GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM), the electronic brake control module (EBCM), and the transmission control module (TCM) communicate control and diagnostic information via a controller area network (CAN) communication bus. The ECM monitors CAN operational status by expecting a constant flow of messages from the EBCM and the TCM. If the ECM fails to receive an expected message from one of the other modules, DTC U2104, U2106, or U2108 will set, depending on what communication is lost.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U2106 Lost Communications With Transmission Control System

Conditions for Running the DTC

    • The ignition is ON.
    • The module supply voltage is in the normal operating range of 9-16 volts.

Conditions for Setting the DTC

The CAN message from the TCM is not received by the ECM for 200 milliseconds.

Action Taken When the DTC Sets

The control module will not illuminate the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

Conditions for Clearing the MIL/DTC

    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • Clear the DTC with a scan tool.

Diagnostic Aids

The modules that communicate on the CAN system are wired parallel to each other until the respective circuits are spliced together for the ECM. An open in the CAN circuits of one module will not affect the other module. A short to ground or short to voltage affects all modules no matter where the failure occurs.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. If the fault is a short to ground, a short together, or an open between the ECM and the CAN circuit splices, DTC U2104 will also set. These failures affect the entire system.

  2. This test is for an open in the CAN HI circuit that will only affect the TCM communications.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics for the 7.8L engine

Connector End View Reference: Computer/Integrating Systems Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Did DTC U2104 also set?

Go to DTC U2104

Go to Step 3

3

  1. Turn OFF the ignition for 30 seconds.
  2. Start the engine.
  3. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC fail this ignition?

Go to Step 4

Go to Intermittent Conditions for the 7.8L engine

4

  1. Turn OFF the ignition.
  2. Disconnect the engine control module (ECM) and the transmission control module (TCM) harness connectors that contain controller area network (CAN) circuits.
  3. Test the CAN high circuit for an open between the splice and the TCM. Refer to Testing for Continuity .

Did the CAN high circuit test open?

Go to Step 12

Go to Step 5

5

Test the CAN low circuit for an open between the CAN harness splice and the TCM. Refer to Testing for Continuity .

Did the CAN low circuit test open?

Go to Step 13

Go to Step 6

6

Test for greater than 50 ohms between the CAN high and CAN low circuits. Refer to Troubleshooting with a Digital Multimeter .

Is the resistance greater than 50 ohms?

Go to Step 7

Go to Step 16

7

Probe the CAN high circuit with a test lamp connected to battery voltage. Refer to Troubleshooting with a Test Lamp .

Does the test lamp illuminate?

Go to Step 15

Go to Step 8

8

  1. Turn ON the ignition.
  2. Probe the CAN high circuit with a test lamp connected to a good ground. Refer to Troubleshooting with a Test Lamp .

Does the test lamp illuminate?

Go to Step 14

Go to Step 9

9

Probe the CAN low circuit with a test lamp connected to a good ground.

Does the test lamp illuminate?

Go to Step 18

Go to Step 10

10

Probe the CAN low circuit with a test lamp connected to the battery voltage.

Does the test lamp illuminate?

Go to Step 17

Go to Step 11

11

Test for an intermittent or poor connection at the TCM. Repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs .

Did you find and correct the condition?

Go to Step 20

Go to Step 19

12

Repair the open in the CAN high circuit between the TCM and the CAN splice. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

13

Repair the open in the CAN low between the TCM and the CAN splice circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

14

Repair the short to voltage in the CAN high circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

15

Repair the short to ground in the CAN high circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

16

Repair the short across the CAN high and CAN low circuits. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

17

Repair the short to ground in the CAN low circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

18

Repair the short to voltage in the CAN low circuit. Refer to Wiring Repairs .

Did you complete the repair?

Go to Step 20

--

19

Replace the TCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 20

--

20

  1. Use a scan tool to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text.

Does the DTC run and pass?

Go to Step 21

Go to Step 2

21

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK