I have a 98 explorer sport, 4.0 6cl sohc. I've been having issues with the A/C and heater. The problem is that the vents are not opening properly. When I turn the knob to select either heat or ac the air only comes out of the defog/defrost vent on dash. It started last summer but I didn't worry about it too much because the air still came out cold. I did however notice that once it started to get cold this winter, the compressor still kicked on when I was using the heater. I finally started messing with it to see if I could fix it. Not knowing anything about the system I just changed out the vacuum switch to see if that's why the vents were not opening and closing the way they're supposed to be. But that didn't work. It's warming up again here in AZ and now the air only comes out hot. Can anyone help me?
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Re: A/C and Heater vents.
You have a vacuum leak. The default postion(no vacuum) is defrost and heat. Check the small black vacuum line coming out of the firewall on the right side near the ac evap and heater core. Trace it back to intake and make sure it has vacuum there and there are no leaks. You should also see a vauum canister when u trace it back,this is the reserve so u have enough vacuum to keep it from changing from floor to def when accelerating.
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when you operates the heater button worm water of your engine's water jackets enter to the evaporator locates inside your A/C blower & air flows through it & air becoming worm. Therefore I proposed you to check your vent linkages for vent doors inside the blower unit.
What is a CO sensor? Never heard of it. a 1999 OBD2 ford has approximately 1600 possible fault codes most of which can turn on the light. If you dont know which one is turning it on, you are guessing randomly which is expensive and usually unsuccessful. get the code(s) and then diagnose the possible causes. The vehicle you list (99 4L explorer) rarely had injector problems. Much more common were EGR sensor(DPFE) sticking at voltage, evap problems, misfires, and lean conditions due to dirty MAF, or vacuum leaks. get the fault code, or you will not fix the problem.
that switch controls a vacuum operated door that lets a/c or heat thru.when accelerating,the vacuum in the engine changes so it sound like a vacuum issue.take apart the center stack and there is a vacuum actuator in there connected to the flap door .i have seen those have issues with the metal tab broken away from the plastic that moves it.is the a/c piping cold under the hood where it goes into the heater a/c assembly?the compressor should be cycling on and off and the pressure tested.
i had this exact same problem with my 98 explorer. your blend actuator (blend doors) are not morking. these are on the passenger side under the dash. getting to them will be dificult, but it can be done. you will need to take apart the heater box, to fix them. (the big black box under the dash).