How To Replace the Duty B Solenoid aka Duty Solenoid B aka Lock Up Solenoid
Car: 1997 Legacy Outback Wagon 2.5L DOHC 4EAT Phase 1 Transmission with 207,xxx miles
Recently, the check engine light went on and I had a code of:
P0743 Torque Converter Clutch System Electrical
This was intermittent for a few days then was on all the time. The car drove normally except for the lack of Torque Converter lockup. When the TC fails to lock at highway speeds, engine RPM is higher than normal cutting into gas mileage and more heat is generated within the transmission. More heat is generally not good for transmission or ATF life.
After a few days, I got an additional code:
P0740 Torque Converter Clutch System Malfunction
Research indicated that the issue was probably with the Duty B Solenoid which receives a signal from the TCU and opens to allow ATF through to lock up the TC. To quote an Endwrench article:
"Duty Solenoid B is located on the lower valve body
next to the ATF temperature sensor. It operates the
lock-up clutch in 3 modes: ON, OFF and a gradual
ON/OFF control of the lock-up clutch during gear
shifting in order to reduce shift shock."
Further research revealed very little from which I surmise it is rare for this solenoid to fail. I could not find an instance where someone had actually replaced this part, though I'm sure it has happened.
For some unrelated work which I didn't want to do myself, I had my car into a local independent shop with a very experienced Subaru mechanic. He told me he had never seen P0743 or P0740 codes and had never replaced a Duty B Solenoid. Yikes.
Auto-transmission work is a scary thing for most DIYers, me included. I had no desire to tear into it. BUT, from the little documentation I found, it appeared that the Duty B Solenoid was accessible by merely dropping the pan, so I decided to forge ahead.
Attached below is an extract from the 1997 FSM showing the diagnosis steps. There are additional steps in the manual but they require equipment not normally available to a DIYer, so I left them out of the extract.
When checking the resistance of the solenoid with a multimeter, I could not get a consistent reading but all readings were well out of spec on the high side. So, I swallowed hard and ordered the somewhat expensive, non-returnable part (part# 31939AA052 $175 list). Since this is a rare failure, nobody stocks the part and I had to wait. It took one week from order to delivery, only one day of which was actual shipping transit time to my home. When I received the new solenoid, it tested steady at 13.9 Ohms ... spec is 9-17 Ohms, so good!
The actual replacement was easy. After draining and dropping the trans pan, the solenoid was easily accessible without having to remove any additional parts, not even the screen filter. I simply had to disconnect the wire connector, remove three bolts and loosen a fourth (all with 10mm heads) and pull the solenoid down out of its hole.
I always seem to have a problem with wire connectors, but this one was easy. Simply pinch and pull.
There are three bolts used to mount the solenoid, 2-28mm ones and a 33mm one. The 33mm one is the one farthest toward the front of the car. This same bolt is also used to hold a bracket and I needed to loosen the other bracket bolt to swing it out of the way and get the solenoid out. When installing the new solenoid, make sure the bracket mounting flange covers the solenoid mounting flange ... if you reverse them, the solenoid o-ring may not seal correctly in its hole.
28mm bolts: 8.3±1.1 ft-lb
33mm bolts: 5.8±0.7 ft-lb
ATF drain plug: 18.1±1.4 ft-lb
Oil pan bolts: 3.6±0.4 ft-lb
The only surprise I had was after dropping the pan. The wiring connector for the solenoid was out of its bracket and dangling down in the pan area. I have no idea if this contributed to the failure or not. I was unable to get the connector to remain in the bracket, so I wrapped the wire around one of the pipes to keep the connector as high as possible.
After that, it was simply a matter of putting the pan back on and pouring in the ATF. I had drained the ATF into a bucket with volume markings and it measured 5.5 quarts. Some ATF also ended up on the floor, so about 6 quarts in all.
In summary, although this is a rare failure, it is an easy DIY fix. I hope this write-up and photos will help the next person with the same problem.
P.S.: All codes are gone and the Torque Converter is locking up as it should on the highway. Success!