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As I’m restoring my 1976 f250 4x4 I replaced the proportioning valve along with all new brakes. The brake warning switch on it only has one black wire. Everyone that I’ve seen also only has one wire. My truck has 2 purple with white wires running down to it. They were not hooked up to the old one. According to the wiring diagram I have the wires are correct.... so I’m not sure what to do. Do I tie them together with the one black wire from the brake warning switch ?
I am not sure what to do, with out some home work. But make sure you figure it out, because you do not want to bypass that warning light ability.
You will know way before that light comes on there is a problem with the brake system when the brake pedal goes to the floor as you are saying OH SXXX!
Dave ----
I just checked on my bone stock 1975 F250 and I have two wires going down to that switch.
Looked up brake pressure warning switch for my 75 on LMC and its PN 47-0904 and it shows correct for 73-79....Not suggesting LMC over any other supplier- was first up on google.
Maybe you have the wrong switch ?
FYI, the purpose of that switch it to tell you there is a pressure imbalance(front to back) in your system.
In a perfect situation the plunger of the switch sits in a notch on the CENTERED(balanced) pressure valve. When the system detects a pressure imbalance due to a leak, the slide piece moves toward the lower pressure side( the leaking side) and the switch plunger comes up out of the divot and completes the circuit illuminating your brake warning light.
Tbruz
Thanks tbruz... I looked up the switch on LMC and that matches the plug on my wires. So I’ll order that and change out the switch. Thanks again you guys.
Make sure the rod or piston inside of the brake balance valve( where the switch screws into) is centered otherwise your light will stay on.
There's a process to re-center the rod/piston after a brake balance issue; you can search for it.
Tbruz
The original Ford switches used 2 wires. I've heard later ones had one wire, but have never seen one.
The GM version (and therefore all the aftermarket models to come after) use a single wire switch. The two do not interchange either, as the threads are different.
However, the two different systems are compatible with each other, wiring-wise. Simply twist the two Ford wires on the body side together and splice to the single switch wire. Voila!
Since the Ford system is literally just a "double-ground" setup, and the new one wire is a switch-to-ground, the dash lamp remains fully functional.
I looked up the switch on LMC and that matches the plug on my wires. So I’ll order that and change out the switch.
Got a link to the switch? Is it a Ford factory replacement, or one designed specifically to fit the aftermarket combination valves?
If not designed specifically for your new combo/prop valve, then it probably won't fit due to the mismatched threads I mentioned before.
Thanks Basecamp... I’ve ordered the new switch from LMC so when it gets here if the threads are different I’ll send it back and just tie the wires together.
That can work. If you're lucky!
I always wondered why they said to turn the ignition switch on though. Has absolutely nothing to do with the function, but maybe it lets you watch the light go out?
And if pressing the brake pedal does the trick, why don't they say to just get in and drive it? Ahh, probably the lawyers saying that we need to watch the light go out to confirm that all is well in the brake system before they take it out on the road.
That's probably it.
But either way, the old H-block valves would almost never re-center. Sometimes even after you re-centered it manually by taking it apart. If your light was not illuminated already, it would sometimes come on after bleeding the brakes and stay on for the rest of your life. Or until you got tired of it and removed the bulb. Hopefully before it melted the lens.
And if you were really lucky, it stayed stuck in the middle and you never had to worry about the light coming on in the first place!
Ok, enough negative-Nelly stuff. Hopefully all the replacement prop/combo valves are more cooperative.
One wire goes to the dashboard 'brake' warning light.
One wire goes to the ignition switch 'prove' terminal, that grounds when the switch is only in the start position.
Inside the actual switch at the connector, both terminals are bridged together.
As designed:
- During starting, the brake light illuminates on the dash- being grounded by the ignition switch, proving the switch is connected and the system is working.
- If there is a brake hydraulic problem, the brake differential valve moves to either side, grounding the center pin, illuminating the brake warning light.
- If the switch becomes disconnected or there is a problem with either wire, the brake warning light on the dash will not illuminate while starting- indicating a problem with the circuit.
This might be a good question to ask you knightfire.
Whether a full combination valve, or just the simple differential valve shown in your pictures, does the shuttle actually block the flow of brake fluid from the affected side? Some people say it does, others say not.
The first time I ever saw one I thought it would be used to block flow to stem the leak, but have for many years now been on the side of it does not block flow (or at least not fully) when it shifts.
If you know for sure one way or the other, please share.
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