Brakes Low Brake Fluid Sensor Connection 86gt With 93 Cobra Master Cylinder 5 Lug Conversion

Mustang5L5 said:

The switch is working on the shuttle valve portion of the combo valve.

When you guy the prop valve portion, you aren't touching this part. So it still remains functional.

I wouldn't put too much trust in it though as a few combo valves I've played with had the shuttle locked in place. 30 years or so, plus brake fluid overdue for a flush can corrode some stuff away.

The newer MCs also have a shuttle feature that acts to restrict flow to an open line. Unsure if the older MCs do this as well but want to say yes.

ford got rid of it when they went to the 3 port setup. Rather than read a shuttle position, they go on fluid level. Either will work, so don't overthink it.


Give me some time to get in garage. Watching kiddos at the moment so might be a delay. I think the 3-wire setup is black wire to ground, 1 wire gets 12v and when the sensor float drops, the sensor works off magnetic inductance, closes, and sends signal to other wire.

Both my wires are same color with same tracer. They look black but FSM says should be pink. Either way they look the same. I'll see if I can dig up 1993 wiring diagrams and see if ford updated the low fluid wiring as I think that is wrong. I've found a couple errors in it, but Obviously a few decades late for ford to correct it

Click to expand...

I'm following your train of thought.... I didn't get a chance to touch the car today, got called out. I'm eager to plat around with to see really if I can parallel the circuit. That is a great point about the fluid corrosion. The only 30+ year part in my brake sys will be the combo valve. I may just disassemble it completely to see if I have any corrosion setting up on the internal PDV.

I guess I'll connect tomorrow evening or so...Thanks friend for the input.

Mustang5L5 said:

Ok got it done.

Black wire (wire closest to rear of car) has continuity to ground and battery neg

Key on, I get voltage to middle wire.

I get voltage from middle wire to black wire

Nothing on other wire closest to nose of car. I assume this is the return signal wire that displays the light


MAkes sense as it appears the sensor is not a contact float type switch, but works on magnetic inductance. So the hot wire and ground power the sensor, and when the float drops, a magnet trips it and you get voltage on the return wire.

Click to expand...

Hey there. Hope you had a great week. All good here, but I was not able to get even close to the car this week. Work was kicking my butt! I only had a few minutes of reserve energy left each night to look at your wiring diagram and to ponder what you discovered on your car the night you went out to do the quick test.

I did get a chance to look at the link to the other site. Those guys have the 64.1/2's. Their PV/Combo Valves are a bit diff. One thing that did stick out in my mind from that read was where one of the guys stated: ...when the pressure valve shift under a failure, it does NOT cut flow to that effected side of the valve... other members didn't object, but I understood that statement to be a bit contradictory.... I mean what's the actual function if it doesn't interrupt, and minimize the bleeding of fluid so to speak... I'm not getting lost on that thought though.

Another thing I found informative was a point made by a poster to NOT use Nitril O-rings when cleaning and replacing the internal of the LPV inside the combo valve. I was thinking about disassembling mine to seen if it had any build-up of the 30 year old brake fluid residue. I only did the gutting; nothing else so far... Might need O-rings, but they are tiny!

In addition, I am on the hunt for the thread size for the pressure switch on the 86 & earlier Fords. There is a tool to assist in the bleeding of the whole system. It get insertede in the port for the switch and it locks the LPV in place so it doesn't shift during bleeding and inadvertently close off one of the sides of the systems. Some guys have been writing about it being a PITA to bleed the brakes. I believe they weren't aware if this tool... I found on that the guy say is for a GM, but he doesn't know the thread size.


Tomorrow:

I'm going to start testing my 86 circuit with the 87+ wiring to see if I can get the float circuit to trip the LPS circuit on the 86. I'm planning to do a simple splice/tap into the 86 Pressure Switch wiring with the harness from the 87. There is one thing you eluded to about your car: You mentioned the forward nose wire you got nothing. I was wondering during your test, did you depress the float in the reservoir to see if you got a complete circuit on that wire? I'm sure you didn't go that far, but I thought I'd ask for clarity. I'm pondering, when I do my test, where I should try to intersect the circuits with that wire closest to the nose...... If it is a return wire to the light, then once the float is depressed, what polarity would that wire have? I haven't ohms tested the float switches yet, nor for continuity. I'll do that tomorrow too. What would you suggest?

Thanks again for all of your help. btw, there is no rush on any of this. I know you have the young ones vying for your attention too....

Mustang5L5 said:

Ok got it done.

Black wire (wire closest to rear of car) has continuity to ground and battery neg

Key on, I get voltage to middle wire.

I get voltage from middle wire to black wire

Nothing on other wire closest to nose of car. I assume this is the return signal wire that displays the light


MAkes sense as it appears the sensor is not a contact float type switch, but works on magnetic inductance. So the hot wire and ground power the sensor, and when the float drops, a magnet trips it and you get voltage on the return wire.

Click to expand...

OK. I got a chance to get to the car today to check wiring paths for the 86Gt OEM wiring and to voltmeter the 87-96 connectors with the pigtail wire that I cut off at the boneyard.

Note: No power was used here as my dash is out so I couldn't test this in an energized condition.

Refer to @Mustang5L5 post above #21 to make any sense to my test below.... Again, a complete, confirmed, direction of current flow and wire each polarity is needed to come to any conclusion here.... Also the wiring diagram in a previous post by Mustang5L5 above is helpful.

My Test was done on the 87-96 pigtails OFF the car, but plugged into the reservoir, (this put the magnet in the float system in play) no power applied to anything. Using a voltmeter I get continuity between the Black wire and BOTH of the purple wires independently but only when I allow the float to drop down to actuate / complete the magnetic circuit. This means the circuit path (through the magnetic float switch) is completed through either of the purple wires.

Further test shows: when testing for continuity between just the purple wires, there is full time continuity there, regardless of float switch position. So, the two wires must simply be a (+) in-out loop. The trace strips on the wires are there to indicate direction of current path -ie. to the light or from the current source.

Now, I'm not certain which goes back to the warning light for-sure, (i don't have it all connected to the car); but that could be tested on the car by depressing the reservoir float to actuate the switch, and, if you care to cut one or the other of the purple wires to see if the dash light goes out, then test for current on the other you will get to the answer. Or...lol, you can just read the wiring diagram if you have it.

All this discovery appears to be good news for my plight as I've figured out on side of my plan. My dash is disconnected from my 86GT right now, so I can't power-up my car to decipher the polarity of the two wires coming to my Brake Pressure Switch on the PV. This knowledge will allow me to determine to witch of those wires will I make my connections to get it all to work.

I PM'd a member to check on his 86 for me, but haven't heard from him yet...we'll see. This is where I'm at right now.

SOLVED - See solution below

Thanks to all you guys that helped out. I appreciate it

Notes:

1. Getting to a concrete solution often involves collaboration of like minds working together. In addition: reading and research, along with a desire to learn.

2. The name "Proportion Valve" (PV) as we know of it is not the correct name for the part as we know it. It is actually the " Brake Pressure Control Valve" or aka "Combination Valve," because of its multi-funtionality. It is a Metering Valve (MV) for fluid to the front brakes, a Proportioning Valve (PV) for fluid to the rear brakes, and it is a Pressure Differential Valve (PDV) to complete continuity in the Pressure Differential Switch (PDS) circuit thar lights up the dash Dual Brake Warning Light under brake system fault or ignition contact during Key-On operations.

3. There are no wired circuit on the Fow and SN95 cars the relate to brake pad wear sensing. Only fluid level / pressure sensing circuits are in play.

4. The term Dual Warning Brake Switch is actually correct for both the LBFL switch and the older line- PDS as the duality is in the parking brake warning and the fluid level / line pressure warning operations.

5. 1987 and up Plastic Reservoirs with 3 wire circuits containing the Magnetic Switch at the bowl performs the same task operation process as does the 1986 & earlier cars in their LPV circuits - A mechanical ground circuit that actuates the dash warning light upon ignition "start" contact and during maluction or fault events. (SEE DIAGRAM BELOW)

6. Read full thread if you want to know about compatibility.

SOLUTION DIAGRAM: