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Low line pressure problems auto awd transmission 91 legacy

Front wheels slip delayed engagement

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12 replies to this topic

#1 mikaleda

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Posted 15 June 2013 - 09:16 AM

Okay I have been dealing with this car a little trying to help my mom out with it.
When we first got it the trans would pause going into gear and especially shifting from drive to reverse, it would slip going into gear and the awd would take a minuet to engage letting the front wheels slip. I found out about a TSB in 91 auto trans for clogging radiator coolers and a magnetic filter that goes inline to the trans cooler. We replaced the filter and the radiator (since the old rad blew up) and it started shifting better and there was no delay in engagement. Now a little ways down the road It's showing the same symptoms again, delayed engagement mainly.

I'm just wondering if the trans needs to be drained and filled (yes I know to do it three times with a drive cycle between drains) before I put another new filter inline and if I should replace the internal screen filter while I'm at it.

Also is it possible the low line pressure could have caused the awd slip when we first got it? It doesn't seem to do it anymore, but I haven't really driven it in slick conditions yet. Or is the rear clutch packs going out clogging up the filters?

#2 CNY_Dave

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Posted 15 June 2013 - 10:51 AM

The AWD is engaged by the pressurized trans fluid, so it's feasible.



#3 NorthWet

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Posted 15 June 2013 - 12:40 PM

First, let's try to get some clarity...

 

Is there a reason that you think that the line pressure is low?  Has someone hooked a gauge to the pressure port, or is this best-judgement based on symptoms?

 

I would assume that you would have already mentioned this if true, but are you getting an error from the TCU (blinking "POWER" light when you turn on the ignition)? Solenoid-A, IIRC, modulates the line pressure and commonly throws a code.

 

The inline "filter" (my local dealership said that it is a screen to catch coarse particles, and that it was a life-time part) is in the return path from the torque converter to the fluid pan.  It would seem to me that any affect it would have on line pressure would be to cause a mild increase rather than a decrease.

 

Replacing the filter screen in the pan is a lot of work and not likely to give you much reward... unless you find something else, such as a leaky o-ring or transfer pipe.  (The pan is pretty heavy and has lots of bolts; R&R'ing the valve body is even less fun.)



#4 CNY_Dave

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Posted 15 June 2013 - 12:58 PM

I read somewhere (here? outback.org?) people had a problem with some aftermarket replacement external filters in the cooler circuit, and it did cause a problem that sounds a bit like this. It might not be low line pressure, but a similar symptom.



#5 mikaleda

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Posted 15 June 2013 - 08:10 PM

I had that thought, I did use an aftermarket filter, it was for a Honda. Probably would be best to do a drain and flush and put an OEM Subaru in line filter in before I start dropping the pan.

To north wet, the AT temp light has never come on other than the initial startup. I would say that the awd problem is being caused by the same thing that is causing the delayed engagement problem.

No I haven't Manuely checked the line pressure, I should have stated it differently I shouldn't have said low line pressure I should have said lowerd hydraulic pressure. What I believe is happining isn't happening in the TC I think it has to do with the valve body as when the old inline fliter was clogged it started leaking out of the hoses and the engagement issue coupled gears slipping in engagement and I assumed this was because of low hydraulic pressure and might also cause the hydraulically controlled awd clutch pack to slip.

You did mentione something I find interesting, the TSB is for another screen filter in line? The main filter is a screen and is supposed be a "lifetime" filter, but I have seen threads on here with these screens were clogged with friction material after 260k. This car has close to 270k miles on it and I would say anything can clog with Time and abuse, this car had not been taken care of when we got it the struts were blown, it had a brake drag, and it had a set of matching bald tires.

What I would like to know is this inline filter supposed to be a screen filter? If so I know what part of the problem is, the parts store told me that the TSB was for "large amounts of metal material clogging the radiator transmission cooler" and was "a magnetic filter". Now in wondering if the parts computer has a 91 Honda mixed up with the 91 Subaru legacy since I got a genuine Honda part and have never heard of high amounts of metal shavings in the 4eat

Edited by mikaleda, 15 June 2013 - 08:11 PM.


#6 NorthWet

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Posted 15 June 2013 - 08:36 PM

I think that your use of "line pressure" is proper, as I think that is what the FSM refers to as the regulated (by duty-A solenoid) hydraulic pressure.  (Sorry, I'm not in a situation where I can verify the term.)  Regardless, I understood what you meant.

 

My recollection is that it is the POWER light that blinks rather than the AT Temp, but I might be confusing the EA version of the "4EAT".  My memory is VERY suspect at the moment, though.

 

Take the accuracy of my inline filter "info" with a grain of salt: It was what a parts person at the dealership told me, and I am not too impressed with this dealership.  However, the explanation did make sense.  The guy said that the issue is caused by friction material from the TC's lockup clutch shedding into the ATF in the TC, then flowing out into the cooler lines where the passages in the in-radiator cooler are narrow enough that the debris tends to get stuck.

 

(The rest of the story is why I called:  I was trying to get a new filter under the TSB to replace one I suspected of being plugged.  He claimed that they were just debris screens and meant as a one-time, lifetime install.  So, a possible conflict of interest on his part.) 

 

Edit:  I don't suppose that you still have the old filter to do a post-mortem on, do you?


Edited by NorthWet, 15 June 2013 - 08:38 PM.


#7 mikaleda

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Posted 15 June 2013 - 09:17 PM

No, I wish I did open it up and look now. I'm wondering if I might be able to look up the acctuall TSB and get info from there myself. They might explain more about the filters and it might shed some light on the subject. Does anyone know of how to find a TSB?

And also the power light has never blinked at me either and I'm a gauge watcher when I test drive a vehicle.
When we first got it I did a fairly thougrogh test drive with it for 18 miles, on highway, dirt road, and off road mixed driving and that was how I found out about the awd slip

That being said my mom (who drives the car mostly) is fairly good at watching for unusual gauge readings, but she might not have noticed something like the power light blinking. I really don't think this is a elctrical problem, I think it's mechanical so I'm going to start with a drain fill procedure (inspecting the fluid) and an oem inline filter and see what happens. If the fluid is good looking with little or no friction material and the delayed engagement goes away and the awd slip is still there, then I will see if I can pull any codes from the TCU. If that fails to release any codes or the delayed engagement doesnt go away I am planning on pulling the internal screen filter and replacing it and cutting the old one open and inspecting it, that should tell me a lot, if it is clean it would lead me to believe a malfunctioning duty c or wiring to the duty c if its clogged with friction material that would lead me to believe that the awd clutch packs need to be replaced.

Edited by mikaleda, 15 June 2013 - 09:30 PM.


#8 NorthWet

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Posted 16 June 2013 - 12:36 AM

One thing you might verify is that the pan bottom has not been dented upwards under the pickup.  Another member usually suggests checking this, as the clearance between the pickup and pan is minimal.



#9 presslab

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Posted 16 June 2013 - 09:13 AM

The TSB filter is in addition to the screen inside the transmission.  It has a pleated paper element.  I don't believe it's magnetic; there's already a magnet in the pan.  There are aftermarket filters called "Magnefine" that have a magnet, but these are not factory filters.

 

The cooler and external filter is not on the main "line pressure" hydraulic circuit, it's on the torque converter return.  Basically if this filter clogs up your transmission will overheat, which could damage a bunch of stuff, primarily the planetary gearsets.  But it won't be the primary cause of delayed engagement, even with a stopped up cooler it should still shift fine until everything burns up.

 

There are dozens of different hydraulic circuits in the transmission, of which the line pressure is only one.  The valves in the transmission can compensate for some wear, so I don't necessarily think line pressure is the problem.  Often there are leaks on the "lathe cut" seals causing loss of pressure (or misdirected pressure) to actuate the clutches.

 

The TCU on these years is fairly dumb and will only report obvious errors like a burned out solenoid.



#10 mikaleda

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Posted 16 June 2013 - 09:15 AM

I did look at the trans pan when I was doing the oil change the other day and it looked good

#11 mikaleda

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Posted 16 June 2013 - 09:19 AM

The TSB filter is in addition to the screen inside the transmission.  It has a pleated paper element.  I don't believe it's magnetic; there's already a magnet in the pan.  There are aftermarket filters called "Magnefine" that have a magnet, but these are not factory filters.
 
The cooler and external filter is not on the main "line pressure" hydraulic circuit, it's on the torque converter return.  Basically if this filter clogs up your transmission will overheat, which could damage a bunch of stuff, primarily the planetary gearsets.  But it won't be the primary cause of delayed engagement, even with a stopped up cooler it should still shift fine until everything burns up.
 
There are dozens of different hydraulic circuits in the transmission, of which the line pressure is only one.  The valves in the transmission can compensate for some wear, so I don't necessarily think line pressure is the problem.  Often there are leaks on the "lathe cut" seals causing loss of pressure (or misdirected pressure) to actuate the clutches.
 
The TCU on these years is fairly dumb and will only report obvious errors like a burned out solenoid.

The TCU should tell me If it's overheating and it hasn't, but when I changed the filter the first time it did solve my problem for a while so I suspect the problem is here.

If its blocking the flow from the torque converter it would decrease the pressure in other places as well I would assume since the pump is connected to the TC

#12 presslab

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Posted 16 June 2013 - 09:37 AM

The TCU will tell you if the fluid at the valve body has overheated, where the sensor is located.  It doesn't know the temperature at the exit of the cooler.  But my point is that any overheating in the past could have damaged things, and everything is not necessarily fine just because it's not overheating now.

 

I assume you changed the fluid the same time you changed the filter.  Changing the fluid could of course would have a marked effect on everything.

 

The output of the oil cooler only lubricates the gears and spragues and then dumps into the pan.  There are detailed diagrams in the service manual.



#13 mikaleda

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Posted 17 June 2013 - 09:27 AM

The TCU will tell you if the fluid at the valve body has overheated, where the sensor is located.  It doesn't know the temperature at the exit of the cooler.  But my point is that any overheating in the past could have damaged things, and everything is not necessarily fine just because it's not overheating now.
 
I assume you changed the fluid the same time you changed the filter.  Changing the fluid could of course would have a marked effect on everything.
 
The output of the oil cooler only lubricates the gears and spragues and then dumps into the pan.  There are detailed diagrams in the service manual.


Actually I didn't change the fluid when I changed the filter (I know I should have) I just didn't think about it. If the trans cooler only does what you are saying then I wonder why when the filter was changed the trans stopped slipping, and the delayed engagement quit




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