SPN 788 - FMI 2 (Diagnostic Code: Transmission Gear Sensor - Erratic, Intermittent, or Incorrect)

Fracturing Transmission TH48-E80

Figure 1.1 - Transmission Position Sensor

MID 027 CID 0700 FMI 02

Conditions Which Generate This Code:

This diagnostic code is recorded when the ECM reads erratic data from the position sensor.

Possible causes of this Diagnostic Code are listed:

Wire 126-PK (Pink), 721-BR (Brown), 722-WH (White), 723-OR (Orange), 724-YL (Yellow), 725-GN (Green) or 726-BU (Blue) are open or shorted to ground between the ECM and the Transmission Position Sensor.
Wire 231-BK (Black) is open between the Transmission Position Sensor and frame ground.
Wire J765-BU (Blue) is open between the Transmission Position Sensor and frame ground.
The Transmission Position Sensor has failed.
The ECM has failed.

Ensure that the Diagnostic Code for the position sensor is active.

Figure 1.2 - Transmission Speed Sensor Schematic

System Response:

The Transmission will not shift properly.

Test Step 1. CHECK THE GROUND CIRCUIT

A. Turn the key start switch and the disconnect switch to the OFF position.

B. Measure the resistance from contact 10 (231-BK/Black) to frame ground.

Expected Result: The resistance that is measured is less than 5 ohms.

Results:

OK - The resistance that is measured is less than 5 ohms. Proceed to Test Step 2.

Not OK - The resistance that is measured is greater than 5 ohms. There is an open in the ground circuit (Repair the harness or replace the harness. Verify that the repair corrects the problem).

Test Step 2. CHECK THE BATTERY VOLTAGE.

A. Disconnect the machine harness from the switch.

B. Turn the key start switch and the disconnect switch to the ON position.

C. At the machine harness connector for the Transmission Position Sensor, measure the voltage from connector contact 4 wire (126-PK/Pink) to frame ground.

Expected Result: The voltage that is measured is approximately (+) Battery.

Results:

OK - The voltage that is measured is approximately (+) Battery. Proceed to Test Step 3.

Not OK - The voltage that is measured is NOT approximately (+) Battery. The harness has failed (Repair the harness or replace the harness).

Test Step 3. CHECK THE HARNESS

A. Use the ET in order to view the status of the Transmission Position Sensor.

Figure 1.3 - Actual Gear (Abnormal/Incorrect Data)

B. Replace the Transmission Position Sensor with CAT P/N 338-1462 (Refer to Figure 1.1 above).

→Remove cover/gasket and bolts (Transmission Hyd Control).

Figure 1.4 - Transmission Hyd Control

→Remove strap-cable.

→Remove the Transmission Position Sensor and connector.

Figure 1.5 - Transmission Position Sensor (Remove)

Note: The ET status screen 8 shows all of these wires. The ground verify (pin 1) should always be grounded. Two other wires should always be grounded in order to indicate the current gear selection. Therefore, three wires will be grounded at the same time and three wires will be open.

Figure 1.6 - Actual Gear (Neutral)

Figure 1.7 - Actual Gear (Park-Stall)

Figure 1.8 - Actual Gear (1F)

Figure 1.9 - Actual Gear (7F)

Caterpillar ET Software: Online Licensing

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