[plain OM]

The attached photos are supposedly showing the change in the extension housing when the 4EAT Phase II was modified to "direct control", which included the relocation of the AWD solenoid and the "reversal" of the control programming. The later tail section appears to have different "ribbing".

I'm not sure if the change was coincident with the introduction of MY2004 cars, or actually effective with MY2005 for the Outback, although I guess that's moot at this time. (Subaru often phases in such changes, with some models getting the newer version earlier.)

The valve bodies certainly would not be interchangeable.

ntippet's waveform explanation of duty cycle % is excellent!

With the 4EAT that has the multi-plate clutch transfer system, the rear VSS is not necessary as far as rear wheel spin is concerned because the rear drive (shaft) cannot turn faster than the drive to the front differential -- it's simply impossible if nothing's broken.

This does not take into account what one wheelmight do as the front and rear differentials will allow one wheel to speed up while the opposite wheel slows down. However, the VSSs are monitoring the drive shaft speeds, not individual wheel speeds.

The rear VSS is used as a reference; the TCM compares the front and rear VSS signals and should adjust the duty cycle if the front VSS is showing a higher speed than the rear VSS. The basic logic of the AWD system is that the two drives be turning at essentially the same speed and if they're not, it increases the clutch pressure to effectively lock the drives together.

Note: The TCU also uses "torque" as an input to it's duty cycle control. In the FreeSSM post that ntippet linked, the third chart shows the front wheel speed (VSS) ramping up while the rear wheel speed (VSS)remains low, yet the duty cycle does not peak out. However, in this particular case, as I recall, the front wheels were intentionally allowed to spin and with the rear drive effectively disconnected, there was little torque being developed in the drive train. Later on we examined the impact of torque as represented by the difference ("delta") between engine rpm and torque converter turbine rpm. This appeared to be a significant, if not overriding determinant of duty cycle.

 

[plain OM]

I have the older style, and have confirmed I have the older valve body.

I presume you're referring to the replacement transmission.

Looking at the photo, I'm wondering: Where's the spin-on filter mount? It looks as if there's a plug over it. Or is it plugged temporarily in the picture?

 

[plain OM]

Ah yes, I didn't remember it's with the H6.

 

[plain OM]

Yes, now that you mention it, I also recall there were some 2.5s in 2004/5 with the filter in the LF fender. It had me way off in a thread some time back. Sorry for the diversion.

 

[plain OM]

I just noticed something else- the FWD fuse does NOT connect the TCU pin to +12V, it connects it to ground.

That's correct. It connects the pin to ground. The fuse is described in the FSM as the 'FWD [or AWD] switch' and is shown connecting to ground in the wiring diagrams.

 

[plain OM]

I also have a set of voltage measurements to compare against (at some point) for all gears, at idle, mid-throttle, and full-throttle, measured AC and DC.

This is weird. Both transmissions appear to be the same, i.e, earlier Phase II, yet they respond in reverse to the duty cycle signal from the TCM. Something just doesn't ring right.

In 2003 and 2004, two 4-speed transmissions were used; one was the 4EAT with the multi-plate clutch transfer system, and the other was the 4EAT with the VTD transfer differential, which was used with "VDC" cars. This has me wondering if it's possible that the VTD version might have used a reversed duty cycle signal-to-clutch pressure relationship all along, and could the replacement tranny actually be different at the back end while still appearing the same in terms of the valve body? Not likely, but what else could explain the reversed response?

Be interested in the comparison between the Voltages measured on your car compared to others.

 

[plain OM]

I obviously misunderstood when/how the change to the later version of the 4EAT was implemented.

It now appears that there were (at least) two changes related to the AWD control.

The earlier change would have been the relationship between the throttle position, duty cycle, and clutch pressure, which is the topic of discussion here. This seems to have been introduced into the 4EAT Phase II for MY2004, and could be the change noted in the referenced TSB (see below, tnx Peaty). But the solenoid/transfer valve remained in the original location in the extension case.

Subsequently, the location of the AWD solenoid was apparently moved from the extension case to the valve body, at least that's according to Subaru documentation. This occurred at the same time the use of "accumulators" was eliminated. The accumulators were used to smooth shifting; the newer system makes use of more precise control of the shift solenoids. Subaru refers to the newer approach as "direct control". This might have been coincident with MY2005 Outback/Legacy production, and would have been reflected in the different valve body and tail section exterior.

I had previously thought the two changes were at the same time.

Nevertheless, it's interesting that according to my 03 FSM, the MPT 4EAT had the inverse relationship, that is, the higher the throttle, the lower the duty cycle, and the higher the pressure. (See below). However, in 03, the VTD 4EAT already had the proportional relationship, that is, the higher the throttle, the higher the duty cycle and the higher the pressure, which is what ntippet clearly has on the 2004 MPT 4EAT.

In regard to swapping the valve bodies, I don't think this will work because I believe the AWD solenoid/transfer valve might still be the tail section area.

 

[plain OM]

I was referring to the small valve assembly that accompanies the transfer solenoid, not the entire transmission valve body.

My misunderstanding. You're right. Take the AWD solenoid/valve from the tail section of old tranny and put it in the new tranny. That way, if the rest of the piping is the same, the valving will respond to the old (2003) TCU's duty cycle signal (inverse throttle/duty cycle), and it should work properly. But that does mean taking down the tail section, which there's some reluctance to do at this time.

Okay, back on track.