Cruise control cannot be set:

Check fuses:

Under dashboard, No. 8 (10A) and No. 4 (7.5A)
Engine compartment, No. 41 (20A)

Check that horn sounds.

Test components and check wiring for open circuit, loose or disconnected terminals; in the sequence as given under each heading.

Cruise control cannot be set:

(a) Main switch or wiring fault on BLACK/RED, LIGHT GREEN or poor earth G402
(b) SET/RESUME switch or wiring fault on LIGHT GREEN/RED, LIGHT GREEN/BLACK or poor earth G402
(c) Brake switch and mounting or wiring fault on LIGHT GREEN, GREY, GREEN/WHITE or poor earth G402
(d) Clutch switch and mounting or wiring fault on PINK, poor earth G402.
Auto selector position switch or wiring fault on PINK, poor earth G401.
(e) Control unit or wiring fault on BROWN/BLACK, BROWN/WHITE.

Cruise control can be set, indicator light not illuminated:

(a) Dimming circuit in gauges, wiring fault on YELLOW or RED
(b) Control unit, poor earth G401*

Cruise speed is noticeably higher or lower than what was set:

(a) Vehicle speed sensor (VSS)
(b) Actuator and cable deflection
(c) Control unit or wiring fault.

Excessive overshooting or undershooting of speed when trying to set speed:

(a) Actuator and cable deflection
(b) Vehicle speed sensor (VSS)
(c) Control unit or wiring fault.

Steady speed is not held even on a flat road with cruise control set:

(a) Vehicle speed sensor (VSS)
(a) Actuator and cable deflection
(c) Control unit or wiring fault.

* See Wire Harness and Earth Locations

Car does not decelerate or accelerate accordingly when SET or RESUME button is pressed:

(a) SET/RESUME switch, cable reel wiring fault LIGHT GREEN/BLACK, LIGHT GREEN/RED
(b) Control unit or wiring fault.

Set speed is not cancelled when clutch pedal is pressed:

(a) Clutch switch and mounting, wiring fault PINK or poor earth G402.
(b) Control unit or wiring fault.

Set speed is not cancelled when Auto selector lever is moved to N:

(a) Auto gear position switch, wiring fault PINK or poor earth G401.
(b) Control unit or wiring fault.

Set speed is not cancelled when brake pedal is pressed:

(a) Brake switch and mounting, wiring fault GREEN/WHITE
(b) Control unit or wiring fault.

Set speed is not cancelled when main switch is pressed to OFF:

(a) Main switch, wiring fault LIGHT GREEN.
(b) Control unit or wiring fault.

Set speed is not resumed when RESUME button is pressed (with main switch on, but set speed is temporarily cancelled):

(a) Brake switch and mounting, wiring fault LIGHT GREEN/BLACK or LIGHT GREEN/RED
(b) Control unit or wiring fault.