The Subaru mystery burn - Transmission Digest

The Subaru mystery burn

The Subaru TR580 transmission is known for having torque converter clutch solenoid failures. An example of this can be seen in Figures 1 (above) and 2 (below).

Figure 2.
Figure 2.

The solenoid resistance should be between 10 to 13.5 ohms. The resistance right at the solenoid itself on the valve body is close to 38 ohms. And as most techs know, this solenoid is not available separately from the dealer, nor are any of the other solenoids available separately for purchase. The entire valve body has to be purchased for a single solenoid repair. Companies like Rostra and Dorman make this TCC solenoid available separately, giving the shop an option for an affordable fix. 

Although the TCC solenoid is the solenoid with the most frequent failures, all the other solenoids are also prone to fail as well. In fact, Subaru issued a bulletin which provides diagnostic steps that conclude with fixing harness issues or replacing the complete solenoid assembly (Bulletin no. 16-131-20R).

By unplugging the vehicle harness from the transmission harness, resistance checks from the T4 connector can be made to isolate the problem, whether it is internal or external. You can see an example of this in Figure 1. 

A T4 connector view and terminal location is provided in Figure 3 with a non-turbo terminal ID chart in Figure 4. This chart provides both circuit ID as well as solenoid and TFT resistance specifications which can be used to make these checks.

Figure 3.
Figure 3.
Figure 4.
Figure 4.

An example of these issues

Sal at Transmission Pros recently had a 2015 2.5L DOHC Forester come to the shop with code P0970 for a Transfer Clutch Solenoid problem along with an illuminated Trans Temp Light. 

Since this wasn’t Sal’s first rodeo in dealing with Subaru solenoid codes, after doing a visual inspection of the harness and connectors, he put a new valve body assembly in as his repair. But this time, it didn’t cure the problem. 

With that, he decided to check the Transfer Clutch Solenoid circuit from the TCM to the transmission. To do this, Sal removed the TCM to gain easier access to its connector. In doing so, he heard a rattling sound coming from inside the TCM. The circuit board was then removed from the housing for inspection (Figure 5). It was at this time that a component of the circuit board fell out as you can see in Figure 6. 

Figure 5.
Figure 5.
Figure 6.
Figure 6.

Figure 7 shows the location of where this component should be on the circuit board. 

Figure 7.
Figure 7.

All that is known of this part is that it is a 0.1 Ohms 5% wire-wound cement-filled ceramic 1- or 2-watt resistor—and that it is very hard to burn, so something else burned affecting this resistor. This is a bit scary knowing that a new TCM needs to be installed. After all reasonable checks were made to ensure that a new TCM would not suffer the same burn problem, it was installed, and the car was delivered. At the time of writing, it seems this mystery burn problem has been resolved, as the vehicle is still on the road.

Read more stories from our Technically Speaking column series here.

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