Intermittent AWD issues - Subaru Outback - Subaru Outback Forums
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post #1 of 23 (permalink) Old 02-22-2019, 10:02 PM Thread Starter
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Intermittent AWD issues

Hi all,

I've been noticing in the last few months that I'm not getting power to the rear axle when spinning. I've been struggling in some snow situations lately and I think I have narrowed the culprits down, but I haven't had much experience with the auto transmissions so I wanted to bounce ideas off a few others. The issue is intermittent, sometimes power is transferred back normally, other times I can spin my front wheels for 15-20 seconds and have no engagement. Usually doing this when standing still will trip a speed sensor code.

What I've done so far:
-Verified the AWD fuse is NOT in place.
-Verified rear axle is NOT powered at problem times
-Full transmission fluid change (12qts, Subaru ATF-HP)
-Datalogged transmission

What I know so far:
-Fluids are clean and at proper levels (problem existed before and after change)
-No codes are stored in the transmission
-A reset and reset level 2 (via FreeSSM) on the transmission does not fix anything
-The air purge procedure outlined in the FSM did not do anything, mostly because without the actual SSM you cannot get the trans to open the solenoids when running.
-Problem seems to exist only when the transmission is hot. First few mins of driving as a general rule has AWD without issues.
-The TCU is sending the correct signals at the proper times, according to the logs. No codes are recurring except the rear speed sensor, only if I'm standing still and spinning the front wheels for several seconds.

My guess is that the Duty C solenoid is failing. Clutchpack seems to be fine, there have been situations while spinning where the rear grabs all of a sudden with a solid thump and lurch forward if rears are on dry pavement. I have not yet done a wiring harness check, but will soon before I drop 150$ on a new solenoid.

Am I missing anything?

Car: 06 Outback Wagon 2.5i
4eat Sportshift
about to roll over 200k.

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post #2 of 23 (permalink) Old 02-23-2019, 08:26 AM
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Has someone actually observed that neither rear wheel was turning while one or both fronts were spinning?

The logs should have separate data for front and rear wheel speed. These are based on sensors in the transmission, not at the wheels. Rear Wheel Speed would actually reflect whether or not the output side of the transfer clutch (which connects to the rear drive shaft) is turning.

If the Transfer Clutch duty cycle % is high, and yet the rear wheel speed is not registering, then that's not right.

The "thump and lurch" could be due to the solenoid, the transfer clutch control valve, or the clutch mechanism itself suddenly changing.

What sensor code is appearing?

The application of line pressure to the clutch piston to engage the transfer clutch can be monitored with a pressure gauge connected to a port on the side of the transmission. This would verify whether or not the solenoid and valve are functioning, but it would require the test at a time when the AWD would normally appear to not be working.

As far as I know, for ~MY 2004/5+, there are no Subaru replacement solenoids available for the 4AT and 5AT valve bodies. However, as cardoc notes below, the 4AT transfer solenoid is available from another supplier. If the current solenoid is found to be defective, or at least is very highly suspect, then this might be a solution.

What type of logs do you have? Romraider? Perhap post them here.

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Last edited by plain OM; 02-23-2019 at 03:16 PM. Reason: correct re solenoid availability
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post #3 of 23 (permalink) Old 02-23-2019, 11:15 AM
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Cobra Transmissions has a replacement solenoid and is easily replaced if it comes to that.

https://cobratransmission.com/soleno...waAgwIEALw_wcB
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post #4 of 23 (permalink) Old 02-23-2019, 04:04 PM Thread Starter
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Quote:
Originally Posted by plain OM View Post
Has someone actually observed that neither rear wheel was turning while one or both fronts were spinning?

The logs should have separate data for front and rear wheel speed. These are based on sensors in the transmission, not at the wheels. Rear Wheel Speed would actually reflect whether or not the output side of the transfer clutch (which connects to the rear drive shaft) is turning.

If the Transfer Clutch duty cycle % is high, and yet the rear wheel speed is not registering, then that's not right.

The "thump and lurch" could be due to the solenoid, the transfer clutch control valve, or the clutch mechanism itself suddenly changing.

What sensor code is appearing?

The application of line pressure to the clutch piston to engage the transfer clutch can be monitored with a pressure gauge connected to a port on the side of the transmission. This would verify whether or not the solenoid and valve are functioning, but it would require the test at a time when the AWD would normally appear to not be working.

As far as I know, for ~MY 2004/5+, there are no Subaru replacement solenoids available for the 4AT and 5AT valve bodies. However, as cardoc notes below, the 4AT transfer solenoid is available from another supplier. If the current solenoid is found to be defective, or at least is very highly suspect, then this might be a solution.

What type of logs do you have? Romraider? Perhap post them here.
The code that appears is the P1706, rear speed sensor malfunction. I was logging to diagnose that, actually. I will try to post the logs (its on another computer that I don't have at the moment) but I watched Transfer Duty, Front and Rear speeds and the operation from the TCU side of this continues to be spot on what it should be. The only time P1706 pops up is if I get stuck and sit there spinning for several seconds, which makes sense if the rear wheels get no power. Both fronts are spinning, rears do not and it looks like a dead sensor. And this has been visually verified several times. Most of our streets have been snowpacked for the last few weeks here in SD, so I've had opportunity to start drive testing on slick surfaces.

I don't have equipment to do the line pressure test for the transfer clutch, but I can look into what it needs and if I can borrow a gauge and such from an auto parts store. But I think the visual confirmation goes a long way in identifying whether or not I have transfer clutch operation, at least in a pinch.

I will either take more logs or post the ones I have soon. Hopefully if I do need to take logs, I can find a slick patch again when this is acting up and be sure to note it. These intermittent gremlins are always the most fun to sort out....


Quote:
Originally Posted by cardoc View Post
Cobra Transmissions has a replacement solenoid and is easily replaced if it comes to that.

https://cobratransmission.com/soleno...waAgwIEALw_wcB
Thanks cardoc, I had found the rostra ones on Amazon, but I'll take a look at cobra for sure. They're a bit cheaper.
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post #5 of 23 (permalink) Old 02-23-2019, 06:10 PM
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I'm not certain, but the P1706 diagnostic/troubleshooting page in the FSM seems to indicate that a symptom of a problem with the rear speed sensor is no lock up, presumably of the AWD transfer clutch.

The AWD transfer clutch control system depends, in part, on both front and rear speed sensor signals in order to detect wheel slip and the need to increase the AWD transfer clutch engagement pressure. Without the rear speed sensor, it seems to me the system wouldn't be able determine where to set the transfer clutch pressure, and might just keep the clutch open, or only partially-engaged depending on other inputs.
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post #6 of 23 (permalink) Old 02-23-2019, 10:26 PM Thread Starter
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Yes, I assumed that if the code is tripped the rear lockup is disabled. But I clear it as soon as I see it (plus if you shut the car down, the code just gets stored and is not active upon restart).
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post #7 of 23 (permalink) Old 02-25-2019, 12:06 AM Thread Starter
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Well I did a few things and had a successful log run today. Thought I ought to try again since I can try to recreate some issues I wasn't as aware of when I last did the datalogging. I have a lot more where this came from but I'll put the highlights up.

I did about an hour of driving tonight. Files are in tab delimited text format, which should go into excel fairly easily (csvs do not attach). Here's what I can say so far:

-Ambient temps are in the low 1s, so ATF temps are notably lower.

-I've done some logging of simply crusing around without trying to spin the wheels and there does not appear to be any signal dropout or abnormal signals. The wheel sensors are reporting as they should. Wanted to save anyone the trouble of sifting through a logfile of 15+ mins each, at ~6ms resolution so I didn't upload those here.

- Cold_Spinwlock_control: After some driveway idling (setting the computer up) I did a quick control wheelspin and as expected the rear axles are breaking loose as the fronts are. (time = 15737)

-Hot_wheelspin_lock: After at operating temps, this was a couple of spins on ice (all 4 wheels) and they do appear to stay together with both attempts. (time = 970, 18723)

-Hot_wheelspin_somelock: Some snowy roads, I do not know if there was trouble with the transfer or the fronts are on ice and rears on dry, which could slow down the speed response. But the last 2 spins, AWD does fully engage. (time = 581, 14058, 31483, 38207)

-Nolockspin_lockup: This is where it gets interesting. Found an abandoned road that was ice covered. Front wheels peeled out (time = 1734) and rears did not kick in for several seconds. At the point where it shifts into 2nd gear, I felt a bit of a thump and the rear started fishtailing. However, there's a clear speed differential front to back for 2-3 seconds on the log. I did a few more unlogged attempts and the rear began fishtailing, confirming my suspicions.

-RearSSCodeSet: Similar to the Nolockspin, I found some ice and this time on an incline. Front wheels spun for almost 5 seconds with no apparent power to rear wheels (time = 14673). Rear Speed Sensor Malfunction code was set during this time (P1706). Note the computer is sending the signal to transfer power rearward.

-HeadingHome_nolock_thenlock: Immediately after, I headed home. There were a couple more wheelspins on the way with no rear power, then it again started working on the last spin (time = 87235). Also note that even with the active speed sensor code, it still was attempting to transfer power.


Looks like the TCU is doing what it is supposed to. Not sure if I'm having an electrical fault or mechanical/hydraulic fault in the transmission. Guess I will have to cruise around for a bit with a multimeter and wait for a time when I know the transfer isn't happening and probe the duty C wires.

If there are other parameters I should log to better assess this problem or any other scenarios I should run, definitely let me know. I'm not well versed in automatics but I hope I'm on the right track.
Attached Files
File Type: zip logfiles.zip (16.5 KB, 3 views)

Last edited by Jagular1785; 02-25-2019 at 12:55 PM. Reason: Update log format
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post #8 of 23 (permalink) Old 02-25-2019, 08:03 AM
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For the logs, instead of converting all of them to post, create a zip file and put all the logs in to the zip and upload the zip.

You are doing everything right.

Two things come to mind. First that the duty solenoid may be failing at operating temperatures after the fluid and solenoid heat up. Think of it in the same manner as an ignition coil that fires when cold and stops firing when it heats up. Same thing. Heat causes expansion and if the winding in the solenoid is bad, when the expansion occurs the solenoid can't function.

Second thing is a worn clutch, but the cold operation and harsh engagement is an indicator that the clutch is in tact and can function. But if this continues, the clutch will burn up due to lack of fluid pressure.

I'm also leaning toward the possibility of a bad ground in the transmission - there's a ground wire on the valve body that can become loose or fall off if the bolt backs out, and lack of ground affects the solenoids. The way the TCM shifts you may not be feeling a difference in shift solenoid operation until it's too late. You may not have a ground issue in the transmission, it's just something to be aware of when you drop the pan; and make sure you get it connected when you put the solenoid bracket back on.


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post #9 of 23 (permalink) Old 02-25-2019, 12:04 PM
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@cardoc

Indeed.

I'd add that it's not the transfer solenoid that could be failing; instead, it could be the transfer valve. The solenoid controls pilot pressure to the transfer valve (spool type). It's the valve that actually controls the line pressure application to the transfer clutch piston. The solenoid could be working fine, but the valve could be sticking in its bore, and not responding properly to the variable application of pilot pressure.

But I'm still puzzled by the P1706. There's nothing that suggests it would be triggered if there's a significant difference between the front and rear speeds. A few years ago I was indirectly involved with a non-functioning rear wheels problem with 2006 Forester. Front wheels would spin while rears didn't do anything. Neither the owner who complained about the problem nor the technicians working on the car and experiencing the fault, found any trouble codes or any warning lights coming on. (It turned out the transfer clutch plates were toast, literally (melted).)

If, in the current case, there's a problem with the wiring in, or to, the solenoid such that it went open circuit at operating temperature, that should trigger a code, specifically, P1707, AT AWD SOLENOID VALVE CIRCUIT MALFUNCTION. Instead, the only code appearing P1706, and that's related to the rear speed sensor, not the solenoid. Based on the FSM troubleshooting for P1706, that can be caused by a bad connection to the sensor or the sensor itself (e.g. weak or no signal). I haven't found anything that suggests the P1706 could be triggered by a difference in the front and rear speed signals.

Incidentally, in this thread, ntippet substituted a manual, variable, control of the duty cycle to the AWD solenoid, and attached an equivalent load to the TCM so that it would still send out a control signal according to the driving conditions. The manual control was set to zero, so that the solenoid remained off. The car was then driven on an ice-covered road. The front wheels, as expected, broke away and there was a significant difference in the front and rear speed, but no codes were triggered. Granted that was with a 2004, but that TCU had the same code available, i.e., P1707 and P1706.

Monitoring of the transfer clutch apply pressure would be the way to determine if the cause is in the valve body or in the clutch. But that's a challenge to do when the problem is somewhat intermittent.
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post #10 of 23 (permalink) Old 02-25-2019, 12:52 PM Thread Starter
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@plain OM: I think the 1706 has more to do with the rear wheels not spinning *at all* when this happens, despite 80-95% duty on the transfer clutch. The differential in speed isn't the big thing, because that's been happening a lot in the snow without the code. But if I am stopped completely and get stuck to where no car movement happens and I still spin the fronts for several seconds, code gets tripped.

I also know that in 2005, there was a significant update to the transfer clutch solenoid as well as some other things like TCU programming and such. Per the FSM, Phase II 4eats use 12V signal to lock the transfer clutch, whereas prior versions used 0V. They *should* fail open more frequently than binding up (which seems to be a common failure on the 4eat).

Could it be that the TCU is more sensitive to these codes for the Phase II 4eat?

EDIT: Looking at the FreeSSM thread, according to my logs it appears that the TCU is more responsive than what is described there. The logs indicate that (depending on how hard you mash the throttle) the TCU responds within .05 to .1 seconds. Of course there's a delay for the pressure to hit the clutches, but it sounds like those schemes are a bit different. Not all that surprising. I am using Romraider, I forgot to mention.

And I would love to have a way to easily read the pressure on the clutch, that truly would be the way to know. I just don't have a great way to do this, especially with how cold it has been. I will have to wait until it gets a bit warmer before I attempt to do anything under the car, as I just have my garage/driveway to do anything.

@cardoc Is this ground located on the valve body or just somewhere nearby? I did have the pan off the car when I did the fluid change, just to ensure there wasn't a lot of built up material in there. It honestly wasn't too bad for having almost 200k, though the fluid was probably the original fluid as dark as it appeared. You think I have enough reason to bite the bullet and order a solenoid?

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