what causes the A/C only to blow throught the defroster
vents when the C/C is on??
The first thought is they are both vacuum controlled, and since it acts up when you have the cruise on, I'd be checking for a vacuum leak in that system. One possiblity would be the vacuum check valve right at the intake manifold.
The second possiblity is that they are both electronic. The AC and cruise systems on Dodge trucks do have one thing in common, it's a ground connection on the RH inner fender. Although the common complaint is a loss of AC (no clutch operation) and a loss of cruise control.
thanks so much where about is the ground in the fender well? i already replaced the chech valve to no avail
John G. has given a good hint: The problem may be electrical or vacuum, or both. The vacuum actuator for the cruise control may be leaking. A quick check: Remove the vacuum line at the actuator and plug the line, then take a test drive. Turn on the cruise control (the cruise control is disabled) and examime if there is any effect on the blend doors in the AC unit. If not, the cruise contol actuator is highly suspect.
If any vacuum lines in the cruise control or AC vacuum circuits have been distrubed recently, check these over carefully. Several of the vacuum line fittings have "ports" built in, i.e., calibrated orifices that limit air flow. Make sure none of these have gotten mixed up, or were replaced with ordinary full bore fittings.
One of these orifices is there to prevent a large vacuum loss in the intake manifold in the event of a vacuum line or component failure which affect engine running and loss of power braking. That orifice will be the circuit between the vacuum accumulator and the intake manifold or throttle assembly. A leaking accumulator can adversely affect vacuum controls.