diesel - Diesel Truck Resource Forums

I think this is the PN. Can anyone confirm?

"Mopar 68057090AB Brake Fluid Level Sensor"

Found this too:
http://4a30d8fd18dae1bf393d-df49f4cedb726ad03ad145d2e3d346bd.r41.cf5.rackcdn.com/fileadmin/meder/pdf/en/Technical_Documents/Technical_Write-Ups/Brake_Level_Sensor_Application.pdf


Also, ABS is still functional with the light on.

CorneliusRox said:

Also, ABS is still functional with the light on.

Click to expand...

What do you want to bet they made it so it would disable the ABS in the next 2 or 3 years after yours? LOL

I've seen them on dead ones before, don't have one, though.

It appears to have started in 2001 one, when the disc brakes hit the 2nd gens.

Mark.

Found one in a reservoir in my pile.
It has a number on it, 4129A.

Mark.

Thanks Mark! I ordered one on Amazon before your post. Should be here Tuesday next week.

If I have issues with that, let me know if the one in your hand is for sale ;-)

Standard FLS175 is the replacement number.

https://www.oreillyauto.com/detail/b/standard-ignition-5753/engine-sensors---emissions-25132/body-sensors-25094/brake-fluid-level-sensor-11310/e9baf9fd5113/standard-ignition-2-terminal-brake-fluid-level-sensor/fls135/5809093?q=Brake+Fluid+Level+Sensor&pos=11

Mark.

Standard FLS175 is the replacement number.

https://www.oreillyauto.com/detail/b/standard-ignition-5753/engine-sensors---emissions-25132/body-sensors-25094/brake-fluid-level-sensor-11310/e9baf9fd5113/standard-ignition-2-terminal-brake-fluid-level-sensor/fls135/5809093?q=Brake+Fluid+Level+Sensor&pos=11

Mark.

CorneliusRox said:

Thanks Mark! I ordered one on Amazon before your post. Should be here Tuesday next week.

If I have issues with that, let me know if the one in your hand is for sale ;-)

Click to expand...

That's fine, but I added the Standard part number and a link above.
FWIW, after looking at my reservoir, it doesn't physically touch fluid in any way.
I suspect it's more of a TEMPERATURE sensor, than an actual level sensor.

Mark.

Thanks for the links. I called O'Reilly's and NAPA. Both couldn't find the part. Oh well.

Mark Nixon said:

That's fine, but I added the Standard part number and a link above.
FWIW, after looking at my reservoir, it doesn't physically touch fluid in any way.
I suspect it's more of a TEMPERATURE sensor, than an actual level sensor.

Mark.

Click to expand...

LOL

Check out that link I have further up. It's a PDF on how these work. They're a float sensor using a magnet I guess.

I tried soldering the broken section to see if that'd work.
It didn't... Looks like I'm waiting until the new one comes in.

FIXED

Alright, I've got this all figured out and wanted to share. The system is very simple.
Master Brake Reservoirs Side:
Instead of a float and switch like conventional systems, this has a magnet on the float. When the float gets too low the magnetic field effects the sensor and sends a signal to the ECM/BCM to display a ‘BRAKE’ light on the dash.
Sensor Side:
The sensor works in two ways. The first way is when the fluid is high and the magnet is nowhere near the sensor. It’s a simple circuit where there’s one resistor (R1), the ECM/BCM sees this specific resistance and you truck is happy. The second way is the magnet makes the two floating plates (seen in green) flex and contact. This makes the circuit see a second resistor (R2) in parallel. The equivalent resistance (Reqv) [(1/Reqv)=(1/R1)+(1/R2), so Reqv=1/((1/R1)+(1/R2))] is then seen by the ECM/BCM and puts a ‘BRAKE’ light on your dash.

What happened in my way was the two parallel plates must have gotten bent when the sensor broke and fell out of its housing. The two plates were always contacting and showing the ECM/BCM bad information.
For the time being, I bent these plates out of the way, and my truck is happy. My new sensor will be here next week.

When the new sensor gets in, I’ll measure the resistance of this system in case anyone wants to build a bypass if they’re ever chopping up a harness for race stuff.

Picture 1 – Brake Reservoir Mechanics And Impact On Sensor


Picture 2 – Sensor Breakdown (Red Lines = wires on A-side of PCB, Blue Lines = wires on B-side of PCB, Green boxes = magnetic plates, R1 Box = Resistor 1, R2 Box = Resistor 2)