Cruise control inop

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texdave

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tex
Truck Year
1976
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C10
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I noticed that my factory cruise control was not working on my 76 3/4 ton Suburban. I took a look at the 76 Supplemental shop manual to learn up on the Cruise system and seems like the major components are: the regulator, the servo unit, brake switch, and the column switch.

The brake and column switch are in place and seem to move fine. The servo unit looks damaged, see the pictures.
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I couldn't readily identify the regulator though. Any tips on where it should be located? Also, I'm not seeing a servo unit part on Rockauto to replace this. Is that part not reproduced anymore? If so, any tips for how to get cruise going again? Thanks.
 
75gmck25

75gmck25

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K25 Camper Special TH350 NP203
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The diagram for my '75 is not really clear, but seems to show the regulator mounted very close to the place where the throttle cable goes through your bracket.

The regulator must be able to measure speed. It should have one speedometer cable running from transmission to regulator, then a 2nd cable that runs from the regulator to the speedometer. The regulator also has an electrical connector, and wires running to the cruise control switch on the turn signal stalk.
 
Snoots

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Cruise Control Troubleshooting and Info

Long story short, I bought a simple lock up kit since my truck didn't have the system installed outside of the transmission. I replaced the wiring inside the transmission (Solenoid and pressure switch to the 4 terminal connector on the top... .it's a 700R4 btw). I ran the wiring up and through...
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Richard B

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1983
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K5 Blazer
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6.2l
1983 k5 diesel cruse is inop. The control switch functions are all correct at the control module. All pins at control module have proper voltage and ohm values, as well as servo measurement values, and speed signal advances to 5v when spedo cable turning. Servo tested and operates correctly to control the vacume bellows. There are no vacume leaks, steady to servo at 18 in. The break and vacuum switches are correct. I don't know what else to do. I have no capability to test the control module . Any suggestions is appreciated, I don't want to just throw money at it.
 
AuroraGirl

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Richard B said:
1983 k5 diesel cruse is inop. The control switch functions are all correct at the control module. All pins at control module have proper voltage and ohm values, as well as servo measurement values, and speed signal advances to 5v when spedo cable turning. Servo tested and operates correctly to control the vacume bellows. There are no vacume leaks, steady to servo at 18 in. The break and vacuum switches are correct. I don't know what else to do. I have no capability to test the control module . Any suggestions is appreciated, I don't want to just throw money at it.
Click to expand...

1983 ACCESSORIES & EQUIPMENT General Motors - Cruise Control Systems - Custom Cruise III​

DESCRIPTION & OPERATION​

DESCRIPTION​

System uses manifold vacuum to power a throttle servo unit. When speed adjustment is necessary, the servo moves the throttle by receiving a varying amount of controlled vacuum from a transducer.

The speedometer cable from transmission drives the transducer. A cable from the transducer drives instrument panel speedometer. There are 2 control switches operating speed control. The first is a "CRUISE" slide switch with "OFF", "ON" and "RESUME" positions.

The second is a "SET-COAST" button switch located on the end of turn signal lever. It controls engagement of transducer. Two brake release switches are provided.

An electric switch disengages transducer. A vacuum valve (switch) decreases vacuum in the servo unit to quickly return throttle to idle position when the brake pedal or clutch is depressed.

OPERATION​

This system is designed to operate at speeds above 25 MPH. "CRUISE" switch must be in "ON" position to activate speed control system. Driver accelerates to desired speed, then partially depresses and slowly releases speed control "SET-COAST" switch button.

To change speed setting to higher speed, depress accelerator until desired speed is reached, then fully depress and slowly release "SET-COAST" switch.

To change speed setting to lower speed, depress "SET-COAST" switch fully. Hold in this position until vehicle has decelerated to new desired speed setting. Then, release "SET-COAST" switch slowly to re-engage speed control.

Speed control system is disengaged by applying brakes. To accelerate vehicle to previously set speed in memory, slide "CRUISE" switch to "RESUME" position.

Accelerator pedal may be depressed at any time to override speed control system. Release of accelerator returns speed to previous level. Sliding "SET-COAST" switch to "OFF" position or turning ignition "OFF" will also disengage the system and erase the "RESUME" memory.

PRELIMINARY INSPECTION​

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

CRUISE SYSTEM INOPERATIVE​

Disconnect brake and clutch switch. Check continuity with pedals in "free" position. If continuity shows an open, adjust or replace the brake/clutch switch as necessary. Check the vacuum release valve and the combination vacuum release valve/converter clutch switch for vacuum operation. If leaking, replace. If misadjusted, adjust.

SERVO CHECK​

  1. Disconnect the electrical connector at the servo. Make sure the connector and pins are free of corrosion. Check resistances. Resistances should be as follows. If not, replace servo unit.
    SERVO PIN RESISTANCES
    Pin Number​
    Ohms​
    A-C​
    10​
    E-C​
    10​
    B-D​
    5​

  2. Turn off ignition switch then disconnect the body harness connector from the Vehicle Speed Sensor buffer amplifier. Connect a Speedometer Signal Generator (J-33431) to amplifier connector. Disconnect the servo throttle linkage at the servo unit. Set the signal generator to 54 MPH and 30 Hertz square wave position. Start the engine and move mode switch slider to "ON" position.
  3. Depress and release "SET/COAST" switch. Servo should pull to approximately 28-42% of servo position travel and hold. Move cruise lever and slider to "RESUME ACCEL" position and hold for 3 seconds. Servo will pull to the full stroke position and hold.
  4. After releasing slider from the "RESUME ACCEL" position, the servo will return to 28-42% of the travel position. Momentarily move the CRUISE lever slider to "RESUME ACCEL" position. Servo will pull to the 28-42% position of travel and hold.
  5. If tests in steps 3) and 4) do not perform as indicated, check the harness for defects, then make electrical and pneumatic tests as needed. If steps 3) and 4) do perform as indicated, a buffer amplifier or pulse modulator generator defect is indicated.

ELECTRICAL CHECK​

Check fuse. Check chassis ground and harness. Perform continuity checks with ohmmeter between controller and servo. Check the Mode Control Switches by replacing with a known good switch or test for continuity as listed in Mode Control Switch Continuity table.

MODE CONTROL SWITCH CONTINUITY

Set/Coast Switch​
O = Open, C = Closed​
Normal​
Position Slider​
Off​
1-2​
O​
1-3​
O​
1-4​
O​
2-3​
O​
2-4​
O​
3-4​
O​
Normal​
Position Slider​
Off​
1-2​
O​
1-3​
O​
1-4​
O​
2-3​
O​
2-4​
O​
3-4​
O​
Normal​
Position Slider​
R/A​
1-2​
O​
1-3​
O​
1-4​
C​
2-3​
O​
2-4​
C​
3-4​
O​
Depressed​
Position Slider​
Off​
1-2​
O​
1-3​
O​
1-4​
O​
2-3​
C​
2-4​
O​
3-4​
O​
Depressed​
Position Slider​
Off​
1-2​
O​
1-3​
O​
1-4​
O​
2-3​
C​
2-4​
C​
3-4​
C​
Depressed​
Position Slider​
R/A​
1-2​
C​
1-3​
C​
1-4​
C​
2-3​
C​
2-4​
C​
3-4​
C​


Fig. 1: Mode Control Switch Wire Identification

PNEUMATIC CHECK​

  1. Disconnect bead chain or rod and electrical connector at the servo unit. Start the engine. Apply 12 volts between A and C. Apply 12 volts between E and C. Servo should move full stroke. If not, check the vacuum hoses to the vacuum supply. Remove voltage from E and C.
  2. The servo should move a full stroke. If not, disconnect vacuum brake release at the servo and plug the servo. Momentarily apply 12 volts between E and C to allow the servo to full stroke. If the servo holds its position, adjust the brake vacuum release valve or replace the valve. If no defect is found in any of the tests, replace the electronic controller.
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Richard B said:
1983 k5 diesel cruse is inop. The control switch functions are all correct at the control module. All pins at control module have proper voltage and ohm values, as well as servo measurement values, and speed signal advances to 5v when spedo cable turning. Servo tested and operates correctly to control the vacume bellows. There are no vacume leaks, steady to servo at 18 in. The break and vacuum switches are correct. I don't know what else to do. I have no capability to test the control module . Any suggestions is appreciated, I don't want to just throw money at it.
Click to expand...
 
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