DTC 68

Cruise Servo Circuit Diagram:

CIRCUIT DESCRIPTION - SOLENOID CIRCUITS
The cruise servo vent and vac solenoids are output lines and are controlled by high side drivers in the PCM (in this case, the circuit is grounded on one side and the "high side drivers" in the PCM provide power.) When the ignition is "ON", the PCM looks at these output lines and will set codes depending on the status of each line.

CIRCUIT DESCRIPTION - SERVO POSITION SENSOR (SPS)
The PCM supplies a 5 volt reference signal to the servo position sensor through circuit 398, and then monitors the signal return through circuit 399. Depending on the actual servo position, the signal on CKT 399 will vary, indicating the actual servo position. The PCM uses this information to control the servo position when cruise is engaged.

CODE 68 DESCRIPTION
The un-intregrated PCM controlled cruise control system is designed to monitor itself to be sure that the desired cruise servo position and actual cruise servo position are equal. Code 68 will set if the actual SPS position is greater that 15% above desired SPS position of 0% for 1.5 seconds.

If a code 68 is stored, the vehicle will operate in "Power Management Mode". While in this mode, the PCM will shut off fuel to 3 cylinders, to avoid over-revving the engine in case the throttle is stuck open due to a sticking cruise control cable, brake vacuum dump valve not venting servo, or failed vent solenoid not venting servo.

TEST DESCRIPTION

Code 68 Diagnostic Chart:

Numbers below refer to circled numbers on the diagnostic chart.

1. Most VAC solenoids leak a small amount of vacuum when closed. But they should not leak enough to allow the servo to reach WOT in 15 seconds with the vent closed.
2. When the VAC solenoid is forced "ON", the vacuum will be vented to the atmosphere unless the VENT valve is stuck closed.


DIAGNOSTIC AIDS:

1. A cruise throttle cable that is sticking or binding can cause a code 68 to set. Also, code 68 can be set if the cruise servo is compressed manually while the cruise switch is "ON".
2. Outside interference such as CB antenna lead near the PCM wiring harness may cause a false SPS reading and set a false code 68.
3. An intermittent may be caused by a poor connection, worn through wire insulation, or a wire broken inside the insulation. Check for:
^ Poor connections or damaged harness. Inspect harness connectors for backed out terminals, improper mating, broken locks, bad terminals, poor terminal to wire connections, and damaged harness.