Difference in Set Speed and Actual Speed and/or Set Speed Hunts

LARGE DIFFERENCE BETWEEN SET SPEED AND ACTUAL VEHICLE SPEED AND/OR SET SPEED HUNTS

System Description
ASCD allows the driver to maintain speeds between 40 kph (25 MPH) and 144 kph (89 MPH) without using the accelerator pedal. ASCD control unit uses inputs from speedometer and controls ASCD pump. ASCD pump will use the signals from ASCD control unit to maintain the driver's desired set speed.
ASCD pump consists of a vacuum motor, an air valve and a release valve. Input signals from the ASCD control unit controls ON/OFF operation of the vacuum motor, air valve and release valve. The pump is connected to ASCD actuator by vacuum hose. When the ASCD pump is activated, the ASCD pump vacuums the diaphragm of ASCD actuator to control throttle cable.

Possible Cause
- ASCD actuator
- ASCD pump
- Vehicle speed sensor
- ASCD control unit

Diagnostic Repair Procedure (for confirmed incidents)
1. Check vacuum hose between ASCD actuator and hose: No cracks or leaks.
2. Check ASCD wire: Smooth operation and adjustment.
3. Check ASCD actuator: Use a hand vacuum pump to apply -40 kPa (-0.41 kg/sq.cm, -5.8 psi). Wait 10 seconds and confirm that vacuum pressure decrease is less than 2.7 kPa (0.028 kg/sq.cm, 0.40 psi).
4. Check resistance of vacuum motor, release valve and air valve: Vacuum motor = approx. 3 ohm, release valve and air valve = approx. 65 Ohm.
5. Check ASCD pump operation: Disconnect vacuum hose from ASCD pump connector. Then connect vacuum gauge to ASCD pump and apply 12 V direct current to ASCD pump.





6. Check vehicle speed sensor input signal: Use CONSULT-II DATA MONITOR to confirm that VHCL SPEED SE indicates actual vehicle speed.
a. Check speedometer operation: If speedometer is not operating properly, repair as necessary.
b. Check circuit between ASCD control unit and combination meter: Continuity/No shorts to ground.