Cruise Control System GMC Cab & Chassis R2500

DESCRIPTION

This system has the ability to maintain cruise speed, resume speed, accelerate, coast-down, tap speed up and tap speed down. System consists of a multi-function control switch, electronic control module, servo unit, vacuum reservoir, speed sensor, vacuum supply, electrical and vacuum release switches, and 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 or resume desired speed. A low speed limiter (contained within control module) prevents system engagement below 25 MPH.

OPERATION

System operation is controlled by electronic controller module. The ECM monitors the vehicle speed through the speed sensor. It also monitors operation of the multi-function control switch, located on end of turn signal lever and servo position. As commands as relieved from the multi-function control switch, the electronic control module either holds the current position of the servo. Applies vacuum to the servo to increase throttle position (accelerates). Or vents vacuum from the servo to reduce vehicle speed (deceleration). System can be disengaged by turning off cruise control switch or by activating brake or clutch (M/T) release switches. A vacuum release valve controls servo vacuum release to the atmosphere as either pedal is depressed. This allows servo to quickly release throttle linkage, returning throttle to idle position.

Scheme 1

MULTI-FUNCTION CONTROL SWITCH

The multi-function control switch turns cruise control system on and off. It also has the capability of returning vehicle speed to the last stabilized speed setting when slide switch is momentarily moved toward "R/A" (Resume/Accelerate) position. If slide switch is held for more than one second in "R/A" position, system enters acceleration mode. Vehicle can be accelerated by holding switch in "R/A" position.

When switch is released, vehicle will maintain selected cruise speed. Slide 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 one MPH for each tap, up to 10 taps. After 10 taps, system must be reset to new speed to continue acceleration 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 at or above 25 MPH (cruise control switch in "ON" position), depress and release set/coast switch. Vehicle speed will then be set at speed vehicle was traveling when "SET" button was released. Vehicle cruise speed should be within one MPH of vehicle speed when "SET" button was released. Speed will be maintained until cruise control switch is turned to "OFF" position, ignition is turned to "OFF" position or set/coast switch is fully depressed and held. Depressing brake or clutch pedal (M/T) releases vehicle speed, but will not shut system off. (Scheme 1)

"COAST" Mode

With set/coast button fully depressed, vehicle set 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. (Scheme 1)

"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 one MPH for each tap. (Scheme 1)

ELECTRONIC CONTROL MODULE

The control module monitors servo position, mode control switches, and output of speed sensor. In response to these input signals, 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 electrically operated vacuum diaphragm. Along with 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. (Scheme 2)

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. (Scheme 2) Vacuum is trapped in diaphragm, therefore holding the throttle in a fixed position.

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. (Scheme 2) 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. (Scheme 2) Vacuum valve remains closed.

SPEED SENSOR

The Vehicle Speed Sensor (VSS) buffer amplifier supplies vehicle speed information to control module. Optic head portion of VSS is located in speedometer frame. (Scheme 3) 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. (Scheme 3)

Scheme 2

Scheme 3

ELECTRICAL & VACUUM RELEASE SWITCHES

These switches are used to disengage cruise control whenever clutch or brake pedals are activated. An electrical release switch is mounted on brake and clutch pedal (M/T) brackets. Switches disengage system electrically if either pedal is depressed. (Scheme 4)

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. (Scheme 4)

Scheme 4

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. (Scheme 4)
2. With brake pedal depressed, insert switch into tubular clip until switch body seats on clip. (Scheme 4) 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.

INOPERATIVE SYSTEM ELECTRICAL

Check power supply to control module terminals "G" and "A". Check for open ground circuits to control module terminals "J", "H" and servo terminal "C". Check for open leads at or to servo terminals "B" and "D" (servo position sensor). Check for open leads to or at servo terminals "A" and "E" (vent and vacuum circuits). Check for no high and low voltage signals at module speed sender terminal "D". Check for incorrect input signals to SET/COAST and/or RESUME/ACCELERATE terminals of control module. Check for poor connections at control module or defective control module.

INOPERATIVE SYSTEM MECHANICAL

Check for lack of vacuum at small hose to servo. Inspect for malpositioned servo spring and/or rod. Check for leaky servo, broken fittings and/or inoperative valves. Check for disconnected servo linkage. Inspect vacuum release valve for leaks or defective hose.

VEHICLE SURGES ELECTRICAL

Check for intermittent ground circuits to servo module and/or vehicle speed sensor. Check for intermittent opens in servo position sensor leads, connections and/or terminals. Inspect for high spikes on positive battery line. Check for turn signal, radio or other electrical component interference. Ensure correct control module application. Make sure that all vacuum and vent electrical leads are correctly routed.

VEHICLE SURGES MECHANICAL

Check for excess slack in servo linkage. Ensure that servo spring is not sticking. Misadjusted or leaky vacuum release valve or hose. Check for excessively dry or overlubricated speedometer cable. Make sure that speedometer cable is in good working order. Check for restricted vacuum supply hose to accumulator or servo. Inspect for binding servo linkage. Check for torque converter clutch cutting in and out or transmission shifting. Check for engine cutting in and out due to mixture ignition problems. Check that 3 port or 2 port check valve is positioned properly.

CRUISE SET SPEED HIGH OR LOW

Check vacuum hose routing for pinches, leaks or restrictions. Check servo linkage for excess slack and adjust as described in this article. Check control module part number to ensure that correct module is in place. Replace control module, if no system problem is found.

NO RESUME FUNCTION

Ensure that operator is operating system properly. Check for an open connection at terminal "A" of control module or that terminal is in wrong cavity. Check that control module is the correct one and is operating properly. Inspect for improper operation of multi-function control switch.

UNWANTED CRUISE OR ACCELERATION

Check for crossed vacuum and vent wires. Check for defective or intermittent turn signal control switch. Inspect for sticking valves in servo. Check for binding throttle linkage. Inspect for grease in speedometer head. Check for floor mat interference. Check for loose retainer on bail of servo. Check for good ground circuit. Ensure that speed sensor is working properly. Check for correct control module application.

DIAGNOSIS & TESTING

Note. For wiring diagrams, refer to appropriate chassis wiring diagram in WIRING DIAGRAMS section.

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 RESISTANCE TEST and SERVO VACUUM TEST. If servo is not okay, repair or replace as necessary.
3. If servo is okay, perform MULTI-FUNCTION CONTROL SWITCH TEST. If switch is not okay, adjust, repair or replace as necessary. If switch is okay, perform a CONTROL MODULE VOLTAGE CHECK. If voltage check results are satisfactory, perform a CONTROL MODULE RESISTANCE CHECK. If resistance check results are okay, but system problems still remain, exchange control module with known good control module and retest system.

SERVO RESISTANCE TEST

1. Turn ignition switch to "OFF" position. Disconnect servo connector and remove servo assembly. Servo has terminal pins that are labeled by letters "A" through "E". (Scheme 5)
2. With an ohmmeter, probe terminal pins "D" to "B" of servo. Resistance should be 15-25 ohms. If resistance is not within specifications, replace servo. If resistance is between 15-25 ohms, go to next step.
3. Probe terminal pins "A" to "C" of servo. Resistance should be 30-55 ohms. If resistance is not within specification, replace servo. If resistance is 30-55 ohms go to next step.
4. Connect ohmmeter leads between terminal pins "E" and "C" of servo. Resistance should be 30-55 ohms. If resistance is not within specification, replace servo. If resistance is 30-55 ohms, proceed to SERVO VACUUM TEST procedure.

Scheme 5

SERVO VACUUM TEST

1. Disconnect linkage, bead chain or cable from servo to throttle. Disconnect electrical connector to servo. Start engine and let idle or apply vacuum to servo vacuum port. 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, check vacuum supply to servo.
3. If servo pulls in, remove jumper wire from pin "E" on servo. Servo should stay in position. If servo stays in position, proceed to step 5). If servo fails to remain in position, disconnect vacuum brake release at servo and plug.
4. Momentarily reconnect jumper wire to pin "E" until servo fully pulls in, then remove jumper wire. If servo stays in position, adjust or replace the vacuum release valve.
5. Turn ignition switch to "ON" position. Vacuum release valve should engage. Turn ignition switch to "OFF" position and disconnect vacuum valve connector at valve. Turn ignition switch to "ON" position. With properly adjusted vacuum release switch, battery voltage should be present across connector terminals. No battery voltage indicates an open circuit.

MULTI-FUNCTION 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. (Scheme 6)
2. With slider switch in "ON" position and set/coast button not depressed, reading should be infinity (open circuit) at all terminals except 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. (Scheme 6)
4. With slider switch in "OFF" position and set/coast button depressed, reading should be infinity (open circuit) at all wire terminals except 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. (Scheme 6)
6. With slider switch in "R/A" position and set/coast button depressed all wire terminals should be 12 volts.

Scheme 6

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 zero 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 zero volts. With slider switch "OFF" and set/coast button not depressed, reading should be zero 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.
6. If vehicle is equipped with a speed sensor, probe terminal "D". Raise and support vehicle. Rotate drive wheels and note voltage output. Voltage should read between 0-4 volts. If not repair or replace speed sensor.

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 zero ohms (continuity).

Note. This previous test checks a blank terminal connector, but it is correct according to GM.