Symptom Related Diagnostic Procedures

On cruise control systems that use a separate vacuum pump, ensure that adequate vacuum is present at pump before attempting to diagnose system.

Fig. 1 Cruise control transducer with resume feature:

CRUISE CONTROL INOPERATIVE
1. Check servo chain or rod adjustment. Adjust as necessary.
2. Check vacuum hose for leaks or restrictions. Repair or replace as necessary.
3. Check drive cables for proper routing, kinks or sharp bends. Repair or replace as necessary.
4. Check throttle linkage for binding or improper adjustment. Repair binding or adjust throttle linkage.
5. Check adjustment of brake pedal release switches. Adjust if necessary.
6. Check engagement switch operation. Unplug connector at bottom of steering column and plug in known good engagement switch assembly. If system operates, replace engagement switch assembly.
7. Check fuse. If blown, replace. If fuse blows again, check for short circuit or short in resume solenoid diode.
8. Start engine and check resume solenoid source vacuum, Fig. 1. Remove two wire connector from resume solenoid. Connect ground wire to terminal that black wire was connected to and supply 12 volts to terminal that had other wire connected to it. Disconnect output vacuum hose from transducer. Vacuum should be present. Replace solenoid if vacuum is not present.
9. Place ignition switch in run position and control switch in ON position and check engage/hold wire harness. Disconnect two wire connector at transducer. Push engage/hold button in part way to engage position and check each terminal with a 12 volt test light.
a. If test light is on at only one terminal, test engagement switch. Check for open circuit in wire. Repair or replace part at fault.
b. If test lights at both wires, check transducer for poor ground. If ground is good, replace transducer.
c. If test light is off at both circuits, check for open in brown/white wire from fuse box to On/Off/Resume switch. If circuit is good, check engagement switch operation.




SPEED INCREASES AFTER ENGAGEMENT
1. Speedometer cable or housing bent, kinked or improperly routed. Repair or replace as necessary.
2. Transducer orifice improperly adjusted. Adjust orifice tube.
3. Transducer inoperative. Replace transducer.

SPEED DECREASES AFTER ENGAGEMENT
1. Throttle linkage not tight enough. Adjust throttle linkage.
2. Vacuum leak or restriction in vacuum hoses. Check for damaged, kinked, disconnected or worn hoses. Replace or repair as necessary.
3. Transducer orifice improperly adjusted. Adjust orifice tube.

Fig. 2 Aspirator vacuum assist cruise control schematic:

CRUISE CONTROL OPERATES ERRATICALLY
1. Vacuum line from source to transducer restricted. Remove restriction.
2. Check servo unit for full stroke. Replace if necessary.
3. Transducer inoperative. Replace transducer.
4. On systems with aspirator vacuum assist, check valve is stuck open or is missing. Install or replace check valve. Refer to Fig. 2.

EXCESSIVE LOSS OF SPEED ON INCLINES
1. Loose throttle linkage. Adjust throttle linkage.
2. Vacuum hose from source to transducer restricted. Remove restriction or replace hose.
3. On systems with aspirator vacuum assist, check valve is stuck open or installed backwards or cruise system to aspirator hose is not connected. Replace check valve or install properly. Attach aspirator hose. Refer to Fig. 2.