Central Control Module (CCM)

Circuit Description

The Serial Data Circuit is an asynchronous, bi-directional serial data link operating at 8192 bits per second. It is a master/slave configuration, with the CCM acting as master.

Code 41 will set if the ECM does not respond to CCM polls for more than 600 milliseconds. The CCM polls the ECM at 200 millisecond intervals; therefore, this code will set if the ECM does not respond for three consecutive CCM polls.

Test Description

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

1. This test determines if the CCM is having a problem communicating with the ECM.
2. This test checks for an open between Serial Data 1 of the CCM and ECM serial data line 1.
3. This test checks for an open between Serial Data 2 of the CCM and ECM serial data line 1.
4. This test checks for a short to ground in CKT 800.
5. This test checks for a short to battery in CKT 800.
6. This test repeats Tests 2 through 5 using ECM serial data line 2 rather than line 1.

Diagnostic Aids

A short circuit on the serial data link will take the whole link down; an open circuit will take down only those devices which become open-circuited.

A serial data fault may also cause an inoperative trip computer, loss of English/Metric temperature display control on the C68 Electronic HVAC control head, and loss of the "Check Gauges" function.

If this fault is present, the C68 Electronic HVAC control head "OFF" LED will flash.

The ECM requires ignition power to send serial data. If the AIR BAG Fuse is blown, or the 10-way I/P-to-DERM harness connector is disconnected, the ECM will not receive power and this code may set.

Check for an ECM Code 51, which will cause this code to set. The ECM Code 51 problem will also be evidenced by the inability to flash ECM codes or to read ECM codes through the CCM.