1986-1989 C68 ECC Bench Testing



Update: You can now purchase a bench test power cable with factory connector Click here


Wiring Diagrams for C68 HVAC - Click Here


Note: The wires which come preinstalled in the black connector housing are clipped into place, and probably don't need to be moved. There are several other wires which have individual plastic sheaths attached. The connectors on these wires have been modified so that they won't catch when inserted into the connector. Their purpose is making temporary connections needed for testing.


Warning: Before connecting power to the wiring harness, make sure that the plastic sheath for each of the unused test wires completely covers the connectors. Also make sure the unused wires/connectors are not resting on the cluster circuit boards.


Note that the External Temp LED will flash because the system is not connected to blend door motor, programmer or blower module. This is expected, and can safely be ignored. The factory procedure for reading error codes doesn't apply here, as those components aren't connected.

  1. Plug the wiring harness connector into the cluster.

  2. Connect the bench test cable to a source of power.

  3. The three illumination bulbs in the faceplate should illuminate brightly, and the green Vacuum Fluorescent Display should light up and show typically "75". If not, look at replacing the power supply capacitors and reshaping or bypassing the faceplate connector, which can become misshapen over time.

  4. Press the WARM button. The button should operate smoothly and the temp indicator display should increase by one degree. Press the COOL button. The button should operate smoothly and the temp should decrease one degree. If not, there may be a problem in the faceplate.

  5. Press the Fan Manual Up button. The button should operate smoothly and the fan speed should increase. Next press the fan manual down button. The button should operate smoothly and the fan speed should decrease. Next press the Fan Auto button.
    The button should operate smoothly and the auto mode should be selected.

  6. Press the mode buttons. The mode should be selected, and the button should operate smoothly.

  7. Select Auto mode and set the temperature set point to 60 degrees using the COOL button.
    Use a multimeter to measure the output voltage from terminal C6 of the controller to ground. It should be approximately 12V (or whatever battery voltage is). Next turn off the unit using the OFF button. 0V when the unit is off. If not, the output transistor for the compressor request is defective.

  8. Press the External temperature measurement button. The displayed temp should read 105-123 degrees (set by a sense resistor in the cable). If not, the power supply in the controller needs to be rebuilt.

  9. Select Auto mode and press the External Mirrors / Rear defogger option (top right button). Measure the resistance between terminal C15 and ground. It should measure low resistance or 0. Next turn off the unit. It should measure very high or infinite resistance. If not, the output transistor for this function in the controller is defective.