Central Control Module (CCM)

Circuit Description

The LCD Dimming Output Circuit, which controls the back lighting on the radio control head (and the HVAC control head when equipped with C68 Electronic Climate Control) receives its ground in the form of a PWM output from the CCM through CKT 1381. Power is supplied to the circuit internally through the radio control head (and HVAC control head, if equipped).

When the CCM is powered up, after 20 milliseconds the CCM pulls CKT 1381 low. After another 20 milliseconds it returns the line to high; after another 20 milliseconds it pulls the line low again. The CCM monitors these low-to-high transitions, and if it finds that all samples were low (transition not made), Code 72 will set.

Test Description

Number(s) below refer to circled number(s) on the diagnostic chart.

1. This checks for the appropriate signal up to the point of connection with the radio control head.
2. This checks if the radio control head is causing the fault; if the code resets, replace the radio control head.
3. This checks if the C68 HVAC control head is causing the fault; if the code resets, replace the HVAC control head.
4. This test checks for intermittents; if none and the code resets, replace the CCM.

Diagnostic Aids

If the code is a history code, the problem may be intermittent. Try performing the tests shown while "wiggling" wiring and connectors; this can often cause the fault to appear.

Check for poor connections at the CCM, the radio control head, and the HVAC control head (if equipped with C68 Electronic Climate Control); these could cause an open or an intermittent fault. Refer to "Intermittents/Poor Connections".

This code will only set if the dimming request IS NOT 0% (full bright) or 100% (full dim). Also, the dimming circuit must be active for the code to set, i.e., the headlamps or parking lamps must be "ON" and some level of dimming must be requested.