I have a 1975 F-250 and I need help with the wiring for the low brake switch. I've recently replaced it because the old one was worn out and no wires were connected to the switch but a cheap alligator clip thing that was just draining my battery and had no purpose.

So I properly but the two wires on the low brake switch soon to find out when I press on the brake the brake light comes on....but so does the dash and parking lights. If anyone has any advice it would be much appreciated the wires that are on the switch are black and red.

 

Which dash lights? Maybe the brake lights are finding their ground path through the dash and parking lights. Check your lighting grounds. There is one at each corner of the truck. Fronts should be visible under the hood, and the rears are behind the tail light 'buckets.'

 

Sorry I meant the instrument cluster lights
Here are the grounds behind the taillights


Two up front

 

How about the ground from the engine to the cab? It's between a bolt on the rear of the engine, and a bolt on the firewall. Those can cause weird problems if missing.
If the wiring is all good, I might start to suspect the turn signal switch.

 

Ive actually just replaced it a few weeks ago but I'll check to make sure it is on tight. It could have something to do with the turn signals but were should I start if it is the turn signals?

 

If your new replacement ground got bolted up against new paint on the engine or firewall, you'll want to take an Exacto knife or something, and scrape down to bare metal where the ring terminal goes.

Not sure how to troubleshoot the switch, but google might uncover something, or hopefully someone here will have some idears.

I did once put a jumper wire somewhere in the TS switch of my old '68 Cougar, in order to restore function to my brake lights which had stopped working. This was before I had the internet, but I believe that I probably consulted the wiring diagram in my Haynes manual.

 

No such thing as a low brake switch. This is probably what you're referring to:

C8AZ-2B264-A .. Brake Pressure Differential Valve Warning Lamp Switch.

1968/69 FoMoCo Passenger Cars; 1968/79 F100/350, Bronco & Econoline.

This switch has nothing to do with the brake (stop) lamps, head lamps, tail lamps, parking lamps or marker lamps.

This switch threads into the Brake Pressure Differential (aka proportioning) Valve. It's also called the low brake fluid warning lamp switch.

This switch operates the low brake fluid warning lamp on the dash.

 

This is what I am talking about

 

I've never seen any switch like this that is described as a "low brake switch."

It's hard to tell from this pic, where is it located?

 

The switch sticking up out of the '67-'72 pressure differential valve or, on the '73-later disc/drum brake valve is a pressure switch. Brake systems didn't get fluid level switches in them until the late 80's/early '90s, and they were located inside the brake MC fluid reservoir.

Later model Ford MC (For a Ford Ranger) with a fluid level warning switch --not the same thing as a pressure differential switch located on the brake valve.



The pressure differential valve spool inside the brake valve assembly will remain centered as long as there is equal pressure acting on the annular ends of the valve spool from both the primary and secondary brake circuits. If there is a pressure loss in the brake system due to a failed caliper/wheel cylinder seal, failed MC seal or a ruptured hard line or flexible brake line, the system pressure will become unbalanced. --system pressure imbalance and the brake warning light coming can also be the result from bleeding the brakes out, after they have been serviced.

The higher pressure exerted by the good circuit will push on the end of the pressure differential valve spool and force it over to the low pressure side (the failed circuit side). The pin on the warning switch will ride up a 'ramp' on the PDV spool. This pushes the switch pin in which closes the switch contacts. This completes an electrical ground path through the body of the brake valve, through the chassis and illuminates the brake warning light on the dash.

I don't know what GVW the F250 in question is but, it could have either a cast iron Kelsey-Hayes disc/drum brake valve (if the truck is 2WD and has single piston front calipers) or, it could have a brass Weatherhead valve (if the truck is 2WD and has twin piston front calipers).

Whatever the case, there are 3 internal functions inside these brake valve bodies: Metering (hold-off) to the front discs, Pressure Differential sensing between the primary and secondary brake circuits and Proportioning to the rear drum brake circuit.

The following diagram shows the internal workings of a PDV for a Bumpside truck. The pressure differential valve spool/switch function inside of a Dentside brake valve works exactly the same as what's inside of this Bumpside PDV. --the Dentside brake valve just has two additional functions (Metering & Proportioning) that the Bumpside valve doesn't have.



The brake valve and the pressure warning switch should have nothing to do with any of the lights on the truck, other than the brake warning light on the dash. Sounds like there are some problematic wiring issues going on and sounds like the PDV spool inside the valve may be shifted, allowing a ground path through it to turn on lights that shouldn't be coming on. If the PDV spool is centered, there should be no completed ground path through the valve. If centered, it should be an open circuit.

Cast iron Kelsey-Hayes disc/drum brake valve for single piston front calipers.



Brass Weatherhead disc/drum brake valve for dual piston front calipers.

 

That's the brake light switch on the brake pedal. --completely different from the pressure differential switch on the brake valve.

 

One thing for certain is...this is not a "low brake switch!"

Brake (stop) lamp switch shown as 13480 in pic:

 

I know this is a very old discussion, but as an FYI, in the old Ford wiring diagram for my '79 E-150 it's labeled as "Low Brake Switch"