When i accelerate past 50 mph, or register greater than 2750rpm, the climate system automatically goes to windshield defrost, no matter what position the control dial is placed on. When i decelerate under 50mph/2750rpm, i regain vent control. Mods are: 4inch siuspension lift, 33x12.50x15 tires, and 1inch throttle body spacer.
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. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ? Air Flow Control
Air flow control is accomplished in the following manner:
Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
The position of the function selector knob determines the manner in which the system will operate.
Each position of the function selector knob is detented for positive engagement.
Vacuum Control Motors
The vacuum control motors (18A318):
are located on the A/C evaporator housing, A/C recirculation air duct and heater air plenum chamber (18471).
direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).
run the fault codes first to check for faulty sensors
what you describe indicates a problem in the exhaust in that when the exhaust gas volume reaches a certain level the sensors are reporting it and the ECM is not able to fix it
have an accredited exhaust shop check the cat converter
for failure and partially blocking the exhaust at speed
if it was a fuel pump you would run into problems long before the speed at which it occurs
AT best it could be an EVAP problem with the vent not working properly but I think that it is exhaust related
This engine is fully electronically controlled, so you need the dealer to do a full diagnostic of the engine control system, it is the only way to get to the bottom of this. Make sure you can show the dealer exactly what is going on otherwise if they can't duplicate the concern and no fault codes are present they can't fix it.
You have a vacuum leak.Defrost is the default position (no vacuum).
You have a vacuum reservoir that mounts behind the battery. This is for the reserve vacuum to the climate control while you are accelerating,since while accelerating you loose manifold vacuum.
You will see a vacuum line that goes from here to the intake manifold then one that goes from here to the fire wall near the heater coolant hoses and into the car to the controls. Check this carefully and you should find a leak some where in the lines or the reservoir.