3126B and 3126E On-highway Engines Caterpillar

Use this procedure under the following situation:

Use this procedure to determine if the AT/MT/HT transmission interface relay (output #7) is operating correctly.

The following background information is related to this procedure:

Allison AT/MT/HT transmissions require an input from the ECM to regulate the transmission shifting. The ECM monitors various operating parameters in order to determine if the transmission should use closed throttle shift modulation or full throttle shift modulation. Programming the "Transmission Style" parameter to "AT/MT/HT Option1", "AT/MT/HT Option2", "AT/MT/HT Option3", or "AT/MT/HT Option4" activates ECM Output #7 in order to control a shift interface relay that is connected to the Allison transmission.

Transmission Style

For engines that have a Full Feature ECM, the "Transmission Style" must be programmed to one of the AT/MT/HT options in order for Output #7 to control the AT/MT/HT transmission relay. For the "Basic (GM) System", Output #7 is always available.

Note: The ECM does not communicate with the Allison AT/MT/HT Transmission through a data link.

Cruise Control or Vehicle Speed Limiting is NOT active.

The AT/MT/HT transmission interface relay will be disabled if any of the following conditions occur:

• The engine is not running.

• The transmission is in neutral and the speed of the vehicle is less than 24 km/h (15 mph) (NOV99 and older Personality Modules).

• The accelerator pedal position is less than 65 percent of the programmed value. Refer to the note below.

Note: JUL99 and newer Personality Modules have a programmable "AT/MT/HT Part Throttle Shift Speed" parameter that provides three different shift modulation settings for the transmission. The "Low" setting will disable the relay below 65 percent throttle, the "Medium" setting below 55 percent throttle, and the "High" setting below 45 percent throttle.

The AT/MT/HT transmission interface relay will be enabled under any the following conditions:

• The engine is running.

• The transmission is in gear or the speed of the vehicle is greater than 24 km/h (15 mph) (NOV99 and older Personality Modules).

• The accelerator pedal position is greater than 80 percent of the programmed value. Refer to the note below.

Note: JUL99 and newer Personality Modules have a programmable "AT/MT/HT Part Throttle Shift Speed" parameter that provides three different shift modulation settings for the transmission. The "Low" setting will enable the relay above 80 percent throttle, the "Medium" setting above 70 percent throttle, and the "High" setting above 60 percent throttle.

Cruise Control or Vehicle Speed Limiting is active.

The AT/MT/HT transmission interface relay will be disabled when all of the following conditions occur:

• The speed of the vehicle is greater than 2 km/h (1 mph) of the cruise set speed or the vehicle speed limit.

• The load factor is less than 75 percent.

• The engine speed is greater than 2000 rpm.

The transmission interface relay will be enabled when the load factor is greater than 98 percent.

Illustration 1	g00729187
Schematic for the AT/MT/HT Transmission Relay

Illustration 2	g00729188
ECM terminal

Test Step 1. Check Electrical Connectors and Wiring.

1. Thoroughly inspect ECM connectors J1/P1 and the firewall bulkhead connector. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.

2. Perform a 45 N (10 lb) pull test on each of the wires that go to P1:20 or P1:12.

3. Check the ECM connector (allen head screw) for the proper torque of 6.0 N·m (55 lb in).

4. Check the harness and wiring for abrasion and pinch points from the sensor to the ECM.

Expected Result:

All connectors, pins and sockets should be completely coupled and/or inserted and the harness and wiring should be free of corrosion, abrasion or pinch points.

Results:

• OK - Proceed to Test Step 2.

• Not OK -

  Repair: Perform the following repair:Repair the connectors or wiring and/or replace the connectors or wiring. Ensure that all of the seals are properly in place and ensure that the connectors are completely coupled.Verify that the repair eliminates the problem.

  STOP

Test Step 2. Use ET to Check the Accelerator Pedal Position Sensor.

1. Ensure that the cruise control switch is off.

2. Connect ET to the cab data link connector.

3. Turn the ignition key switch to the ON position.

4. Use the following procedure to check the accelerator pedal position sensor:

   1. Monitor the duty cycle of the throttle sensor on ET. Access the display screens in the following order:
      • "Service"

      • "Calibrations"

      • "Throttle Position"

   2. While the duty cycle is being monitored on ET, depress the accelerator pedal and release the accelerator pedal. Repeat this action several times.

Expected Result:

The duty cycle of the accelerator pedal position sensor is between 3 percent and 100 percent.

Results:

• OK - Proceed to Test Step 3.

• Not OK -

  Repair: Perform the following diagnostic procedure:Troubleshooting, "Accelerator Pedal (Throttle) Position Sensor Circuit - Test"

  STOP

Test Step 3. Use ET to Check the Solenoids and the Relays.

1. Connect ET to the cab data link connector.

2. Turn the ignition key switch to the ON position.

3. Access the "AT/MT/HT XMission Interface Test" through the following menus:
   • "Diagnostics"

   • "Diagnostic Tests"

   • "Special Tests"

   Note: The "Transmission Style" must be programmed to "AT/MT/HT Option 1", "AT/MT/HT Option 2", "AT/MT/HT Option 3", or "AT/MT/HT Option 4" for the "AT/MT/HT XMission Interface Test" to be available. This will allow output 7 to control the AT/MT/HT transmission relay.

4. Begin the "AT/MT/HT XMission Interface Test" while you listen for the solenoids to click. You may need to be closer to the transmission in order to hear the click of the relay.

Expected Result:

The relay activates when the test is enabled.

Results:

• Yes - The ECM is functioning properly at this time.STOP

• No - Proceed to Test Step 4.

Test Step 4. Use ET to Check the ECM.

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Illustration 3	g00770060
Breakout T

1. Turn the ignition key switch to the OFF position.

2. Disconnect the ECM engine harness connector J1/P1.

3. Connect a breakout T to the ECM connector J1/P1.

4. Connect a voltage test lamp to terminal 20 (Output 7) and terminal 65 (-Battery) of the breakout T.

5. Turn the ignition key switch to the ON position.

6. Access the "AT/MT/HT XMission Interface Test" through the following menus:
   • "Diagnostics"

   • "Diagnostic Tests"

   • "Special Tests"

   Note: The "Transmission Style" must be programmed to "AT/MT/HT Option 1", "AT/MT/HT Option 2", "AT/MT/HT Option 3", or "AT/MT/HT Option 4" for the "AT/MT/HT XMission Interface Test" to be available. This will allow output 7 to control the AT/MT/HT transmission relay.

7. While the voltage test lamp is being observed, cycle the special test on and off.

8. Stop the special test.

Expected Result:

The voltage test lamp behaves in the following manner:

• The voltage test lamp turns on when the test is active.

• The voltage test lamp turns off when the test is not active.

Results:

• Yes - The ECM is OK. There is a problem in the harness.

  Repair: Perform the following repair:Repair the engine harness or replace the engine harness. Verify that the repair eliminates the problem.

  STOP

• No - Temporarily connect a test ECM. Repeat this test step. If the problem is resolved with the test ECM, reconnect the suspect ECM. If the problem returns with the suspect ECM, replace the ECM.

  Repair: Perform the following repair:

  1. Temporarily connect a test ECM.

  2. Remove all jumpers and replace all connectors.

  3. Recheck the system for active diagnostic codes.

  4. Repeat the test step.

  5. If the problem is resolved with the test ECM, reconnect the suspect ECM.

  6. If the problem returns with the suspect ECM, replace the ECM.

  7. Verify that the repair eliminates the problem.

  STOP