Diagnosis Of Cruise Control System - GMC G Service Manual

9.4 ACCESSORIES

DIAGNOSIS OF CRUISE CONTROL SYSTEM

PROBLEM
Inoperative System
(Electrical)
2. Open ground circuits to the module, servo or
3. Open leads to or at the servo terminals "B"
4. Open leads to or at servo terminals "A" and
Inoperative System
(Mechanical)
Vehicle Surges
(Electrical)
Vehicle Surges
(Mechanical)
POSSIBLE CAUSE
1. No power to module terminals "G" and "A"
(figure 5).
buffer. Module terminal J, H and servo termi­
nal C.
and "D" (servo position sensor).
"E" (vent and vacuum circuits).
5. No high and low voltage signal at the module
speed sender terminal "D".
6. Incorrect input signals to the module, Set/
Coast or Resume/Accelerate terminals (fig­
ure 8).
7. Poor connections at the module.
8. Cruise control module.
1. No vacuum at the small hose to the servo.
2. Cocked servo position rod and spring. Leaky
servo, broken fittings, and/or inoperative
valves.
3. Disconnected servo linkage.
4. Misadjusted or leaky vacuum release valve
or hose.
1. Intermittent ground.
2. Intermittent open in the vacuum, vent, SPS
(Servo Position Sensor) leads or connectors
or terminals or servo.
3. Voltage spikes on the positive battery line
(Transient).
4. Turn signal, radio or other electrical device
interference.
5. Wrong module part number.
6. Crossed vacuum and vent electrical leads
between the module and servo.
1. Excess slack in servo linkage.
2. Servo valve sticking.
3. Misadjusted or leaky vacuum release valve
or hose.
4. Dry or overlubed speedo cable.
CORRECTION
1. Check, repair, adjust or replace the fuse,
brake and clutch switches, turn signal lever,
wires in the steering column.
2. Check the ground circuits for loose or broken
connections. Clean and repair as needed.
3. Repair the leads or replace the servo (figures
6 and 7).
4. Repair the leads or replace the servo.
5. Check, repair, replace: open leads, bad
printed circuit foil at the cluster, bad buffer
amplifier, terminal "B" of servo to "B" of buffer,
ground for buffer, foreign material or grease
in speedometer head.
6. Check, repair, replace: open leads, open
connector, bad turn signal lever and terminal
in the proper cavity.
7. Clean, repair the module connector as
needed.
8. Replace the module.
1. Check for plugged, pinched, disconnected
vacuum hoses, incorrectly positioned 3-port
or 2-port check valve, restricted engine vac­
uum fitting.
2. Replace the servo.
3. Reconnect.
4. Check for leak in vacuum release hose or
valve or misadjusted vacuum dump switch.
1. Check for poor grounding to the servo mod­
ule, VSS (Vehicle Speed Sensor) buffer. If
chassis ground is suspect, attach a new
ground strap between the cowl and engine
block. Retorque all ground connections.
2. Do a physical and an electrical inspection as
shown in the test charts (figures 5 through 8).
3. Check for any hot wires intermittently making
contact with chassis ground. Repair as
required.
4. Reposition electrical leads away from cruise
or VSS (Vehicle Speed Sensor) buffer leads.
Check for poor grounds. If none are found,
replace the module.
5. Replace the module with a correct part
number.
6. Rewire.
1. Adjust to requirements outlined in the On-
Vehicle Service portion in this section.
2. Replace the servo.
3. Check for leak in vacuum release hose, or
valve, or misadjusted vacuum dump valve.
4. Lubricate to specifications.
GMTB-0946-2L