CCM Input and Output Testing
Courtesy light problems are a common issue. The CCM manual control can be very helpful in diagnosing the problem. When cycling the courtesy lamps in CCM manual control mode, we can prove that the courtesy lamp relay and all courtesy lamps are functioning properly.
When there is an intermittent courtesy lamp problem, try cycling the control relay repeatedly in an attempt to coerce it into acting up. If you listen carefully, you can hear the relay click as you cycle the courtesy lamps. If the relay clicks, the CCM and wiring to the relay are working.
There is a possibility that the relay is clicking, and the lamps are not coming on. Relays use a set of points to make the high amp power connection.
The set of points can become corroded and prevent relaying the power to the lamps. If the relay does not click, check the relay itself. In most cases, the relay or possibly wiring from the CCM is to blame. Wiring issues usually come up replacing the heater core or sound system installation.
Door switch input is also helpful. Go to CCM inputs 1.3 04 and 05. Try moving the doors when they are closed to simulate bumps during travel. If the switches show that the door is open, the courtesy lights will come on unexpectedly.
TIP: the CCM uses an ambient light sensor input for courtesy light control. When the CCM is used to cycle the relay, the ambient light sensor input is not used. Whether there is enough ambient light available or not, the courtesy lights should cycle on and off during testing.
If all else fails and the courtesy lights will not come on in full daylight, test the ambient light sensor operation. Use a piece of electrical tape and a flashlight.
Place the tape over the light sensor in the DIC button panel’s upper left corner while monitoring CCM 1.2 data at 03.
You should see approximately 240 counts. Shine the flashlight on the sensor, the counts should drop to 0.
We have never seen one of the ambient light sensors fail, but as time goes by, they will.
VATS or Pass-key problems occur in the 1990–1996 Corvettes from time to time.
The CCM has more tolerance for ignition key pellet resistance changes than the 1986–1989 Corvettes, making them more reliable. The ability to access Pass-key trouble codes helps troubleshoot issues more quickly. A good example would be CCM trouble code 52 (VATS key circuit is shorted).
The most logical explanation is that the wiring insulation has been damaged, allowing the wires to touch each other. The steering column may be the culprit.
The wiring could be damaged from either tilting the column or rotating the key cylinder. CCM code 51 (invalid key detection) would point to an ignition key resistor pellet that is dirty or damaged.
CCM codes 61-65 indicate that the wrong resistance key was being used during programming.
We have found that when CCM codes 41 or 54 display, it has always been an ECM (Engine Control Module) failure. The first thought is that the CCM would be the culprit, but to date, we have never seen this happen. The law of averages shows that CCMs have an excellent track record. We have only seen one failed CCM (in a 1995 Corvette) in all the years we have worked with them.
CCM (Central Control Module) Codes 72 and 74 usually occur after the radio control head was replaced. The radio head LED and LCD open circuits are noticed by the CCM, setting the code. There is a simple fix while the radio head is being replaced.
Resistors and one diode must be installed to alleviate the problem. A 1K 1/2 watt resistor is installed in the following manner: one side of the resistor must be powered by the radio head C2 connector in position 8 (pink wire). Connect one end of the resistor to this wire.
This will send the correct voltage signal to the CCM once the other side of the resistor is connected.
At radio head connector C1, locate the gray/black striped wire at position 3. Connect the other end of the resistor here. Be careful: the radio head has two gray/black striped wires in connector C1. The gray/black striped wire at position 4 is the incandescent lamp control.
It will not set a code when left disconnected.
To recap: the resistor will be installed at connector C2 position 8 on one end and connector C1 at position 3 for the other end.
LED dimming circuit
The purple/white striped wire at the radio head connector (C1 position 2) requires a 10 Ohm resistor and diode in series with the pink wire. The same pink wire from connector C2 position 8 is used for power. Connect one side of the 10 Ohm resistor to the pink wire. The diode should be installed in series with the resistor. Diodes are an electrical check valve, allowing current to flow in one direction only.
Diodes have an arrow on their barrel denoting direction of current flow. The arrow should point away from the resistor towards the purple/white striped wire. One side of the diode connects to the resistor.
The side with arrow pointing out goes to the purple/white striped wire. This will prevent “SYS” from flashing while you’re enjoying the new sound system.
