Modulated (Internal) EGR System (Federal Vehicles)
EGR valve opening rate is modulated by a signal from the Vacuum Regulator Valve (VRV) mounted on the diesel fuel injection pump. See Fig 1. EGR valve opening is regulated from fully open at idle to fully closed at wide open throttle. EGR is reduced when the TCC is engaged, the reduction being controlled by the EGR control assembly.
The control assembly is located between the VRV and the EGR valves. When the TCC is disengaged, the solenoid of the EGR control assembly is de-energized and VRV regulated vacuum is directed to the EGR valve. When the TCC is engaged, an electrical signal energizes the solenoid in the control assembly, allowing the vent to open. The reducer valve portion of the control assembly then reduces VRV regulated vacuum (otherwise equal to EGR valve vacuum) by 2.5" Hg.
Normally open during driving, the EPR valve is closed at idle to increase exhaust pressure. This increases EGR flow to the engine. Above idle, the EPR valve opens to provide normal EGR level for driving.
Throttle position is sensed by the VRV on the injection pump. The VRV then sends a vacuum signal proportional to throttle travel. When VRV regulated vacuum is above 12 in. Hg, the EPR switch closes the circuit. This energizes the solenoid, allowing full vacuum pump vacuum to be directed to the EPR valve, closing it.
The closed EPR valve results in additional exhaust back pressure, which boosts EGR for reduced emissions. When vacuum is below 12 in. Hg, the switch opens and vacuum to the solenoid is blocked. The EPR valve is vented at the solenoid portion of the EPR switch, opening the EPR valve.