Recirculated/Fresh Air Operation in '88-'90 C/K Vehicles

[stockypoo]

Hello,

I wanted to add a writeup on the operation of the recirculate/fresh air door operation for 88 - 90 C/K models. This info only pertains to vehicles that do not have a 'Max' button on the electronic climate control unit.

I searched for this information on this and other forums and there seemed to be much confusion over the operation of these style units. The common misconception is that recirculated air is only activated when the fan is on Hi or Med with temp control full cold. If your system is stock and includes the high pressure sensor (actually called the Air Inlet Valve Motor Pressure Switch (AIVMPS)) on the back of the AC compressor, then the operation is as follows from the 1990 Electrical Wiring Manual:

"The Air Inlet Valve is normally in the position which allows outside air to enter the system. In this position, the Air Inlet Valve Potentiometer returns about 4.9 volts to the Heater and A/C Controller. When the A/C Button is depressed, and if the temperature setting is at full Cold and the blower is set to MED or HI, the Air Inlet Valve Motor will move the valve to the recirculating position 30 seconds after the Air Inlet Valve Motor Pressure Switch closes. The Air Inlet Valve is opened again, 5 minutes after the Air Inlet Valve Motor Pressure Switch opens. It opens immediately if the temperature setting is raised, the blower speed is reduced to Low, or the air flow is directed to the floor outlets"

Details on the AIVMPS are as follows: Closes at 210 - 255 PSI, Opens Above 165 PSI, Minimum pressure differential of 45 PSI. This is a single wire unit, so when the switch is closed at 210-255 ground is sensed on input 157 in the Climate Control head.

If you add a new compressor and do not install the old AIVMPS sensor, then recirculate mode will never be activated. To bypass this, I simply cut the vehicle side connector on the AIVMPS pigtail, added a terminal, and grounded it to the engine block. Now, recirc mode is enabled 30 seconds after A/C is turned on Hi/Med with the temp control on full cold. If I ever want fresh air on Hi or Med fan speed, I move the temp control one notch in the warm and fresh air is activated.

I also found it interesting that the AIVMPS is not a high pressure cutoff switch (HPCO) for the compressor. As far as I can tell from limited understanding of wiring diagrams, the AIVMPS does not have any input to the compressor clutch to prevent overpressure.

One last note is that the fresh air door actuator on 88 - 90 C/K units has a 5 wire input rather than the 2 wire seen on '91 and later

Thanks for reading and I hope this helps someone