DTC 74

Circuit Description

The LED Display Dimming Output Circuit receives its ground in the form of a PWM output from the CCM through CKT 1382. The power for the circuit is supplied internally from the light switch, HVAC control head, and radio control head.

When the CCM is powered up, CKT 1382 is pulled low. After 20 milliseconds it sends the line 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 74 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 test checks for proper dimmer control operation.
3. This removes the Radio Control Head from the dimming circuit. If the test light varies, the control head was the short to ground and should be replaced.
4. This removes the Light Switch Assembly from the dimming circuit. If the test light varies, the Light Switch was the short to ground and should be replaced.
5. This removes the HVAC Control Head from the dimming circuit. If the test light varies, the control head was the short to ground and should be replaced.
6. This removes the CCM from the dimming circuit. If the test light varies, the CCM was the short to ground and should be replaced.
7. This test determines whether an open circuit condition is due to faulty wiring or connectors, or the CCM itself.

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 light switch, 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.

If CKT 1382 is shorted to ground, the LEDs will stay at full brightness regardless of changes in ambient light level and/or dimmer control lever position.

If CKT 1382 is open, the LEDs will always be "OFF."

If there is an open or short to ground in CKT 1382's wiring found using the diagnostic chart, it is most likely between the CCM connector and the splice point where it branches to the individual devices.