Cruise Control System GMC Cab & Chassis S15

DESCRIPTION

This system has the ability to cruise, resume speed, accelerate, or coast-down. System consists of mode control switches, control module, servo unit, speed sensor, vacuum supply, electrical and vacuum release switches, and an electrical wire harness.

A vacuum operated servo unit is used to maintain desired speed. System control module monitors vehicle speed and servo position. Control module operates vacuum and vent valves in servo to maintain desired speed. A low speed limiter (contained within control module) prevents system engagement below 25 MPH.

OPERATION

The operation of control module is controlled by mode control switches, located on end of turn signal lever. System may be disengaged by turning off cruise control switch or by activating brake or clutch (M/T) release switch. A vacuum release valve, vents servo to atmosphere as either pedal is depressed, allowing servo unit to quickly return to throttle idle position.

Multi-Function Lever. Scheme 1

CRUISE CONTROL SWITCH

The cruise control switch turns system on and off, and returns cruise control to last speed setting when momentarily moved toward "R/A" (Resume/Accelerate) position after braking. If slider switch is held for more than 1 second in "R/A" position, system enters acceleration mode. Vehicle may be accelerated by holding switch in "R/A" position.

When switch is released, vehicle will maintain selected cruise speed. Slider switch may be used to resume vehicle speed. With cruise control engaged and operating, quickly press switch to "R/A" position and release. This feature increases vehicle speed by approximately 1 MPH for each tap, up to 10 times. After 10 times, system must be reset to new speed to continue function.

SET/COAST SWITCH

The set/coast switch is located on end of turn signal lever and has 2 positions, normal and depressed. Set/Coast switch has the following functions

"SET" Mode

With vehicle speed above 25 MPH (cruise control switch in "ON" position), depress and release set/coast switch. Speed will be set at speed vehicle was at when button was released. Vehicle cruise speed will be within 1 MPH of actual speed at engaged speed. Speed will be maintained until cruise control switch is turned off, ignition is turned off or set/coast switch is fully depressed and held. Depressing brake or clutch pedal (M/T) releases cruise control, but will not shut system off. (Scheme 1)

"COAST" Mode

With set/coast switch fully depressed, control speed may be raised or lowered. To increase speed, accelerate to new speed, fully depress set/coast switch and release. New speed is set as switch is released. To decrease speed, depress and hold set/coast switch. When vehicle has slowed to desired speed, release switch to set new speed.

"TAP DOWN" Mode

With cruise control engaged and operating, slow vehicle speed by "tapping" set/coast switch. Do not hold switch in depressed position as it will place system in coast mode. This feature decreases vehicle speed by approximately 1 MPH for each tap.

ELECTRONIC CONTROL MODULE

The control module monitors servo position, mode control switches, and output of speed sensor. In response to these inputs, control module electrically signals opening and closing of vent and vacuum solenoid valves in servo. Control module is usually mounted on accelerator pedal bracket.

SERVO UNIT

The servo unit consists of a vacuum operated diaphragm, a normally open solenoid valve that vents diaphragm chamber to atmosphere, a normally closed solenoid valve that connects diaphragm chamber to vacuum source, and a variable inductance position sensor.

Steady Cruise State

Servo alters throttle position in response to signals from control module. With vacuum and vent valves closed, servo has constant vacuum on diaphragm and places no-flow requirement on vacuum source. Vacuum is trapped in diaphragm.

Vehicle Losing Speed

Servo alters throttle position in response to signals from control module. Control module energizes vacuum solenoid to open vacuum valve to vacuum source. This increases vacuum level in servo to increase throttle opening. Vent remains closed.

Vehicle Gaining Speed

Servo alters throttle position in response to signals from control module. Control module de-energizes vent solenoid to open vent valve to atmosphere, which reduces vacuum in servo and allows throttle return spring to decrease throttle opening. Vacuum valve remains closed.

SPEED SENSOR

The vehicle speed sensor (VSS) buffer amplifier supplies vehicle speed to control module. Optic head portion of VSS is located in speedometer frame. A reflective blade is attached to speedometer cable/head assembly. Reflective blade spins like a propeller, with its blades passing through a light beam from an LED in optic head.

As each blade enters LED light beam, light is reflected back to a photocell in optic head causing a low power speed signal to be sent to buffer for amplification and signal conditioning. This amplified signal is then sent to control module.

Servo Unit. Scheme 2

Vehicle Speed Sensor (VSS) Buffer is located on back of instrument panel. Scheme 3

ELECTRICAL & VACUUM RELEASE SWITCHES

These switches are used to disengage cruise control. An electrical release switch is mounted on brake and clutch pedal (M/T) brackets. Switches disengage system electrically as either pedal is depressed.

A vacuum release valve is also mounted on brake pedal bracket. This valve vents trapped vacuum in servo to atmosphere as brake pedal is depressed, allowing throttle to quickly return to idle position. This is done by routing a separate hose directly to servo from normally closed vacuum switch.

BRAKE/RELEASE SWITCH ADJUSTMENT

1. With brake pedal in fully released position. Brake light plunger should be fully depressed against pedal shank. Adjust switch by moving in or out. Make sure tubular clip is in brake pedal mounting bracket.
2. With brake pedal depressed, insert switch into tubular clip until switch body seats on clip. Audible clicks can be heard as threaded portion of switch is pushed through clip and toward brake pedal.
3. Pull brake pedal fully rearward against pedal stop until audible clicking sounds can no longer be heard. Switch will be moved into tubular clip providing adjustment. Release brake pedal and repeat step 3), to make sure that no audible clicking sounds remain.

CLUTCH/RELEASE SWITCH ADJUSTMENT

Position switch slider forward on switch shaft. Fully depress clutch pedal to set switch.

2.5L & 2.8L Engines

With cable assembly installed on cable support and carburetor/throttle body, install cable assembly to servo bracket, using second ball of servo chain on cable. With throttle fully closed (and ignition and fast idle cam off), adjust cable jam nuts until .04" (1 mm) clearance exists between throttle lever stud and end of cable slot. Tighten jam nuts.

4.3L & 4.8L Engines

With cable assembly installed and using second ball of servo chain, install servo assembly chain on cable assembly. With throttle fully closed (and ignition and fast idle cam off) adjust cable assembly jam nuts until .04" (1 mm) clearance exists between lever pin and end of cable assembly slot. Tighten jam nuts.

V8 Gasoline Engines

With throttle fully closed (and ignition and fast idle cam off), adjust length so that rod assembly is over end of stud. Install retainer.

V8 Diesel Engine

With engine off and idle screw against stop, assemble lower end of rod link to throttle lever and upper end to hole closest to servo. This adjustment should provide a minimum of .04" (1 mm) slack.

SYSTEM SURGES

Servo and throttle linkages should operate freely, smoothly and be properly adjusted. Check vacuum hose routing for pinches, leaks or restrictions. Check for dry speedometer cable. Check for excessive grease or dirt on the reflective blade on speedometer head. Perform SERVO TEST . Replace control module if no system problem is found.

CRUISE SET SPEED HIGH OR LOW

Check vacuum hose routing for pinches, leaks or restrictions. Adjust or replace as necessary. Check servo linkage for excess slack and adjust as described in this article. Replace control module, if no system problem is found.

EXCESSIVE CRUISE SPEED LOSS ON HILLS

Check hoses for vacuum leaks. If applicable, determine if check valve is functional. Check vacuum hose routing.

RESUME OR COAST DOES NOT OPERATE

If all other cruise control functions are working properly, except resume or coast, replace control module.

PRELIMINARY INSPECTION

Check system to make sure there are no bare, broken or disconnected wires or any pinched, damaged or disconnected vacuum hoses. Servo and throttle linkage should operate freely and smoothly. Servo linkage should be properly adjusted. If preliminary inspection does not reveal any problems, further testing is required.

CRUISE CONTROL DIAGNOSIS

1. Check throttle linkage to servo for proper adjustment. If not okay, adjust linkage. If adjustment is okay, check all electrical and vacuum connections for proper engagement. Check fuse.
2. If connections or fuse are not okay, repair or replace as necessary. If system is okay, perform «SERVO TEST»(/gmc/cab-chassis-s15/1985-1988/remont/cruise-control-systems/#cruise-control-system__servo-test) . If servo is not okay, repair or replace as necessary.
3. If servo is okay, perform «CRUISE CONTROL SWITCH TEST»(/gmc/cab-chassis-s15/1985-1988/remont/cruise-control-systems/#cruise-control-system__cruise-control-switch-test) . If switch is not okay, adjust, repair or replace as necessary. If switch is okay, see CONTROL MODULE, SERVO & CRUISE CONTROL SWITCH CHECK chart.
4. If switch is not okay, repair or replace as necessary. If switch is okay, see CONTROL MODULE, SERVO & CRUISE CONTROL SWITCH CHECK chart. If set/coast switch is not okay, repair or replace as necessary.
5. If set/coast switch is okay, perform «SPEED SENSOR TEST»(/gmc/cab-chassis-s15/1985-1988/remont/cruise-control-systems/#cruise-control-system__speed-sensor-test) . If speed sensor is not okay, repair or replace as necessary. If speed sensor is okay, replace control module.

SERVO TEST

1. Turn ignition off. Disconnect servo connector and remove servo assembly. Servo has terminal pins that are labeled by letters "A" through "E".
2. With an ohmmeter, probe terminal pins "D" to "B". Resistance should be 15-25 ohms. If resistance is higher or lower, replace servo. If resistance is 15-25 ohms, go to next step.
3. Probe terminal pins "A" to "C". Resistance should be 30-55 ohms. If resistance is higher or lower, replace servo. If resistance is 30-55 ohms go to next step.
4. Probe terminal pins "E" to "C". Resistance should be 30-55 ohms. If resistance is higher or lower, replace servo. If resistance is 30-55 ohms, go to SERVO VACUUM TEST procedure.

SERVO VACUUM TEST

1. Disconnect linkage, bead chain or cable from servo to throttle. Make sure electrical connector to servo is still disconnected. Start engine and let idle. Manually actuate servo vent and vacuum control valves by connecting jumper wires from battery positive post to pins "A" and "E" on servo assembly.
2. Connect another jumper wire between pin "C" of servo and ground. With brake and clutch pedal (M/T) in free position, servo should fully pull in. If servo fails to pull in, go to step 5).
3. If servo pulls in, remove jumper wire from pin "E" on servo. Servo should stay in position. If servo stays in position, check connection of electrical connector at servo. If servo fails to remain in position, remove larger of two hoses and plug orifice of servo.
4. Reconnect jumper wire to pin "E" until servo fully pulls in, then remove jumper wire. If servo fails to stay in position, replace servo. If servo stays in position, check brake and/or clutch release valve for adjustment. Check hoses and valves for leaks, and repair or replace as necessary. Check for proper system operation.
5. Remove larger of two hoses and plug orifice of servo. Servo should pull in. If servo fails to pull in, remove smaller hose from servo and check for vacuum. If vacuum is present, inspect connectors for leaks. If okay, replace servo.
6. If no vacuum is present, check connectors between servo and vacuum source. Repair or replace as necessary. If servo pulls in, check brake and/or clutch release valve for adjustment. Check hoses and valves for leaks, repair or replace as necessary.

CRUISE CONTROL SWITCH TEST

1. Using a voltmeter or test light, test cruise switch at connector mounted on steering column. With slider switch in "OFF" position and set/coast button not depressed, reading should be infinity (open circuit) at all wire terminals.
2. With slider switch in "ON" position and set/coast button not depressed, reading should be infinity (open circuit) at all 5 wire terminals and 12 volts at Green/Blue wire terminal.
3. With slider switch in "R/A" position and set/coast button not depressed, reading should be infinity (open circuit) at wire terminals Yellow/Red, Green/Red, and Red/Blue. Reading should be 12 volts at wire terminals Yellow/Green, Yellow/Blue, and Green/Blue.
4. With slider switch in "OFF" position and set/coast button depressed, reading should be infinity (open circuit) at 5 wire terminals and 12 volts at Green/Red wire terminal.
5. With slider switch in "ON" position and set/coast button depressed, reading should be infinity (open circuit) at wire terminals Yellow/Green, Yellow/Red, and Yellow/ Blue. Reading should be 12 volts at wire terminals Green/Red, Green/Blue, and Red/Blue.
6. With slider switch in "R/A" position and set/coast button depressed all wire terminals should be 12 volts. See CONTROL MODULE, SERVO & CRUISE CONTROL SWITCH CHECK chart.

CONTROL MODULE VOLTAGE CHECK

1. With control module connected, ignition switch in "ON" position, and slider switch in "ON" position, probe terminal "G" with a voltmeter or test light. With brake/clutch switch not depressed, reading should be 12 volts. With brake/clutch switch depressed, reading should be zero volts.
2. Probe terminal "A" with slider switch in "ON" position. Reading should be 12 volts. With slider switch in "OFF" position, reading should be zero volts.
3. Probe terminal "M" with slider switch in "R/A" position. Reading should be 12 volts. With slider switch in "ON" position and set/coast button depressed, reading should be zero volts. With slider switch in "OFF" position and set/coast button depressed, reading should be 0 volts.
4. Probe terminal "L" with slider switch in "ON" position and set/coast button depressed. Reading should be 12 volts. With slider switch in "ON" position and set/coast button not depressed, reading should be 0 volts. With slider switch "OFF" and set/coast button not depressed, reading should be 0 volts.
5. If vehicle is equipped with a cruise control lamp, probe terminal "B" with slider switch in "ON" position and set/coast button depressed. Reading should be 12 volts.

CONTROL MODULE RESISTANCE CHECK

1. With control module disconnected and servo connected, probe terminal "C" to ground with ohmmeter. Reading should be 30-55 ohms. With servo disconnected probe terminal "C" to ground. Reading should be infinity (open circuit).
2. With servo connected, probe terminals "F" to "H". Reading should be 15-25 ohms. With servo disconnected, probe terminal "F" to ground. Reading should be infinity (open circuit). With servo disconnected, probe terminal "H" to ground. Reading should be infinity (open circuit).
3. With servo connected, probe terminal "K" to ground. Reading should be 30-55 ohms. With servo disconnected, probe terminal "K" to ground. Reading should be infinity (open circuit).
4. With servo connected or disconnected, probe terminal "J" to ground. Reading should be 0 ohms (closed circuit).

SPEED SENSOR TEST

1. Turn ignition switch on and place cruise control switch in "ON" position. With speedometer turning (either through wheels or by hand), measure voltage by probing terminal "D" of control module with all connections mated. If voltage varies at around 4 volts, replace control module.
2. If voltage is not present, disconnect connector at VSS buffer. Measure voltage at connector cavity of wire that goes to terminal "D" of control module. Voltage reading should be 8.2 volts. If reading is correct, service vehicle speed sensor.
3. If voltage is incorrect, probe wire at terminal "D" with connector at VSS buffer still disconnected. If voltage reading is 8.2 volts, locate open in wire from control module to VSS buffer and repair as necessary. If voltage is incorrect, replace control module.

General Motors Custom Cruise III Wiring Diagram. Scheme 4

CONTROL MODULE, SERVO & CRUISE CONTROL SWITCH CHECK. Scheme 5

See also:
• SERVO TEST
• CRUISE CONTROL SWITCH TEST
• SPEED SENSOR TEST