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Brake Fluid Pressure Sensor


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Brake fluid pressure sensor mounts in the top of the distribution manifold - triggers the brake warning light on the dash. Two pin on my '91. Anyone have a part number for one of these ? Any brand, Mopar preferred. My local AZ can't find a number for it.  Any help appreciated -

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It's not a pressure sensor. It's a mechanical switch with a plunger that's actuated by the shuttle valve in the distribution block (or "manifold," as you called it).

 

f287a83d865c9429d79df3df69f4149e.jpg

 

The factory parts manual doesn't list the switch as a separate part -- it comes as part of the "proportioning valve" assembly (so-called, although in the MJ it doesn't do any proportioning). Your best bet is a junkyard, or one of our members who has a few spares.

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1 hour ago, josehuerta said:

Brake fluid pressure sensor mounts in the top of the distribution manifold - triggers the brake warning light on the dash. Two pin on my '91. Anyone have a part number for one of these ? Any brand, Mopar preferred. My local AZ can't find a number for it.  Any help appreciated -

 

https://www.carolinaclassictrucks.com/proportion-valve-sensor.html

 

proportioning%20valve%20sensor_250x188.j

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I appreciate the input.

 

I used my own spare today and hoping someone has discovered an interchange that will work in it's place. I spent the afternoon looking for one of these with no luck. I know I could source a replacement block from a salvage yard, but was hoping to find just the switch with fresh plastic that doesn't just crack as my original did. Frustrated with throwing away otherwise good parts (and being jacked up by dealerships).

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7 hours ago, Ωhm said:

Since the two pin switch ties the wires together internally, can't you just tie the wires together externally and use the single pin switch?

 

Short answer: No.

 

Long answer: The switch connects the two wires to complete the circuit. If you tie the two wires together, the circuit will be closed and the light will be on permanently. The switch won't be doing anything.

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39 minutes ago, Ωhm said:

Looks like switch ties two wire together internally. A$$umption based off 88 Elect Manual.

 

673686206_BRAKEWARNINGSWITCH.jpg.e226415b6583487b2fecba8d7b18fc04.jpg

I would not even describe it as internal. there appears to be a bonding strap between the two pins visible in the cavity holding the pins. 1st pic shows switch without the actuating pin in place (remember, my switch is broken), the 2nd shows it with it in place as it would likely appear when engaged by a pressure differential. That same pin would at that point be contacting the body of the shuttle valve. Could this then ground either/both circuits to trigger the light?

 

PB140040.jpg

PB140041.jpg

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1 hour ago, josehuerta said:

Could this then ground either/both circuits to trigger the light?

Pin_B goes to the grounding side of the brake bulb in the IP cluster. Pin_A connect to the grounding side of IP brake bulb through that bonding strap, but circuit continues to IGN SW_G1 (bulb check; G1 to ground) or Park Brake Switch (to ground). Any one of these three things when grounded will turn IP brake bulb ON.

 

1824897892_BRAKEWARNINGSWITCH_1.jpg.2b966b08a40cc6cb40c957664e341a9e.jpg

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OK, not as simple as I always want things to be. I'm an old guy and this quickly made my brain tired. Bottom line: I seek a new replacement switch that will work without having to replace the complete proportioning valve... for my use and anyone else ..in this situation to keep these old Comanches running (it would not require pulling the block, bleeding brakes, etc.). Any salvage switch is 20 year old plastic. Plus I live in a small town with no really good salvage yard. Would  a single pin work? My take on the schematic and your explanation is that yes... both/either lead triggers the light. Also, I see some 2 pin switches on the internet but assume not all switches are the same - different engagement pressures for different vehicles. Tracking that down is a challenge in itself. Thoughts? Anyone? I always find a way to make things more difficult -

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An additional thought: if the pins both go to the ground side of the bulb then what is the power source? Do the two pins both go the ground side of the bulb... or come from the ground side? In other words, are these what I call negative switched, meaning the bulb is hot but not on until grounded through one of the leads? Afraid I'm not very "electric" -

 

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51 minutes ago, josehuerta said:

are these what I call negative switched, meaning the bulb is hot but not on until grounded through one of the leads?

 

Exactly. Most "trouble" or "alarm" indicators are energized by the application of a ground signal. It's much easier and safer to switch ground signals than hot voltage signals.  The HO's use ground switching to change the state of relays almost exclusively. Renix, not so much.

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Thank you! Ground switching. I have minimal experience troubleshooting electrical issues on a vehicle but have already run into this before on the Comanche. I reasoned that it had to be negative switched - ground switched - but with no experience was not sure it was even feasible. This information in itself is a great thing to know for any troubleshooting on the vehicle.

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