Service Engine Soon Light is On

The illumination of the Service Engine Soon light indicates that there is a malfunction in the solenoid, control module, control relay or wiring. Before diagnosis, check the solenoid and module connectors for proper connection.

1. This step is to determine if there is power to the Air Temperature Switch.
2. This step will insure that there is power to the circuit by eliminating the Air Temperature Switch. This step will also check the wiring between the Air Temperature Switch and the solenoids.
3. This step will check the wiring between the solenoids and ground including the circuit inside the Control Module.
4. This step will determine if the problem is a short to ground in the "Service Engine Soon" light circuit or a malfunction in the Air Management System.
5. This step checks for Bat(+) at "A" and a ground at "B", to power up the Control Module.
6. This step will determine if a ground signal is sent to the relay because CKT 919 is grounded outside the module.
7. This step checks for a Tach signal to the Control Module.

Fig 1: Chart C-6HD-2, Air Management System Schematic (7.4L)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Flow Chart C-6HD-2, "Service Engine Soon" Light "On" At All Times (7.4L-Carbureted)

Courtesy of GENERAL MOTORS CORP.

Fig 3: Chart C-6HD-2, SES Light Always On (7.4L-Carbureted) (1 Of 2)

Fig 4: Chart C-6HD-2, SES Light Always On (7.4L-Carbureted) (2 Of 2)