On my 82 F150 I have this switch that appears to be like a brake light switch but has a huge factory vacuum hose that goes to it.

I am curious what this switch is and what does it do?

I looked on alldata for 1982 but it has no vacuum diagrams to find out what this is.

I am asking cause I have a vacuum leak in my brake booster and I will be replacing that with the hopes of fixing my stumble on acceleration but if this is a vacuum leak as well I want to fix this too.

 

I can only think of one other area which is connected to a "huge" vacuum hose [3/8"?] and that is the PCV valve.

Is the "switch" you are describing near the brake booster or the engine?

 

The only trucks I have seen with vacuum/electrical switches on them are the diesel equipped trucks. It's mounted on the pass side inner fender.

 

What comes to my mind is something under the dash that's part of the speed control.

 

At first glance like op says, it appears to be a normal brake light switch, just vacuum operated. However I think the brake light switch has a couple wires going to it and it is oriented about 90 degrees from this vacuum switch.

What ctubutis said makes sense though, probably cruise control. The line goes through the firewall and to the vacuum "manifold" on the firewall behind the passenger valve cover iirc. I was getting a major hiss in that general area constantly, but much worse when I put on the brakes. Replaced the line and no more hissing.

A pic of the switch in question is below.

 

The photo above is the switch I am asking about. This one on mine hisses till you press the brake pedal then the hissing goes away. It doesnt always do it but it always does when the engine is switched off.

I asked my boss at work and he said that was for cruise control that when you hit the brakes it deactivated the vacuum servo. My only guess is, upon hitting the brakes it creates a vacuum leak in the cruise control servo to release it.

How ever it should not be hissing all the time and now I need to figure out if replacing just the switch shown in the picture will stop the hissing or if I have to replace something under the hood that is sending vacuum to this switch when cruise control isnt even activated.

I am thinking this coupled with my leaking brake booster could be causing enough of a leak that the signal at the carb is not correct and is resulting in the truck stumbling when you really step down on the throttle.

After 5 carbs being put on the engine since 1994 and every carb resulted in the same stumble out of the box reguardless of tuning. I am starting to look else where. First item on list is to fix theses vacuum leaks. Then I will move on to ignition if that doesnt solve the problem.

 

Yes I agree, the switch in the picture is for the cruise. If you follow it out it should go to the servo for the cruise unless the lines have gotten mixed up.

If you have the cruise control on, and then hit the off button on the steering wheel to de-activate it, you will find the truck slowly loses speed. This is because you electrically told the cruise brain to turn off, but the servo has to bleed the vacuum down.

If you are in a situation where you suddenly hit the brakes while using the cruise, the brake pedal has a electrical switch to tell the cruise to turn off, but the switch in the picture above is a vacuum "dump valve" and it will quickly bleed the vacuum from the servo into the atmosphere under the dash, making the truck stop quicker.

If it's hissing, I doubt your cruise will work very well, if it does at all. I am not sure why it's hissing though if the cruise is not activated, unless during a "get rid of these vacuum lines" spree, it got hooked up to the wrong place.

 

The path for that hose is manifold vacuum (metal tube at back of engine) to firewall manifold/ports to switch so nothing besides the switch regulates vacuum. Just remember to change the hose whilst you are in there.

Oh and one thing that made a medium sized stumble on mine was indeed a vacuum leak. The caps that plugged into the charcoal canisters by/beneath the battery had popped off. Pushed them back on, stumble home.

 

I don't think that is correct. I believe this hose goes to the servo, not manifold vacuum. See my post above.

 

If that's true franklin then my hoses are still messed up. Lol. I'll need to dig into the manuals to find out where the lines should be.

 

I never tried the cruise control on the truck. But I know when there is a hissing that is coming from this valve it is while driving with the pedal not being applied. Applying the pedal stops the vacuum hissing. This has always been like this so I might have to track the hose down. I dont think this one is plugged into the plastic 3 port T on the passenger side. Those are all plain black hoses that I have replaced as they get limp as a boiled noodle and they fall off. This one still has the red factory stripe on it just like my 78 Mercury which the Red stripe like this was used for the cruise control vacuum motor. I will check into this in the morning when I am replacing the brake booster.

Might need to invest in a vacuum diagram for this truck like I did for my mercury.

I also wish I didnt have to put the booster on just yet cause id like to paint this bare metal booster underhood black but I doubt Eastwood could get the paint to me by tomorrow if I order it tonight lol.

 

I checked all the hoses today, they are all hooked up as they should be. The vacuum leak hissing that I heard was obviously coming from the brake booster itself. Makes sense now that I think of it since applying the brakes resulted in the hissing getting quieter as well as slower.

Thats one problem resolved, engine still does the bog but its not wanting to drop dead now like it was doing every time. I need to reset the carb as the engine wont take throttle now when you try to rev it. I will do this after I replace the leaking vacuum modulator on monday. I will also be double checking the timing as well to verify its not a combination of timing carb out of adjustment now.

 

I know this is really old but my 92 7.3 idi has hissing from that same area only mine does it when I push the brake pedal and it will hiss the entire time I hold the brakes... also my brakes dont work well .. they will stop good the first press of the pedal then it gets harder.. I think its losing vacuum... I replaced master cylinder and bled brakes... does this sound like a brake booster problem? I was going to replace that next but dont want to spend another $100 if that's not the fix... thanks

 

It sure sound like a bad booster.
A hard pedal is lack of vacuum boost just like of the engine was off you pressed the pedal a bunch of time to use all the vacuum boost up.
Dave ----

 

I agree, booster also. And since you have a diesel, your engine does not have vacuum, so they use a belt driven vacuum pump. This system works ok but can quickly be overwhelmed, dropping vacuum levels. Even when everything is working like it should, you can pump the pedal a few times and tax the system out, getting a hard pedal.