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Cruise Control Troubleshooting

Contributed By: Bill Kelsey

On the cruise control -- well, this rates some 13 pages, including wiring diagrams, in the 1978 TSM, but assuming that your cruise control is the same as or similar to that used in the late 70s, here is the part of the diagnostic chart for when the Cruise Command does not engage:

  1. Cruise Command harness fuse burned out (check for cause; replace with 1.5 amp fuse only)
  2. faulty brake lamp switch (replace brake lamp switch)
  3. no current to brown wire (check for loose connection of repair wiring harness)
  4. vacuum leak (repair leak)
  5. bad ground at regulator (check regulator for ground, using ohmmeter -- check from regulator to mounting bracket)
  6. bad relay ground (check ground wire for loose connections (use ohmmeter -- check from relay ground to known good ground)
  7. faulty connections (check conections, repair as necessary)
  8. brake lamp fuse burned out (check for cause and repair, replace fuse)
  9. brake lamp bulb(s) burned out (replace bulb(s))
  10. control switch inoperative (see Circuitry Tests -- steps 8-15 (I've included these below))
  11. faulty regulator (replace regulator)
  12. solenoid valve deformed (replace regulator)
  13. relay inoperative (replace relay, check ground, look for open in white wire)

Circuitry Tests, for #10 above:

(Note: It is not always necessary to remove the regulator in case of inoperative Cruise Command).

  1. disconnect push-on connectors (singel and tripel) at regulator
  2. turn ignition switch to ACCESSORY position
  3. move slide switch to ON position
  4. Using test lamp, ground one test lamp lead and touch other lead to brown wire and then dark green wire at connectors. Test lamp should light. If test lamp down not light on either wire, check fuse, relay, control switch and connection at power source.
  5. Push SET SPEED button in and hold. Ground one test lamp lead and touch other lead to each wire connector. Test lamp should light on brown and yellow wires and should not light on dark green or blue wire.
  6. release SET SPEED button
  7. Move slide switch to RES position and hold. Ground one test lamp lead and touch other lead to each wire in connector. Test bulb should light on all wires except blue wire (blue connects to brake lamp side brake lamp switch).

    (Note: To make an independent check of engagement switch before removal from the vehicle, disconnect switch from wiring harness at the multiple connector in passenger compartment, and make the following checks.)

  8. attach jumper wire from 12-volt power source to red lead of control switch
  9. move slide switch to the OFF position
  10. Using test lamp, ground one test lamp lead and touch other lead, in turn, to brown wire, dark green wire, and yellow wire. Test lamp should not light on any of these wires.
  11. move slide switch to ON position
  12. Touch test lamp lead to brown wire and then dark green wire. Test lamp should light on these wires. Touch lead to yellow wire. Lamp should not light.
  13. Push SET SPEED in and hold. Test lamp should light on brown wire and on yellow wire. Test lamp should not light on light green wire.
  14. release SET SPEED pushbutton
  15. Move slide swith to RES position and hold. Touch test lamp, in turn, to brown wire, yellow wire, and then to light green wire. Test lamp should light on all three wires.

(Note: If steps 1 through 7 do not check out and steps 8 - 15 do not check out, replace Cruise Command wire harness. If steps 8 through 15 do not check out, replace control switch.)

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