Lexus Car ES, IS, LS 1998-06

Vehicle Speed Control

Print

Description & Operation



The speed control system is electronically controlled and vacuum operated. Electronic control of the speed control system is integrated into the Powertrain Control Module (PCM). The controls consist of two steering wheel mounted switches. The switches are labeled: ON/OFF, RES/ACCEL, SET, COAST, and CANCEL.

The system is designed to operate at speeds above 30 mph (50 km/h). When speed control is selected by depressing the ON switch, the PCM allows a set speed to be stored in PCM RAM for speed control. To store a set speed, depress the SET switch while the vehicle is moving at a speed between 35 and 85 mph. In order for the speed control to engage, the brakes cannot be applied, nor can the gear selector be indicating the transmission is in Park or Neutral.

The speed control can be disengaged manually by:



Stepping on the brake pedal
 
Depressing the OFF switch
 
Depressing the CANCEL switch.
 
Depressing the clutch pedal (if equipped).
 


NOTE
Depressing the OFF switch or turning off the ignition switch will erase the set speed stored in the PCM.

For added safety, the speed control system is programmed to disengage for any of the following conditions:



An indication of Park or Neutral
 
A rapid increase rpm (indicates that the clutch has been disengaged)
 
Excessive engine rpm (indicates that the transmission may be in a low gear)
 

The speed signal increases at a rate of 10 mph per second (indicates that the coefficient of friction between the road surface and tires is extremely low). The speed signal decreases at a rate of 10 mph per second (indicates that the vehicle may have decelerated at an extremely high rate).

Once the speed control has been disengaged, depressing the RES/ACCEL switch (when speed is greater than 30 mph) restores the vehicle to the target speed that was stored in the PCM.

While the speed control is engaged, the driver can increase the vehicle speed by depressing the RES/ACCEL switch. The new target speed is stored in the PCM when the RES/ACCEL is released. The PCM also has a tap-up feature in which vehicle speed increases at a rate of approximately 2 mph for each momentary switch activation of the RES/ACCEL switch.

A tap down feature is used to decelerate without disengaging the speed control system. To decelerate from an existing recorded target speed, momentarily depress the COAST switch. For each switch activation, speed will be lowered approximately 1 mph.

Overshoot/Undershoot

If the vehicle operator repeatedly presses and releases the SET button with their foot off of the accelerator (referred to as a lift foot set), the vehicle may accelerate and exceed the desired set speed by up to 5 mph (8 km/h). It may also decelerate to less than the desired set speed, before finally achieving the desired set speed.

The Speed Control System has an adaptive strategy that compensates for vehicle-to-vehicle variations in speed control cable lengths. When the speed control is set with the vehicle operators foot off of the accelerator pedal, the speed control thinks there is excessive speed control cable slack and adapts accordingly. If the lift foot sets are continually used, a speed control overshoot/undershoot condition will develop.

To unlearn the overshoot/undershoot condition, the vehicle operator has to press and release the set button while maintaining the desired set speed using the accelerator pedal (not decelerating or accelerating), and then turning the cruise control switch to the OFF position (or press the CANCEL button if equipped) after waiting 10 seconds. This procedure must be performed approximately 1015 times to completely unlearn the overshoot/undershoot condition.

Diagnosis & Testing



Road Test

Perform a vehicle road test to verify reports of speed control system malfunction. The road test should include attention to the speedometer. Speedometer operation should be smooth and without flutter at all speeds.

Flutter in the speedometer indicates a problem that might cause surging in the speed control system. The cause of any speedometer problems should be corrected before proceeding.

If a road test verifies a system problem and the speedometer operates properly, check for:



A Diagnostic Trouble Code (DTC). If a DTC exists, conduct appropriate test.
 
A misadjusted brake (stop) lamp switch. This could also cause an intermittent problem.
 
Loose, damaged or corroded electrical connections at the servo. Corrosion should be removed from electrical terminals and a light coating of Mopar Multipurpose Grease, or equivalent, applied.
 
Leaking vacuum reservoir.
 
Loose or leaking vacuum hoses or connections.
 
Defective one-way vacuum check valve.
 
Secure attachment of both ends of the speed control servo cable.
 
Smooth operation of throttle linkage and throttle body air valve.
 
Failed speed control servo. Do the servo vacuum test.
 


WARNING
When test probing for voltage or continuity at electrical connectors, care must be taken not to damage connector, terminals or seals. If these components are damaged, intermittent or complete system failure may occur.

Vacuum Reservoir
  1. Disconnect vacuum hose at speed control servo and install a vacuum gauge into the disconnected hose.
  2.  
  3. Start engine and observe gauge at idle. Vacuum gauge should read at least ten inches of mercury.
  4.  
  5. If vacuum is less than ten inches of mercury, determine source of leak. Check vacuum line to engine for leaks. Also check actual engine intake manifold vacuum. If manifold vacuum does not meet this requirement, check for poor engine performance and repair as necessary.
  6.  
  7. If vacuum line to engine is not leaking, check for leak at vacuum reservoir. Remove vacuum reservoir.
  8.  
  9. Disconnect vacuum line at reservoir and connect a hand-operated vacuum pump to reservoir fitting. Apply vacuum. Reservoir vacuum should not bleed off. If vacuum is being lost, replace reservoir.
  10.  
  11. Verify operation of one-way check valve and check it for leaks.
  12.  
  13. Locate one-way check valve. The valve is located in vacuum line between vacuum reservoir and engine vacuum source. Disconnect vacuum hoses (lines) at each end of valve.
  14.  
  15. Connect a hand-operated vacuum pump to reservoir end of check valve. Apply vacuum. Vacuum should not bleed off. If vacuum is being lost, replace one-way check valve.
  16.  
  17. Connect a hand-operated vacuum pump to vacuum source end of check valve. Apply vacuum. Vacuum should flow through valve. If vacuum is not flowing, replace one-way check valve. Seal the fitting at opposite end of valve with a finger and apply vacuum. If vacuum will not hold, diaphragm within check valve has ruptured. Replace valve.
  18.  

Removal & Installation



Servo Cable

The speed control servo is attached to a bracket. The bracket and servo assembly are located below the battery tray.

  1. Disconnect negative battery cable at battery.
  2.  
  3. Disconnect positive battery cable at battery.
  4.  
  5. Remove air cleaner housing at top of throttle body and disconnect servo cable at throttle body.
  6.  
  7. Remove battery from battery tray.
  8.  
  9. Disconnect wiring at battery tray.
  10.  
  11. Disconnect positive battery cable at Power Distribution Center (PDC).
  12.  
  13. Loosen PDC at battery tray.
  14.  
  15. Remove 4 battery tray bolts. One of these bolts attaches to speed control bracket flange that supports battery tray. While removing battery tray, disconnect battery temperature sensor electrical connector at sensor.
  16.  
  17. Disconnect vacuum line at servo vacuum hose fitting.
  18.  
  19. Disconnect electrical connector at servo.
  20.  
  21. If servo and mounting bracket are being removed as one assembly, remove two mounting nuts. These are located above right-front tire. Remove inner fender clips and pry inner fender back slightly to gain access to mounting nuts.
  22.  
  23. If servo is being removed from its mounting bracket, remove 2 mounting nuts holding servo cable sleeve to bracket.
  24.  
  25. Pull speed control cable sleeve and servo away from servo mounting bracket to expose cable retaining clip and remove clip.
  26.  
  27. Remove servo from mounting bracket or, remove servo and mounting bracket as one assembly.
  28.  

To Install:

  1. Position servo to mounting bracket.
  2.  
  3. Align hole in cable connector with hole in servo pin. Install cable-to-servo retaining clip.
  4.  
  5. Insert servo mounting studs through holes in servo mounting bracket.
  6.  
  7. Install servo cable mounting nuts and tighten to 8.5 Nm (75 inch lbs.) torque. If servo and bracket is being installed as one assembly, install 2 mounting nuts and tighten to 28 Nm 6 Nm (250 inch lbs. 50 inch lbs.) torque.
  8.  
  9. Connect vacuum line at servo.
  10.  
  11. Connect electrical connector at servo.
  12.  
  13. Connect servo cable to throttle body.
  14.  
  15. Install battery tray and battery temperature sensor.
  16.  
  17. Connect wiring to battery tray.
  18.  
  19. Install battery to battery tray.
  20.  
  21. Connect positive battery cable to Power Distribution Center (PDC).
  22.  
  23. Connect positive battery cable to battery.
  24.  
  25. Connect negative battery cable to battery.
  26.  
  27. Before starting engine, operate accelerator pedal to check for any binding.
  28.  

Servo Switch

CAUTION
Before beginning any airbag system component Removal & Installation, remove and isolate the negative (-) cable from the battery. This is the only sure way to disable the airbag system. Then wait two minutes for system capacitor to discharge before further system service. Failure to do this could result in accidental airbag deployment and possible injury.

  1. Disconnect and isolate negative battery cable.
  2.  
  3. Remove airbag module.
  4.  
  5. Remove electrical connector at switch.
  6.  
  7. Remove switch-to-steering wheel mounting screw.
  8.  
  9. Remove switch.
  10.  

To Install:

  1. Install switch and mounting screw.
  2.  
  3. Tighten screw to 1.5 Nm (15 inch lbs.) torque.
  4.  
  5. Install electrical connector to switch.
  6.  
  7. Install airbag module.
  8.  
  9. Connect negative battery cable.
  10.  

Vacuum Reservoir

The vacuum reservoir is located in the right/front corner of the vehicle behind the front bumper fascia.

  1. Remove front bumper and grill assembly.
  2.  
  3. Remove 1 support bolt near front of reservoir.
  4.  
  5. Remove 2 reservoir mounting bolts.
  6.  
  7. Remove reservoir from vehicle to gain access to vacuum hose. Disconnect vacuum hose from reservoir fitting at rear of reservoir.
  8.  

To Install:

  1. Connect vacuum hose to reservoir.
  2.  
  3. Install reservoir and tighten 2 bolts to 3 Nm (25 inch lbs.) torque.
  4.  
  5. Install front bumper and grill assembly.
  6.  

 
label.common.footer.alt.autozoneLogo