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Duty C replacement


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39 replies to this topic

#26 johnceggleston

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Posted 30 August 2011 - 12:53 PM

when working properly, the duty c receives voltage in a cycle, on, off, on, off. it is not 4 volts most of the time and 12 volts when you need more rwd. when the voltage hits the duty c it activates and directs fluid pressure to the clutch plates. when there is no voltage it is in-active and no pressure goes to the clutch plates, no fluid pressure means no rear wheel drive, only fwd.

the rate of the on/off cycle determines how much pressure and therefore how much rear wheel drive.

if you get the flashing AT Temp light the TCU thinks the duty c is bad. could be the duty c, but probably not in your case, or the wiring or the TCU. since yours is intermittent and your duty c is new i would suspect wiring / connectors or the computer.

if the duty c fails, or the voltage does not get to it, you cannot make it go into FWD. the duty c has to be good and getting voltage all the time to do that.

I THINK,
the power wire to the duty c (1995) is pin 3 on connector b55. but i don't know off hand if b55 is the 16 pin connector or the 20 pin connector. EDIT: it is pin 3 on the 20 pin connector.

if you have a TCU swap that in. if not, measure the voltage on pin 3 when the TB is occurring. does it have voltage? this might tell you if the solenoid is bad or the tcu.

if you cut the wire on pin 3 , on the 20 pin connector you will have 4wd ''locked'' and TB. generally this will cause your AT Temp light to flash the next time you start your car. if you put a 15 ohm, (iirc) resistor on it the light will not flash for a bad duty c. (i think, i haven't tried it yet.)

i had /have an intermittent duty c failure on my 95 legacy. i put the fuse in and sometimes it works and sometimes it doesn't. iirc, it tends to fail when hot. there is a good thread that addresses the TB issue due to a bad, scorched, burnt, ''something'' on the TCU circuit board.

i pulled the rear section of the drive shaft and just drive it in FWD. but i kept the fuse in just to see. some times FWD lights up and sometimes the AT Temp light flashes, but either way i never have TB. i also never have awd.

Edited by johnceggleston, 30 August 2011 - 01:55 PM.


#27 theburro

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Posted 30 August 2011 - 01:20 PM

Well I've followed a bunch of different threads about this Torque binding problem as I am experiencing it in my 97 outback legacy 4EAT.

Here's what I have done so far.

Got the code 24 and went ahead and did a duty-c change. The only thing I think could have been done wrong was putting the spring it its correct location as it sprung out when I took the casing off. If anybody thinks that could be an issue let me know. I'm not sure it is in the correct location.

The fwd fuse has never done anything. I've done several atf changes.

The lastest that I've done is going thru the diagnostice manual for the code 24 as follows it failed 1.4) and 3. It did pass the other ones.

1. CHECK HARNESS AND CONNECTORS BETWEEN
TCM AND DUTY SOLENOID C.
1) Turn ignition switch to OFF.
2) Disconnect connectors from TCM and transmission.
3) Measure resistance of harness connector between
TCM and transmission.
Connector & terminal / Specified resistance:
(B52) No. 3 — (B9) No. 11 / 1 Ω, or less

4) Measure resistance of harness connector between
TCM and body to make sure that circuit does not short.
Connector & terminal / Specified resistance:
(B52) No. 3 — Body / 1 MΩ, or more

2. CHECK DUTY SOLENOID C’s GROUND LINE.
Measure resistance between transmission connector
receptacle and transmission case.
Connector & terminal / Specified resistance:
(T4) No. 4 — Transmission / 1 Ω, or less

3. CHECK DUTY SOLENOID C.
Measure resistance between transmission connector
receptacle’s terminals.
Connector & terminal / Specified resistance:
(T4) No. 11 — No. 4 / 9 — 17 Ω

I am now at a loss, as I think that there is a short circuit issue and I don't know how to go about fixing it. Let me know what you guys think. I also did just go out and pulled the code again got 24, but what I did notice is that when I restarted the car after pulling the code, the fwd light was one for about 1 sec, but only after pulling the code (this is of course with a fuse in) not sure if this means anything.

#28 eulogious

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Posted 31 August 2011 - 06:56 AM

Code 24 is for the duty C. I wish I knew how the TCU determines that the duty C is bad. I'm guessing that it might be blaming the duty C for mechanical issues. I rented a lift by the hour to change the duty C and didn't have time to address the grooves in the clutch basket and hub. I gambled that the grooves could be left alone.

Anyway, since I really don't care about having full time AWD but want AWD on demand I'm thinking of removing duty C control from the TCU and using a simple switch to control it. As I understand things the TCU sends a varying voltage to the duty C to control the torque bias. What if I only send full voltage or no voltage to the duty C? It would either be open or closed. Much like installing the FWD fuse except that the torque bias would either be 90/10 or 50/50. I'd only use AWD off-road or in snow/sleet, etc.

I'm assuming that the wiring is still good but if not, I'll have to address that whether I install a switch, fwd fuse or new TCU. I'm not very optimistic that a new TCU will solve this issue without addressing the 178k wear in the MPT clutch assy. Do you know what voltage is sent to the duty C to open it fully? 5v or 12v? I also need some guidance on the wiring. I'm sure this has been done before but it could take a lot of searching to find it. Thanks for your help.


Replace the TCU first. They fail more often than you would think. I got 6 tcu's in my living room right now, and only 2 of the work. 2 of the 4 have bad duty c circuits, 1 has a bad duty b, and the last is just old. So they do fail, and they fail somewhat often I have found. The duty c failures are hardware failures, as my trans works just fine with a different tcu, and the duty c code will not clear no matter what I do, even with my select monitor it won't clear. It's a hardware issue with the tcu.

Replace the tcu, then wire up the switch if that doesn't work. My money is on the tcu going bad...

#29 SubaruFred

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Posted 31 August 2011 - 12:30 PM

Thanks guys! Just a couple more questions:

1) Where can I find a replacement TCU that isn't almost 20 years old with a lot of miles on it already and doesn't cost a small fortune?

2) Any chance that my 93 TCU wiring is the same as a 95+?

Even if a new TCU fixes the TB I still want to do the wiring mods so I can select between FWD and 50/50 AWD with a switch. I don't like how long the TCU takes to react when rear traction is needed nor do I care about having AWD during normal driving conditions.

#30 SubaruFred

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Posted 31 August 2011 - 09:33 PM

So I installed the FWD fuse again today and no TB on the drive to work. On the trip home I had TB from the start. The FWD indicator light stayed on but I got the duty C code again. Does this sound like the TCU or binding clutches in the MPT?

BTW, does anyone have a good TCU for a 93 Legacy?

#31 eulogious

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Posted 01 September 2011 - 07:16 AM

I would be leaning towards a bad tcu for sure.

On one of the tcu's that I fixed the transistor that actually drives the duty c was physical blown. You could see the marks on it and the char. So I replaced that transistor and it started working again.

So one theory that I came up with is that if you do actually have a duty c issue like you did, it MIGHT affect the actual tcu itself by blowing out that transistor. 15+ year old electronics don't like stress at all, and a bad duty c could put a lot of stress on that transistor. So it wouldn't surprise me if you fix the problem with the trans, to only have more issues down the road from the tcu. The rub is that you can't usually tell if the transistor is bad by just looking at it, you have to test it. The one I fixed was a very unusual case, since transistors usually short, not blow like that.

This is all just a theory of course. I am in the process of trying to determine if there is a common failure with these things, but that is going to take some time and belongs in it's own post. That will come in a bit.

So I would swap out the tcu and see where it gets you, since you know your duty c is good. The other thing it could be is a short in the wiring harness of the trans. That's always possible as well, but swapping out the tcu is much easier then figuring out a wiring harness short :)

I would start looking around in the JY as well. Any tcu up to about 1994 should work for you, as long as the rear diff of the donor car matches your rear diff. For testing purposes you could get one that is close to the ratio that you have, but not far off. So like if you have 4.11's you could get a 4.44, or a 3.9, and it would probably work, for testing purposes. I personally wouldn't run it as my DD that way, but for testing it should work.

If any of my spares worked, I would just give you one of mine, but sadly that's not the case :-\

Edited by eulogious, 01 September 2011 - 07:20 AM.


#32 SubaruFred

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Posted 01 September 2011 - 12:06 PM

I think you're probably right about the TCU going bad. I suppose a wiring short is possible but not very consistent with the symptoms. With the FWD fuse installed the FWD indicator stays lit all the time, even when TB is present. I can fix the TB 95% of the time by shutting the engine off and restarting. The TB behavior hasn't changed at all since replacing the old duty C which makes me think the old duty C was fine.

So basically I have an intermittent duty C failure failure not caused by the C solenoid or wiring. Must be the TCU. However, rather than replace the TCU I think I may just install a switch to the duty C using an alternate ign-on +12V source. Cheap, easy, driver selectable 50/50 AWD and no searching for a replacement TCU. All I have to do is find the correct duty C wire, run some wire and install a switch. What do you think?

#33 eulogious

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Posted 01 September 2011 - 12:39 PM

Nothing wrong with the switch at all. I would personally get a working tcu AND install the switch, but it really doesn't matter, the effect is the same. In fact I have done the switch in my loyal (4eat). Here's the thread I made for it:

http://www.ultimates...ad.php?t=105077

I even made a resistor pack, so when I threw the switch, I wouldn't get the 16 flashes on startup. Worked great!

The switch was my inspiration for my next project which was paddle shifters and a DCCD knob. If I could lockup the AWD, why not have complete control over it's functions instead? Here's my paddle shifter thread for a good read:

http://www.ultimates...ad.php?t=110222

That's now in my legacy.

The only thing to watch out for with the switch is to not accidentally turn the switch on, as you could wear out your MPT clutch pack MUCH more quickly if it's on all the time, and it will cause TB all the time :lol: So wire up a light or something like that, so when the switch is on, it's lit up. Other than that, ain't nothing wrong with the switch!

Edited by eulogious, 01 September 2011 - 12:42 PM.


#34 johnceggleston

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Posted 01 September 2011 - 12:43 PM

Must be the TCU. However, rather than replace the TCU I think I may just install a switch to the duty C using an alternate ign-on +12V source. Cheap, easy, driver selectable 50/50 AWD and no searching for a replacement TCU. All I have to do is find the correct duty C wire, run some wire and install a switch. What do you think?


interesting idea. and it would help diagnose what is wrong, the TCU or the duty c , which is new.

BUT, you seem to forget where you are working, under the dash. replacing the TCU is a pain, but doable and it does not take long. but wiring in a switch wire may also be a pain, but doable.

there may be an easier location to cut and splice the wire if you knew its routing, but it probably goes thru the fire wall pretty quickly. you could always cut and splice at the trans connector, pin #11?, under the hood. you'd just have to run the wire back inside for the switch.

just a thought.

#35 SubaruFred

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Posted 01 October 2011 - 08:24 PM

Thanks again to everyone for your help. I've been side-tracked recently by hurricanes, tropical storms, flooded basements, my other car and work but I did manage to read a little bit about duty solenoids. I was concerned that applying a constant +12v to an intermittent duty solenoid might damage it and it can. Apparently there is a good reason why Subaru recommends not driving around with the FWD fuse installed. It can overheat and damage the duty C solenoid.

So much for my constant +12V on/off duty C switch idea! If I haven't fried my new duty C already with the FWD fuse, I think perhaps I should look for a replacement TCU or in the worst case, remove the drive shaft.

FWIW, my TB is almost non-existent at ambient temps of 75F+ and almost constant at ambient temps of 60F-. I can't say whether this is an electrical resistance issue or a hydraulic pressure issue but I'm leaning toward the notion of excess hydraulic pressure from cold ATF overcoming the duty C.

#36 SubaruFred

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Posted 03 November 2011 - 02:45 PM

Just an update to close the thread out. Although my TB symptoms and TCU codes all pointed to a bad duty C, the duty C was fine. Presumably the TCU is bad, specifically the part that controls the duty C.

I cut the duty C control wire (+V) to the TCU and wired it to a 2-pole LED switch I installed in the dash. For +12V power I tapped into the main ign-on switched feed wire on the steering column. Now when the switch is on the duty C receives +12V and remains in FWD mode without the slight trace of TB. Flip the switch off and I have 50/50 torque split for snow and off-road use.

It's been a few weeks and a few thousand miles now and I'm very happy with this solution. Thanks to whoever came up with this idea originally, to those who suggested it to me and those who provided me with the TCU pinout, etc. It works like a charm. :)

#37 porcupine73

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Posted 03 November 2011 - 02:54 PM

Thanks for the update. Maybe the output transistor for the duty c in the TCU failed open so that it never gives the duty c any juice, even though it thinks it is.

#38 SubaruFred

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Posted 04 November 2011 - 12:19 AM

Thanks for the update. Maybe the output transistor for the duty c in the TCU failed open so that it never gives the duty c any juice, even though it thinks it is.


Could that transistor work fine sometimes but fail intermittently? Sometimes everything worked fine, no TB at all and no TCU codes and then randomly (more often in cold weather) I'd suddenly get full TB and a bad duty C code. Resetting the TCU with an ign off/on cycle always fixed the TB for seconds, minutes or sometimes hours. I had thought that the root cause was the TCU incorrectly reading a bad duty C but maybe it was the TCU itself causing the duty C to fail and then compounding the problem by not even trying to send voltage to it.

IMO the default mode should have been 90/10 rather than 50/50.

#39 porcupine73

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Posted 04 November 2011 - 07:49 AM

Sorry about that, I didn't read back the previous couple posts and see that the transistor frying has already been addressed.

I don't remember if you said you already inspected the transfer clutch packs? There were some good pictures on here somewhere of people servicing them and it showed the groves that get worn into the drum by the clutches. Then the clutch plates can get stuck sometimes in there and cause torque bind. I guess it might get intermittent if they get stuck then release, or something like that.

I think it isn't suggested to drive normally with the FWD fuse in, maybe not so much because of the duty c itself, but because it does change the handling characteristics, probably moreso in an emergency maneuver. With the FWD fuse in, it doesn't give the duty c power all the time, it is something like 95% duty.

#40 SubaruFred

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Posted 04 November 2011 - 12:40 PM

Sorry about that, I didn't read back the previous couple posts and see that the transistor frying has already been addressed.

I don't remember if you said you already inspected the transfer clutch packs? There were some good pictures on here somewhere of people servicing them and it showed the groves that get worn into the drum by the clutches. Then the clutch plates can get stuck sometimes in there and cause torque bind. I guess it might get intermittent if they get stuck then release, or something like that.

I think it isn't suggested to drive normally with the FWD fuse in, maybe not so much because of the duty c itself, but because it does change the handling characteristics, probably moreso in an emergency maneuver. With the FWD fuse in, it doesn't give the duty c power all the time, it is something like 95% duty.



When I replaced the duty C I looked at the clutch assembly and it showed some grooving and wear but I didn't have the time, parts or desire to do anything about it. It still locks up solidly when engaged and slips freely when not. I suppose it's possible that momentarily sticking clutches could be causing the TCU to determine that the duty C is bad but the bigger problem IMO is that once the TCU makes thie determination it will not send power to the duty c until the ign is turned off and back on. This makes a moment of TB turn into full blown TB for the rest of the trip or until the ign is cycled off and on. I tired very quickly of cycling the ign while driving or pulling over frequently to cycle the ign. It was especially bad at night since the headlights go out with the ign off. Hence the duty C switch.

I know that the correct way to fix this would be to rebuild the MPT clutch assembly and possibly the TCU if the rebuilt MPT didn't fix the issue but the car has 179k on it, rusted rear 1/4 panels and I don't plan on keeping it forever. It's true that running in forced FWD mode probably changes the handling characteristics a little but I think it's less than the 205/75/14 A/T tires and 3" suspension lift have done. :)

So while this manual duty C switch may not be the correct way to fix TB, it is cheap, easy and works great. If the clutches ever do stick in the basket grooves, I can't tell. To me it feels like any other FWD car until I switch into AWD mode. My only concern is possibly shortening the life of the duty C but if it fails, I'll either replace it or just pull the drive shaft.




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