If you encounter a late-model D4AT-equipped ­vehicle with an erratic shifting complaint, and ­possibly a DTC P0712 in memory, there may be a poor connection in the transmission wiring ­harness. There have been cases of open circuits or high resistance isolated to the white plastic pass-through section of the transmission harness or the harness itself. It’s important to note that the P0712 may not be set when the condition occurs.

One way to check for trouble in the transmission harness is to road-test the car with the SSMIII connected and data for the transmission fluid temperature (TFT) sensor selected. It’s best to check this after a cold start. Normally, the temperature should read at or close to the ambient temperature when the vehicle is started cold.

While road-testing, if you see the TFT readings are way out of line or fluctuating up and down as the ­vehicle comes up to temperature, it’s likely that the transmission harness would be a good place to start your troubleshooting.

Service Procedure:

1. Disconnect the transmission harness connector at the bulkhead connector. “Wiggle test” the harness and the harness pass-through while monitoring the resistance of the TFT circuit with a DVOM. Pin locations and connector numbers will be different, ­depending on the model. Always refer to the applicable Subaru service manual’s wiring diagram for circuit identification. If the resistance changes while wiggling the harness, further investigation of the ­circuit should be done.

2. SSMIII — You can also perform the wiggle test while monitoring the transmission fluid ­temperature with the SSMIII. Look for any ­fluctuations in the transmission fluid temp while wiggling the harness at the pass-through. In some cases, it may be necessary to remove the ­transmission pan and split-half each leg of the TFT sensor circuit individually. It should also be noted that depending on the condition of the ­circuit, a check engine light may or may not be triggered.

In another case, the customer stated, the ­“transmission has a harsh shift and is slipping.” In this ­instance, the excessive resistance in the transmission harness caused an incorrect TFT input to the TCU. This resulted in the TCU never “seeing” a fluid temperature above 68°, therefore, the operational characteristics were never adjusted accordingly once the fluid reached normal ­operating temperature.

This incorrect input resulted in the customer’s ­concern, but did not set a check engine light. By recording transmission data with the SSMIII while duplicating the concern, it was easy to identify a fault with the TFT circuit. Further investigation revealed the harness problem.

The above are not the only causes of DTC P0712. It’s ­important to use proper diagnostic methods to fully check the entire circuit from the TCU to the TFT ­sensor.

Courtesy of Mitchell 1.