Brake light came on, no reason

[madpogue]

Originally Posted by Warrennc

Anyone have any ideas what symptoms would occur with low or no vacuum?
Just so I can compare to what it is doing now. Crossin my finners that it aint the vacuum system.

Hard pedal, HVAC control malfunctions, poss. noise (chirp, squeak, hiss) from the vacuum pump.

[Warrennc]

Thanks. None of those symptoms exist so...must be the switch or wiring.
Much releaved.

[Patrick Feeley]

With low vacuum, the brakes generally get very stiff. If you put your thumb on the end of the hose off the vacuum pump, it should suck on it pretty hard at idle. Cheers!

[Warrennc]

Thanks everyone. The shop called and they have an opening for Friday. Gonna get a few things fixed. Fuel leak and block warmer first. Since he got an opening before I could check on the switch and wiring, I think I will just add that to his list.
This mechanic is worth waiting for. He is good and inexpensive. Doesn't fix things that don't need fixin.

[vandy]

let me know. ive got about the same issue. basically when my truck is parked ill start it up, disengage the parking brake. light stays on. about 10 seconds later it will go out. i think its a vac. issue. havnt had time to start looking into it yet

[Warrennc]

Mine definetly is not the same symptoms as yours. Mine stays on period, won't go off at all. Brakes work just fine. Definetly not lack of vac there.
I will let you know what he finds out about it.
Yours may be a leaking vac line so check all your rubber and hard plastic lines for deterioration. If they look bad or the black from the rubber ones rubs off on your hands...replace the line.
I probably should do that on mine, rubber lines and hoses only lasts about 10 to 15 years on any vehicle. Went through that on my old 89 suburban (gasser). Had to replace just about Every vac line it had. I just know on more modern vehicles, the vac system can be complex and me not knowing how the vac system on a diesel lays out........

[madpogue]

Vacuum on these trucks is very simple. There is no "vacuum system on a diesel". A diesel engine produces no vacuum. To deal with this, Ford could have used hydroboost (hydraulic fluid from the power steering pump) for power brakes, and simple electrical or cable-based mechanical HVAC controls. But given Ford's predilection for hanging onto archaic technology, they added a separate vacuum _pump_, just so they could keep the old-school vacuum brake boost AND HVAC controls. So all we have is a pump, a distribution box, the brake booster and the HVAC box. Oh, and there's a sensor on the distribution box; if it senses inadequate vacuum, it turns on the BRAKE(!) light, which means you still have solid brakes, but it'll take monster pedal effort to use them.

[Warrennc]

Wow, archaic is right. At least it is fairly simple. To bad it aint so simple or cheep to replace it all with something better.

Thanks. I'm learning something every day.

[vandy]

I haven't noticed any change in my brake pedal.

[Warrennc]

The truck is due to go in this morning to the shop.
Won't know anything till this afternoon or evening...I hope it is at least that early and not like...Monday. Got a few minor things I want to do to the truck.
Things I can do my self. Or at least give it a good try.

I will let yall know what he comes up with about that brake light on the dash for sure.