hey guys I've got a 98 ranger 4wd 4.0 auto. the problem i'm having is when I switch the a/c to max it switches between vent and defrost still blows cold just switches between the two not sure if this is a switch problem or a vaccum problem any help would be appreciated

 

Do you mean vent defrost vent defrost vent defrost?

 

When air blows out of the defrost registers when it's not supposed to, then there is a vacuum leak somewhere in the HVAC air distribution system.

See: Previous FTE Post and refer to the linked HVAC diagram.

On your '98, when MAX A/C is selected:

- The air inlet duct door is at full vacuum, closing off outside air and admitting only recirculated air.
- The heater water control valve is at full vacuum, preventing hot coolant from reaching the heater core.
- The panel/defrost door is at full vacuum and the floor/panel door is at no vacuum, directing airflow to the A/C registers.
- Temperature control is usually set for maximum cold but because the heater water control valve is preventing hot coolant from reaching the heater core, the air cannot be heated in any temperature control position.
- Air will be picked up at the recirc opening by the blower motor. With the temperature control set for maximum cold, airflow across the A/C evaporator core will be diverted past the heater core and then directed into the passenger compartment through the instrument panel A/C registers. There is also some airflow to the side window demisters.
- The A/C compressor will be enabled when MAX A/C is selected.
- The blower motor is on.

 

max a/c blows through dash vents until rpms go up then it switches to defrost vent but still blows cold thinking maybe a vacuum leak but i'm puzzled as to why it does it only in max a/c i'v been told on these trucks that max a/c is recirculated air is there a separate vacuum line for only max a/c

 

The vacuum lines & motor that move the normal/max flap or the heater control valve may have just enough leakage that there is not enough vacuum to hold the panel/defrost door in position. It is also possible the heater control panel may be losing vacuum in certain positions also.
tom

 

pulled the a/c controls yesterday switched to max a/c with the truck running and could hear a vacuum leak at the switch local for dealer says there is a diaphragm inside the switch that can rupture went ahead and ordered new switch about 30 bucks should be in tomorrow I will post results this weekend

 

after changing the a/c switch I still had the same problem finally tracked down a small hole in the vacuum line going to the heat exchange valve on the heater hoses under the hood fixed that and everythings working again

 

Good to hear you found the vacuum leak and fixed the problem.