System Diagnosis

V8-379 DIESEL ENGINES
For Diagnosis on 1984-87 California engines and all 1988 engines, refer to ``Diesel Electronic Control (DEC) System.''
1. Start engine, run until it reaches normal operating temperature and ensure that curb idle speed is within specifications.
2. Remove air cleaner and observe EGR valve. Valve head should be up (valve fully open) and exhaust noise in intake should be noticeable.
3. Disconnect vacuum hose from EGR valve. Valve head should fall and exhaust noise should be noticeably reduced.
4. Connect suitable vacuum gauge to EGR valve supply hose. At idle gauge should read approximately 20 inches Hg.
5. If specified vacuum is available to EGR valve, but valve fails to perform as outlined in steps 2 and 3, repair or replace EGR valve.
6. If vacuum available to EGR valve is less than 20 inches Hg, ensure that vacuum hoses are properly routed and free from cracks or breaks, then check vacuum at pump.
7. If output at vacuum pump is less than 20 inches Hg, repair or replace pump as needed.
8. Reconnect vacuum hoses to pump and EGR valve, then operate injection pump throttle lever through approximately 20° of travel.
9. EGR valve should remain open until throttle passes calibration point of TPS switch, then valve should close.
10. If valve fails to perform as outlined in step 9, stop engine and test control circuit.
11. With engine stopped and ignition switch in on position check for battery voltage at pink wire terminal of TPS switch. If battery voltage is not available at pink wire terminal, repair system feed circuit as needed.
12. Check for battery voltage at TPS switch blue and yellow wire terminals with throttle lever at idle position and while moving throttle through approximately 20° of its travel.
13. TPS switch should operate as follows:
a. At idle battery voltage should be present at blue wire terminal, but no voltage should be present at yellow wire.
b. When throttle is moved approximately 15°, no voltage should be present at blue wire terminal.
c. When throttle is moved approximately 20°, battery voltage should be present at yellow wire terminal.
d. If TPS switch fails to perform as outlined, switch is defective.
14. If TPS switch operates satisfactorily, check continuity of wiring between TPS switch and EPR/EGR solenoids.
15. If wiring is satisfactory, but EGR valve still does not operate as outlined in step 9, test solenoid valve assembly and replace as needed.