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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.
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."
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.
Disconnect battery ground cable.
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
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:
The PCM will inhibit cruise control:
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
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.
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.
You are welcomeCustomer
An Accept is appreciated if I helped in any way.