- Subject: Hydraulic information and diagnosis
- Unit: Subaru 4AT Phase 2
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
The first installment of this article, published in the July 2006 issue of Transmission Digest, provided information regarding leaf-spring locations, solenoid identification and specifications as well as the transfer-clutch solenoid update and its operation.
Parsing hydraulic information is the dominant aspect of Part 2 on Subaru’s Phase 2 transmission, which is used in the Baja, Forester, Impreza, Legacy and Outback models behind either a four- or six-cylinder engine. Transmissions used with four-cylinder engines will have an external filter on the case, as shown in Figure 1. Six-cylinder vehicles have the filter in the left-front fender well.
Figure 2 provides a clutch-application chart. Figures 3, 4 and 5 provide pressure-tap locations and their identification. As you can see, you have a tap for every clutch pack in this transmission in addition to TCC apply, TCC release and main line pressure – a diagnostician’s dream! And to make testing even sweeter, the manufacturer provided an access port through the floorboard near the gas pedal (see Figure 6).
Line-pressure specs in the D position are 44 to 60 psi at idle and 164 to 189 at stall. In the R position the specs are 73 to 97 psi at idle and 220 to 249 at stall. If line pressure checks correctly, you then can compare it with a specific clutch pressure. There should be a difference of no more than 8 to 10 psi. In other words, say you have a delayed engagement into Drive but Reverse is OK. Line pressure at idle is around 52 psi. Now put a gauge on the low clutch. When you place the selector lever into Drive, low-clutch pressure should be as close to line pressure (52 psi) as possible. If it reads below 40 psi, a leak in the low-clutch circuit is causing the delay. These units have had issues with the low-clutch piston seals wearing down, causing this type of problem. Some have been so bad they have produced a complaint of no forward movement.
After a rebuild, the vehicle also may exhibit a delayed engagement first thing in the morning. Leaving the spring and ball out of the front cooling line can cause enough of a converter drain-back to produce this complaint (see figures 7 and 8).
Another area that could cause converter-fill issues and lubrication problems is the input shaft, as there is a plastic plug inside the shaft that separates the converter and lubrication circuits (see figures 9 and 10). It would be a good idea to check the plug, especially if the transmission came into the shop overheating because of a low-fluid condition or because the converter clutch had stopped applying. This type of heat could cause the plastic plug to distort and not seal properly.
Another lube passage that you should inspect is in the oil-pump cover (see Figure 11). Under the lube orifice is a spring-loaded ball (see Figure 12) that could get jammed with debris, blocking the lube passage and causing repeated front-lubrication failure.
The valve body contains a pressure-modifier valve that has three adjustable steps on the end plug, as shown in figures 13 and 14. In all the valve bodies I have seen, the factory has had the plug set on the middle step. If you go one higher you will firm up the shifts; go one lower and you will soften the shifts.
Since this unit replaced the band with a 2/4-clutch pack, be sure to replace the seal between the valve body and case (see Figure 15) during overhaul even if the old seal looks good, as a new seal will provide a better crush and be less likely to leak.
Another area to inspect is the outer seal on the balance piston in the transfer-clutch drum (see Figure 16). This piston holds centrifugal head oil that is used to neutralize residual oil behind the transfer-clutch piston, giving great control over the application of the piston when the solenoid is controlled to put the clutch on. It also helps prevent the clutch from being centrifugally dragged on and causing premature wear.
For additional diagnostic assistance, use figures 17 and 18 for checkball function and location, and Figure 19 for identification of case passages.
The following is helpful information that Subaru has issued:
Delayed engagement or shudder felt during shift into Reverse or Drive
Normal operation takes 1.5 seconds to engage an internal clutch or clutches. When the driver shifts into Reverse or Drive and applies the accelerator too quickly, delayed movement or a shudder/judder will occur. To determine whether there is an internal problem perform a time-lag test. If the average is less than 1.5 seconds the unit is operating normally. If it is more than 1.5 seconds, an internal problem exists.
Shock felt during light acceleration when the converter clutch is applied
When the driver tries to lightly accelerate the vehicle while driving at a constant speed in 4th gear with lockup engaged, a slight shock may be felt through the body of the vehicle. The shock results from the small clearances in the drivetrain gears, axle spline etc. If the lockup clutch is not engaged, the fluid coupling in the torque converter absorbs the shock. Under certain conditions, the driver also can feel this same shock when activating cruise control.
Transmission delays downshifting during low to middle acceleration.
The driver wants to accelerate and starts applying the throttle, but the transmission will not downshift to a lower gear until almost full throttle.
The TCM’s normal shift logic (shift map) that controls gear selection is trying to keep the transmission in the highest gear possible for fuel economy. It directly corresponds to throttle opening, vehicle speed, engine speed and gear selection. The TCM monitors not only the position of the TPS but also how fast the accelerator pedal was depressed.
Depending on vehicle speed, if the driver pushes the accelerator pedal slowly – even to the floor – the TCM may not downshift the transmission. If, however, the pedal is depressed quickly, the transmission will certainly downshift into whatever gear the TCM deems appropriate.
- This shift logic gives the driver some ability to operate the vehicle on the basis of either power or economy.
- Another consideration is in regard to shift change from one clutch to another. If a clutch is turned on or off too soon it will cause a harsh shift and also could cause premature wearing of the clutches. So this logic provides a balance of shift feel and wear characteristics.
- Fluid temperature is also a consideration. Cooler, thicker fluid takes longer to move through a given passage than warmer fluid.
2nd-to-3rd shift flare after vehicle is parked
When the vehicle is started after being parked for a prolonged period – typically overnight – and the transmission upshifts into third gear for the first time, engine speed may flare slightly. This can be an intermittent condition depending on how the vehicle is positioned when parked, temperature of the transmission when parked and ambient temperature.
The shift flare occurs because the hydraulic circuit for the high clutch occasionally drains. When the transmission upshifts into 3rd gear for the first time, the hydraulic circuit must fill before it will apply the high clutch. The time needed to fill the circuit slightly delays the application, causing the flare. The transmission then will function normally for the rest of the driving cycle.
Clicking noise during shift from 2nd to 3rd
When the transmission shifts from 2nd to 3rd gear under light acceleration, the driver may hear a click from under the vehicle. Most customers will notice this noise only when they have a driver-side window open and are driving close to some structure that will reflect the noise back to the vehicle. The noise occurs when the 2-4 brake is released during a 2-3 upshift. At this time, the steel clutch plates generate a metallic clicking noise against the case grooves.