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My AWD Does Not Work, Sort Of


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#1 U R A BUS

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Posted 23 February 2010 - 08:53 PM

Well people, I sure hope someone can help me out. My AWD does not work completely. I have covered the following bases and the result is the same in each case. First of all checked for fuse installed (none), light is not on, on the dash. I have tried running it in low gear, by the way this is an automatic, I have disconected the plug at the transmission under the hood on the passengers side, after all this the result is as follows. When the car is first started the AWD works for about 30-60 seconds, then stops. I restart the car and again the AWD works for about 30-60 seconds, I turn the car off and restart and again the AWD works for about 30-60 seconds and I can keep doing this all night. If your wondering how I can tell all this, I live on an icy, snowy road and have people watching the rear wheels and it very clear to me driving when the AWD stops working. the car is a 1998 Legacy GT sedan. Any help for this problem is greatly appreciated. I am totally lost on this one.

#2 johnceggleston

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Posted 23 February 2010 - 10:56 PM

I have disconected the plug at the transmission under the hood on the passengers side, If your wondering how I can tell all this, I live on an icy, snowy road and have people watching the rear wheels and it very clear to me driving when the AWD stops working. the car is a 1998 Legacy GT sedan. Any help for this problem is greatly appreciated. I am totally lost on this one.


how did the car drive with the wiring harness unplugged.? i gues i forgot to mention that you need to drive tight turns on dry pavement to see if it has torque bind.

what do the observers report when they are watching the wheels on the snow and ice??

how do you know it is working for 60 seconds?? what changes?

#3 Bad Brian

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Posted 24 February 2010 - 12:32 AM

It seems that you misunderstood about the fuse under the hood BUT this does not sound like an issue testable by forcing the car into 2wd anyway

#4 eulogious

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Posted 24 February 2010 - 05:10 AM

Well, I can tell you this much, the reason the AWD works for a min or so and then quits is because the TCU errors out, and then stops the AWD from working, basically it's going into limp mode. There's a couple of things that you can try to check. I would check ALL the speed sensors, VSS1, VSS2, for sure. If either one of the VSS' are broke, the TCU can't calculate if there is any difference in the speed of the 2 sensors so that it can transfer more power to the rear, or to the front. The TCU needs both of those sensors working in order for the AWD to function properly. If one of them is broke, the TCU automatically puts the car in FWD mode so that you can drive it home. That's where I would check first. I find it really weird that it is not giving some sort of error code though, that is strange. Unfortunatley I don't know how to check those sensors, but maybe some else can help with that? I think that this might be your problem. If some of the solenoid fail, they have different symptoms, and the one VSS' being broke is the first thing that pops into my mind when dealing with AWD.

After that, I would start measuring the resistance of all the solenoids in the trans. Something is failing, which is causing the TCU to go into limp mode, which is causing your AWD not to work.

You mentioned that you are using a trans from a different car, are the diff ratios the same for the two cars? I know that if the diff ratios are different between the two trans, and you use the wrong TCU, that MIGHT cause an error because it's expecting a different speed out of one of the sensors, and since it's not getting what it expects the TCU thinks the VSS failed, and then puts it into FWD mode. Just a thought...

Edited by eulogious, 24 February 2010 - 05:14 AM.


#5 U R A BUS

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Posted 25 February 2010 - 10:05 PM

how did the car drive with the wiring harness unplugged.? i gues i forgot to mention that you need to drive tight turns on dry pavement to see if it has torque bind.

what do the observers report when they are watching the wheels on the snow and ice??

how do you know it is working for 60 seconds?? what changes?


Observers see rear wheels spin after starting the car, after waiting about a minute and trying again, they see just the front spin. I will turn the car off long enough to restart and the same thing happens all over again. I tried it again tonight after driving for about a half hour, same thing all over again.

#6 U R A BUS

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Posted 25 February 2010 - 10:07 PM

It seems that you misunderstood about the fuse under the hood BUT this does not sound like an issue testable by forcing the car into 2wd anyway


I understood just fine just wanted everyone to know that there was no fuse installed so that wasn't the reason for AWD not working.

#7 U R A BUS

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Posted 25 February 2010 - 10:15 PM

Well, I can tell you this much, the reason the AWD works for a min or so and then quits is because the TCU errors out, and then stops the AWD from working, basically it's going into limp mode. There's a couple of things that you can try to check. I would check ALL the speed sensors, VSS1, VSS2, for sure. If either one of the VSS' are broke, the TCU can't calculate if there is any difference in the speed of the 2 sensors so that it can transfer more power to the rear, or to the front. The TCU needs both of those sensors working in order for the AWD to function properly. If one of them is broke, the TCU automatically puts the car in FWD mode so that you can drive it home. That's where I would check first. I find it really weird that it is not giving some sort of error code though, that is strange. Unfortunatley I don't know how to check those sensors, but maybe some else can help with that? I think that this might be your problem. If some of the solenoid fail, they have different symptoms, and the one VSS' being broke is the first thing that pops into my mind when dealing with AWD.

After that, I would start measuring the resistance of all the solenoids in the trans. Something is failing, which is causing the TCU to go into limp mode, which is causing your AWD not to work.

You mentioned that you are using a trans from a different car, are the diff ratios the same for the two cars? I know that if the diff ratios are different between the two trans, and you use the wrong TCU, that MIGHT cause an error because it's expecting a different speed out of one of the sensors, and since it's not getting what it expects the TCU thinks the VSS failed, and then puts it into FWD mode. Just a thought...



Okay, that makes perfect sense. But what are the VSS1 and VSS2 (vehicle speed sensor, maybe?) and where are they located? Only the motor was changed, nothing in the rest of the drivetrain. Thanks for you input and please feel free to add as you think of anything. Is there a possibilty that the TCU could be bad? Is that what controls the Duty C?

#8 eulogious

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Posted 26 February 2010 - 03:13 AM

Okay, that makes perfect sense. But what are the VSS1 and VSS2 (vehicle speed sensor, maybe?) and where are they located?


You are correct about what the VSS' are, they are the vehicle speed sensors. Here's a link to a site that has some pdf's on the 4EAT:

http://www.endwrench...e/sysdrive.html

There should be lots of info in there for you about the trans, where the sensors are, and probably how to diagnose problems as well.

Is there a possibilty that the TCU could be bad? Is that what controls the Duty C?


Of course there is a possibility that the TCU is bad, but it is rare that it would fail, and usually if it did fail, the whole thing wouldn't work, just not one part of it, usually. Yes the TCU controls the Duty C solenoid (AWD), and basically everything else with the trans. That's why it's called the 4EAT, 4spd Electronic Automatic Transmission. Instead of using mechanical means to control the trans, it uses a computer, for most of it anyways. Of course there are still mechanical aspects to the trans, but a majority of the trans is control by the TCU.

What you could do is go to a pull a part, grab a TCU out of a car that is similar and try it. Most of the time the PAP will let you try the part to see if it fixes the problem, and if it doesn't you can return it and they don't charge you for it. Mine does that anyways. Just a thought.

I wish I new more mechanically about the 4EAT to help you out more, but I don't, so I would look through some of those docs and see if they can help you test the sensors. I might have the info in my FSM on how to replace and test the sensors, but it's at home and I won't be there for a while. Like I said, hopefully someone will chime in who has done this before!

#9 johnceggleston

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Posted 26 February 2010 - 08:50 AM

AWD basics (for US auto trans), maybe it will shed some light on your situation.

the trans fluid pressure causes the awd transfer clutch to engage the rear wheels and the pressure is regulated by the duty c solenoid. no pressure, fuse in, (all fluid pressure is dumped off) causes FWD. full pressure, not regulated by the duty c or a bad duty c, causes 4wd locked (this would include torque bind in tight turns on dry pavement.)

if you have awd drive for a bit and then you do not, something is changing, obviously. either the fluid pressure is bleeding off somewhere or electronically something is telling it to.

there were / are some old posts in torque bind threads that talk about ''worn leaking housings or something or other" which would cause AWD problems but i didn't pay attention because it didn't apply to me. and i assumed the leaky what ever caused torque bind, but i don't really know. i guess it could cause no fluid pressure and therefore no AWD, i guess?

the Tcu compares front and rear speed / sensors to control the AWD and regulate the duty c. the rear sensor is a very simple magnetic pulse type thing very very similar to the abs sensor on a wheel or a cam sensor. the front sensor is only slightly more complicated, but it sends its info to the speedo which then sends it to the ECU & TCU. you should have a cel if either one is bad. if the front fails the ecu will use the info from the rear to operate the car. but again you should have a cel.

front speed sensor (vss2) should be readily available for pretty cheap at parts yards and easy to replace. but if your speedo is working i do not think it is bad. if electric info to the tcu is the problem it maybe could maybe be bad / dirty connections from the speedo cluster to the ECU/TCU... but who knows. if the rear speedo sensor (vss1) is bad it too is easy to replace but you have to splice the wires, there is no easy wire connector.

the tcu is also easy to replace, challenging only because it is under the dash, drivers side. late 90s, all of the circuit boards are the same and will plug in, but some have a different mounting bracket on the case (95 to 96 i know there is a change). but you can swap the boards in the case and install what ever year you have around. but the closer the new is to the old, the better. outback to outback is good, a gt may have different shift points then a lego.

the tcu is a low labor try at a fix, but i'm leaning towards a fluid pressure problem in the rear extension housing. but that is more labor and parts, more money and more risky if you are not sure.

for those who know more, could the transfer clutch discs work / grab and then fail in a matter of a minute? every time you start the car?

Edited by johnceggleston, 26 February 2010 - 09:07 AM.


#10 eulogious

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Posted 26 February 2010 - 08:58 AM

^^^ Very good explanation!

late 90s, all of the circuit boards are the same and will plug in, but some have a different mounting bracket on the case (95 to 96 i know there is a change). but you can swap the boards in the case and install what ever year you have around. but the closer the new is to the old, the better. outback to outback is good, a gt may have different shift points then a lego.


But I think you forgot one thing, the diff ratio should be the same, ie if you have a 4.11 diff, get a tcu out of another car with a 4.11 diff. Don't get me wrong, it will work even if the diff ratios aren't the same, but the AWD COULD get messed up if the ratio's aren't close enough together, so if you are trouble shooting, just get the same diff ratio TCU just to make sure you aren't causing any more problems.

#11 johnceggleston

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Posted 26 February 2010 - 09:36 AM

^^^ Very good explanation!

But I think you forgot one thing, the diff ratio should be the same, ie if you have a 4.11 diff, get a tcu out of another car with a 4.11 diff. Don't get me wrong, it will work even if the diff ratios aren't the same, but the AWD COULD get messed up if the ratio's aren't close enough together, so if you are trouble shooting, just get the same diff ratio TCU just to make sure you aren't causing any more problems.


i've never heard this, but i guess it could be. it would be much simpler for the tcu just to compare front and rear speed info. if they match do one thing if not, depending on the degree of difference, do another. if one goes out then default to NO awd.

how would the rear diff play into it and mess up the AWD?

thanks.

#12 eulogious

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Posted 26 February 2010 - 10:01 AM

i've never heard this, but i guess it could be. it would be much simpler for the tcu just to compare front and rear speed info. if they match do one thing if not, depending on the degree of difference, do another. if one goes out then default to NO awd.

how would the rear diff play into it and mess up the AWD?

thanks.


I guess the better to question to ask would be, how else would the TCU detect if the rear needed power? By slippage, or the difference between the speed of the front wheels versus the back wheels. The back wheels are connected to the drive shaft, which is then turning inside the trans via the rear output shaft, so VSS1 i believe is reading shaft speed out the back, but that is essentially how fast the back tires are turning. With that being said, having a 3.7 rear diff compared to a 4.11 is a 6% difference in how fast the wheels are turning, therefore possibly throwing out a error and disabling the TCU.

The actual TCU code (yes I have seen it, out of a legacy anyways, and they didn't change the code that much across product lines or years for that matter, still same CPU and memory) is expecting a certain tire size (actually a certain number of tire revolutions per min to be exact) and diff ratio to calculate the speed differences between the two. It has to determine how fast the front wheels are turning, then compare it to how fast the rear should be turning. If either one of those numbers are not what the TCU is expecting, it COULD mess it up and disable the AWD because it thinks one of the VSS' is off/broke because it's expecting a different number than it is getting. Not saying it will always, but just saying it could.

The reason why this comes up is when presslab installed his paddle shifters for his 4eat into his loyale, the TCU he was using was expecting a 4.11 (it was out of a legacy) and his loyale had a 3.7, and the AWD would not work for him until he figured out the difference (6%) and change the value in the TCU rom code itself, then the AWD started working again. Again I only really bring this up because he is trouble shooting his trans, so it would be wise to use a TCU with the same diff ratio just to be on the safe side and not add anymore problems to the ones he already has.

I am not disagreeing with you at all, just pointing out what could POSSIBLY happen :)

Edited by eulogious, 26 February 2010 - 10:08 AM.


#13 johnceggleston

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Posted 26 February 2010 - 01:24 PM

The reason why this comes up is when presslab installed his paddle shifters for his 4eat into his loyale, the TCU he was using was expecting a 4.11 (it was out of a legacy) and his loyale had a 3.7, and the AWD would not work for him until he figured out the difference (6%) and change the value in the TCU rom code itself, then the AWD started working again. Again I only really bring this up because he is trouble shooting his trans, so it would be wise to use a TCU with the same diff ratio just to be on the safe side and not add anymore problems to the ones he already has.

I am not disagreeing with you at all, just pointing out what could POSSIBLY happen :)


i'm impressed, i never would have thought it would matter. thanks for the info.
so an outback TCU from the late 90s would be best.

#14 eulogious

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Posted 26 February 2010 - 01:38 PM

i'm impressed, i never would have thought it would matter. thanks for the info.
so an outback TCU from the late 90s would be best.


Ya, presslab is a smart guy when it comes to that stuff. I got a cool project that I am in the middle of right now and presslab has helped me out ALOT in understanding the code to make it work, so thats the only reason why I kinda get it :rolleyes:

It's because I have recently been working with the TCU's that I thought to chime in as well, so I hope that some of this info helps the OP out!

#15 U R A BUS

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Posted 26 February 2010 - 02:40 PM

^^^ Very good explanation!



But I think you forgot one thing, the diff ratio should be the same, ie if you have a 4.11 diff, get a tcu out of another car with a 4.11 diff. Don't get me wrong, it will work even if the diff ratios aren't the same, but the AWD COULD get messed up if the ratio's aren't close enough together, so if you are trouble shooting, just get the same diff ratio TCU just to make sure you aren't causing any more problems.


the car I would take out of is identical to this one only a year newer. I would assume the ratios would be the same but nothing is is for sure. How do I tell what the ratios are?

#16 eulogious

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Posted 26 February 2010 - 02:44 PM

I should be marked on the back of the LSD. There should be a silver label on it that tells what the ratio is, at least it's that way with the ea82 body series, so unless it changed...

#17 johnceggleston

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Posted 26 February 2010 - 02:51 PM

the car I would take out of is identical to this one only a year newer. I would assume the ratios would be the same but nothing is is for sure. How do I tell what the ratios are?


the 98 GT has the same final drive ratio as the 96 - 99 GT auto trans and the outback 96 - 99 auto trans.

#18 nipper

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Posted 26 February 2010 - 03:17 PM

I'm just going to lurk here for a bit.


Just one question. When you did the spinng tire test, did the car start moving forward? If it did, the TCU will start backing off on the amount of power to the rear wheels.


nipper

#19 johnceggleston

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Posted 26 February 2010 - 08:00 PM

rear extension housing w/ transfer clutch and duty c solenoid



Posted Image

duty c

Posted Image

Edited by johnceggleston, 26 February 2010 - 08:03 PM.


#20 eulogious

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Posted 27 February 2010 - 02:56 AM

^^^Nice pics, thanks for that!

#21 U R A BUS

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Posted 27 February 2010 - 07:01 PM

I'm just going to lurk here for a bit.


Just one question. When you did the spinng tire test, did the car start moving forward? If it did, the TCU will start backing off on the amount of power to the rear wheels.


nipper


The car did move forward when the AWD works but does not when just the front wheels spin.

WOW this is really gettin wierd. I didn't mention earlier that you didn't need to step on the brake to shift this car out of park. Today it wouldn't shift unless I stepped on the brake. And guess what the AWD works fine today.:banana: It worked for as long as I drove the car about an hour. So logic would tell me the clutch pack is ok and it has got to be some kind of electrical problem. It does not have any CEL or flashing lights. If anyone has an idea please share it with us.

#22 eulogious

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Posted 27 February 2010 - 07:05 PM

Then I would swap out the TCU and see what happens... I hope that it's "just" an electrical problem... at least an easy TCU swap to fix it. Electrical problems can be more of a pain than mechanical ones sometimes. Got love computers in our cars :rolleyes:

#23 nipper

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Posted 27 February 2010 - 07:06 PM

The car did move forward when the AWD works but does not when just the front wheels spin.

WOW this is really gettin wierd. I didn't mention earlier that you didn't need to step on the brake to shift this car out of park. Today it wouldn't shift unless I stepped on the brake. And guess what the AWD works fine today.:banana: It worked for as long as I drove the car about an hour. So logic would tell me the clutch pack is ok and it has got to be some kind of electrical problem. It does not have any CEL or flashing lights. If anyone has an idea please share it with us.


I doubt it, the two are not related.

Logic tells me it is a hydraulic issue. When the fluid is cold, its thick. It can transmit more force and not leak as quickly thorugh seals. As it warms up it thins out and losses the ability to transmit force as well and can leak through seals.

If the At Temp light is not flashing it is not electrical.

Also could you please pick just one thread to walk off of, it gets confusing.

Edited by nipper, 27 February 2010 - 07:08 PM.


#24 U R A BUS

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Posted 28 February 2010 - 01:53 PM

I doubt it, the two are not related.

Logic tells me it is a hydraulic issue. When the fluid is cold, its thick. It can transmit more force and not leak as quickly thorugh seals. As it warms up it thins out and losses the ability to transmit force as well and can leak through seals.

If the At Temp light is not flashing it is not electrical.

Also could you please pick just one thread to walk off of, it gets confusing.



I would agree with you except for the fact that when the AWD is not working properly I can drive the car all day so every fluid is hot, shut the car off restart it and the AWD works again for 30 seconds. Yesterday the AWD started working and I drove the car for 20-30 minutes and it worked the whole time because I was on snowy/icy roads and checked it numerous time and the car always had good traction. If it was leaking through seals I dont thinks it would have worked that long. I am leaning toward a bad ground.

#25 nipper

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Posted 28 February 2010 - 02:25 PM

I'm bowing out of this one. KNowing very well how the 4eat works and the AWD if you wish to chase things down knock yourself out.


Goodluck.




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