Part 1 of 2

Chart C-7, EGR System Check (1 Of 2):

EGR Control System Circuit Diagram:

CIRCUIT DESCRIPTION
The linear EGR valve is an electronically controlled emission control device. It meters small amounts of exhaust gas into the intake system when the engine is under load, to help reduce peak combustion temperatures, thereby reducing the formation of oxides of nitrogen emissions (NOx.) The valve operates on a duty cycle signal ("ON" time vs "OFF" time). The solenoid assembly inside the valve gets battery power when the ignition is turned "ON" and becomes energized when the signal modifier completes the circuit to ground. The ECM outputs a varying frequency signal to the EGR valve signal modifier module, which varies the pulse width (duty cycle) of the signal to the EGR valve solenoid to raise and lower the pintle. The signal modifier generates the duty cycle signal by alternately grounding and opening the EGR valve solenoid circuit. The longer the pulse width, the farther the valve opens and the more exhaust is metered into the intake. A sensor is mounted on top of the valve to determine the pintle position for self diagnostics.

TEST DESCRIPTION
Numbers below refer to circled numbers on diagnostic chart.
1. Actual EGR valve position should be 0%. If the EGR valve was stuck open, engine would idle rough. Monitoring long term fuel trim can help identify a partially open EGR valve.
2. To gain access to the EGR connector, the right side of the engine should be raised slightly.
3. The EGR valve signal modifier is mounted to the underside of the intake plenum. The Plenum must be removed to gain access to the connector for testing.
4. The signal from the ECM to the EGR valve signal modifier is 128Hz when the EGR valve is commanded to 100% duty cycle.

DIAGNOSTIC AIDS
The ECM monitors short term fuel trim during a code 32 test and must detect a change to pass the test. If short term fuel trim is at a fixed value (lean or rich) when a code 32 test is run, a code 32 may set.