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GM-Frank
GM-Frank, ASE Master Certified
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Experience:  37 Years Automotive Diagnosis & Repair. Experience with all makes and models..
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1996 Roadmaster: Wagon..cruise control runs fine, except it..loses

Resolved Question:

My 1996 Roadmaster Wagon''s cruise control runs fine, except it loses its speed setting after awhile, and I have to reset it. This has been going on for years. Could there be a vacuum leak?
Submitted: 5 years ago.
Category: Car
Expert:  GM-Frank replied 5 years ago.

 

HelloCustomer

I am seeing an electro mechanical cruise control system on your vehicle. Don't believe they are using vacuum on this system. There is a very extensive diagnostic chart available along with special testing equipment, but I feel we can isolate in a much simpler method since you indicate the cruise functions. Just won't maintain the setting.

CIRCUIT OPERATION

The Electro-Motor Cruise Control is a speed control system which maintains a desired vehicle speed under normal driving conditions. The system has the capability to CRUISE, COAST, RESUME SPEED, ACCELERATION, TAP-UP and TAP-DOWN.

Depressing the Set and R/A Switches simultaneously will suspend cruise control operation without removing memory speed.

An Electronic Controller and Electric Motor are contained in the Cruise Control Module. The Controller monitors vehicle speed and operates the Electric Motor. In response to the Controller, the motor moves a connecting strap that is attached to the Cruise Control Cable. The Cable moves the Throttle Linkage to vary throttle position in order to maintain the desired cruise speed. The Cruise Control Module contains a low speed limit which will prevent system engagement below a minimum speed, about 25 mph . The module is controlled by mode control switches located on the LH Steering Column, part of the Turn Signal/Headlamp Dimmer Switch/Cruise Control Actuator/Windshield Wiper/Windshield Washer Lever.

With the Ignition Switch in "RUN," battery voltage is applied to terminal "F" of the Cruise Control Module. When the Slider Switch is moved to the "ON" position, battery voltage is applied to terminal "A" of the Cruise Control Module Connector. If the brake pedal is not depressed, battery voltage is present at module terminal "D." If the brake pedal is depressed, battery voltage is present at module terminal "G." The module must see terminal "D" change state once each Ignition cycle. (The brake pedal must be depressed once each Ignition cycle for cruise operation to occur). (Some Cruise Modules will use either terminal "D" or "G"). With the brake pedal not depressed, terminal "0" must see current flow through CKT 17 for cruise to operate properly. When the Slider Switch is moved to the "R/A" position, battery voltage is applied to terminal "C" of the module. With the Set Switch depressed, battery voltage is present at module terminal "B." Cruise Control Module Connector terminal "K" is the speed signal terminal. In operation, voltage will oscillate between a high of 4 to 5 volts and a low of near ground. Cruise Module terminal "H" is not used. Ground is at module terminal "E."

General Description

DESCRIPTION
The main parts of the cruise control system are the functional control switches, cruise control module assembly, vehicle speed sensor assembly and cruise control release switch, stop lamp and torque converter clutch (TCC) switch, and clutch on 5.7L engines (Camaro and Firebird) or clutch anticipate switch assemblies on 3.4L engines (Camaro and Firebird).

OPERATION
The cruise control system uses a cruise control module assembly to obtain the desired vehicle cruise speed. Two important components in the module assembly assist in this. The first is electronic controller and second is an electric stepper motor, The controller monitors vehicle speed and operates the electric stepper motor. The motor moves a ribbon and throttle linkage, in response to the controller, to maintain the desired cruise speed. The cruise control module assembly contains a low speed limit which will prevent the system from operation during speeds below 25 mph. The operation of the controller is controlled by functional switches located on the turn signal and headlamp switch and cruise control actuator and windshield wiper/washer lever assembly.

Cruise control release switch, stop lamp and clutch or clutch anticipate switch assemblies are provided to disengage the cruise system. The switch assemblies are mounted on the brake and accelerator and clutch pedal bracket. When the brake pedal or clutch pedal is depressed, the cruise control system is electrically disengaged and the throttle is returned to idle position.

I'm leaning heavily to the cruise module as being the problem. Be sure there is no binding in the cable that leads to the throttle body.

DESCRIPTION
The module has an electronic controller and an electric stepper motor to vary the throttle with each different cruise mode. The module is not serviceable and must be replaced as an assembly.

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Disconnect battery ground cable.

  1. Disconnect electrical connector from module, Fig. 36 .
  2. Disconnect cruise control cable from module, then remove bolts and screws.
  3. Remove module and plugs.
  4. Remove plugs from module if a new module is being installed.
  5. Reverse procedure to install noting the following:
    1. Align holes in accelerator and cruise control adjuster bracket to holes in wheel house panel and position module on wheel house panel.
    2. Torque module bolts and screws to 71 ft. lbs.
    3. Adjust cable, if necessary.

graphic

My database is not giving me a detailed cable adjustment procedure. Usually, there is a button type of release near either end of the cable housing that can be pressed or lifted in order to vary the length of the cable housing. My thoughts are that the cable is stretched and the module and cable should come as a unit. I wouldn't spend much time trying to make adjustments on a 12 year old cable.

I hope the information I have provided is helpful

Have a Great Day

Frank

GM-Frank, ASE Master Certified
Category: Car
Satisfied Customers: 431
Experience: 37 Years Automotive Diagnosis & Repair. Experience with all makes and models..
GM-Frank and 7 other Car Specialists are ready to help you
Customer: replied 5 years ago.
You're saying replace module and its related cable to which it is attached?
Expert:  GM-Frank replied 5 years ago.

 

HelloCustomer

Just a quick followup.

Double check to be sure there are no trouble codes stored that would lead us to a VSS or vehicle speed sensor problem. If the Speedometer is accurate, then the VSS is most likely OK.

First make sure there is no binding or anything that would keep the unit from adding throttle when needed. Say on a slight incline.

You need to determine if the system dis-engages completely, or if it simply doesn't maintain a constant speed. Say you set at 60 and in a few miles of level travel it holds at 60 then goes to 55 and holds there. Then 50 and holds there. That tells me the module is not working correctly.

You indicated the speed drops off....if it abruptly drops out, then you'll want to look at the system brake switch adjustment...which could dis-engage the cruise.

Important To avoid misdiagnosis:

 

^Check for proper operation of brake lamps.

 

^Check throttle linkage for mechanical binding which could cause the system to malfunction

 

^Check for stored Diagnostic Trouble Codes (DTCs) in the PCM. Refer to Powertrain OBD System Check in Engine Controls.

 

The PCM will inhibit cruise control:

^When vehicle speed is less than 40 km/h (25 mph).

 

^When PARK, REVERSE, NEUTRAL, or 1st gear is indicated by the Park/Neutral Position Switch,

 

^When an over/undercharged battery condition exists.

 

^With low engine RPM.

 

^With high engine RPM (fuel cut-off). Refer to Powertrain Control Module Description in Engine Controls for additional information.

Frank

Expert:  GM-Frank replied 5 years ago.

 

Allow me to explain why I would be willing to go with the module.

Here is the process to diagnose:

File In Section: 9 Accessories

Bulletin No.: 83-90-08

Date: June, 1998

INFORMATION

Subject:
New Tool J 42958, Revised Charts and Schematics for Cruise Control Diagnosis

 

Models:
1988-98 Passenger Cars and Trucks with Electro-Motor (Stepper Motor) Cruise Control Systems

 

A new Cruise Control Tester, J 42958, has been sent to all dealers as part of the 1998 Essential Tool Package. This tool will work on all Stepper Motor Cruise Control systems for past model years.

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When I see diagnostic steps that will take 4 hours and a GM special tool to possibly find a solution, I am willing to use my better judgement and if wiring and voltages are good, replace the most common or probable cause.

Frank

Customer: replied 5 years ago.
Well done, GM Frank: make it 120/120, Roadmasters are wonderful. 113,000 on ours. Just did 24.7 mpg high speed NYC to NH. Nothing quite like it presently available--towing package, big car comfort ride, big load, big tow, big mpg. Too bad it was not a hit in the nineties.

BestCustomer
Expert:  GM-Frank replied 5 years ago.

 

You are welcomeCustomer

An Accept is appreciated if I helped in any way.

Thanks

Frank

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