1984-85

Fig. 45 Low altitude EGR vacuum control system schematic. 1984-85 V6 & V8 diesel engines exc. front wheel drive models & models w/DEC system:

Fig. 46 High altitude EGR vacuum control system schematic. 1984-85 V6 & V8 diesel engines exc. front wheel drive w/V6-262/4.3L & models w/DEC system:

The EGR vacuum control circuit on these engines includes an injection pump mounted Vacuum Regulator Valve (VRV), TCC vacuum cut solenoid, Quick Response Valve (EGR-QRV), Thermal Vacuum Switch (EGR-TVS), and an altitude compensation circuit,Figs. 45 and 46. The VRV and TCC solenoid provide high vacuum at idle and decreasing operating vacuum as the throttle lever angles increase, and cutting-off vacuum to the EGR valve when the converter clutch is engaged. The QRV allows the EGR valve to respond rapidly to decreeasing vacuum by venting vacuum trapped in the EGR valve diaphragm chamber when vacuum input to the QRV decreases. The EGR-TVS operates in a conventional manner, venting EGR valve operating vacuum when coolant temperature is below switch calibration temperature. The altitude compensation circuit uses an altitude sensitive switch to operate an Altitude Trim Solenoid (ATS) and an Altitude Vacuum Reducer Valve (AVRV). The components in the altitude compensation circuit allow vehicles calibrated for low altitude operation to meet emission standards when operated at high altitudes and vehicles calibrated for high altitude operation to meet emission standards when operated at low altitudes.
On vehicles calibrated for low altitude operation, vacuum signals transmitted through the EGR-TVS are applied to port 3 of the ATS and port 3 of the AVRV. During low altitude operation, the ATS is not energized and vacuum is transmitted through the ATS to AVRS port 2. Vacuum at AVRV ports 2 and 3 is equal and full output from the VRV is applied to the EGR valve through the QRV. At altitudes above 4000 feet the ATS is energized and vacuum applied to ATS port 3 is vented to atmosphere. Vacuum applied to AVRV port 3 is greater than vacuum at port 2, causing the vacuum applied to the AVRV to be routed through the valve's reducing circuit, and vacuum applied to the EGR valve is reduced by 2-2.5 inches Hg.
On vehicles calibrated for high altitude operation, vacuum signals transmitted through the EGR-TVS are applied to port 1 of the ATS and port 3 of the AVRV. During high altitude operation, the ATS is not energized and vacuum applied to port 1 is vented. Vacuum applied to port 3 of the AVRV is greater than vacuum applied to port 2 and is routed through the valves reducing circuit, reducing vacuum applied to the EGR valve by 2-2.5 inches Hg. At altitudes below 4000 feet, the ATS is energized, the ATS vent is closed and vacuum is transmitted through the ATS to port 2 of the AVRV. Vacuum at ports 2 and 3 of the AVRV is then equal, the reducing circuit is closed and full VRV vacuum is applied to the EGR valve through the QRV.