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Troubleshooting 99 Legacy "AT Oil Temp" Code 76

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I'm a car noob, but have been learning a bunch about them in the past month. I'm a computer engineer, so I like to mess around with things, and have done so since I was little. Anyway, I have a 1999 Legacy Brighton SE. It has a 4EAT and right now has a constantly flashing "AT Oil Temp" Light, that came on randomly on the highway. I've left it parked since then. Whenever I turn the car on, it starts flashing again and never stops. I noticed while driving it home, it seems to go straight from 1st to 3rd gear, and never go into 4th/overdrive. Besides that, everything feels fine.


I did the magic handshake to get the code off (found here: http://www.subaruoutback.org/forums/99-do-yourself-illustrated-guides/44383-199-2004-how-read-diagnostic-trouble-codes-dtcs.html), and it's a code 76, which means " 2-4 brake duty solenoid. Detects open or shorted output signal circuit."




I pulled off the air intake duct/chamber, and unplugged the transmission control unit's plug, and did some quick tests based of the factory service manual.


Here are the two things of interest from the fsm:






One thing I noticed was that there are 3 pins missing on the male end of the plug. 2 are listed as not connected in the FSM, but the other is listed as the "Torque Converter Turbine Speed Sensor". There is not even a wire running to that spot, so I can only assume my model does not have one.




Edit: Removed most of my incorrect troubleshooting. Would just confuse people in the future


There are two typos in the FSM. First off there are two pin 6's on the manual. The ground should be labeled "16". Second off, the picture makes it look like you can measure from the female side of the harness. This measures back towards the TCU and won't work at all. You want the male side (which is incredibly hard to reach). It's much easier to pull the plugs out of the TCU and probe from there.


The TCU plugs are b54 and b55 in the DIAGNOSTICS section of the FSM.

Edited by cegli

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you are probably right about all those things going bad at once, but you never know.


did you read all the current codes?


did you read the past codes?


is the oil pan for the trans dented?


i would pull the TCU under the dash above the brake pedal and look at the circuit board. it is very rare for them to go bad, but they can. burn or scorch marks would be bad as well as any water damage.


been through any high water lately?


originally posted by general disorder:


if the AT Temp light flashes 16 times on startup then you have stored TCU codes. There is a 6 pin black connector above the gas pedal and two grounding wires wrapped into the harness directly above that. Insert one grounding pin into the center pin on the black connector (blue with yellow trace typically), then follow this process:


1: Turn ignition on, apply brake, and place gear selector in 1. Turn ignition off.

2: Turn ignition on.

3: Move selector to 2.

4: Move selector to 3.

5: Move selector to D.

6: Depress accelerator pedal slightly.


7: Read morse codes flashes on the AT Temp light. 24 will indicate a bad duty-c. These are "stored" history codes from previous drive cycles.


8: Turn ignition off, then back on.

9: Move selector to 3.

10: Move selector to 2.

11: Move selector to 1.

12: Depress accelerator pedal slightly.


13: Read codes again. Codes given here are current faults active in the TCU on the current drive cycle.





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I've checked all the current/past codes and all I've got is the "Code 76", which is the same as the one solenoid I measured that was shorted.


I checked the transmission pan and it looks fine.


The TCU under the dash could be a good thing to check. I haven't been through any high water, but I do think I might have a leak somewhere, because the floors are damp. That was my next thing to check out after this...


The one thing that gives me a bit of confidence in the TCU is the fact that it is correctly outputting the code for the blown solenoid, and the fact that the car still drives if I ignore the fact I'm missing two gears.


If the TCU were to blow, what would be the symptoms? I would think the car wouldn't drive at all, or give off a bunch of codes, but maybe I'm wrong...


Thanks for the help!

Edited by cegli

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If you want to know what happens when the TCU goes poof, you can disconnect the TCU under the hood and find out.


What happens when you shift the car manually?


I doubt all those are bad at the same time, but since we know one has failed and the driving symptoms confirm it, I would replace that and see what happens.


Check your engine grounds too as that can sometimes make for wierd readings. It shouldnt but sometimes i sear electrons have not read the same electrical engineering books that the rest of us have.

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Okay, thanks for the help guys. Scratch everything I've said above. It looks like the FSM is showing the pinout for the wrong end of the plug... That's why it was so confusing. I found a "DIAGNOSTICS SECTION" on the internet that my manual didn't have, and it has been incredibly helpful.


It has a full schematic of the connections from the TCM to the transmission:




I checked the connections at the TCM:

A17 -> A8 = 13ohms (dropping resistor)

b19 -> a8 = 3.6ohms (solenoid)


uh oh!


They both measure correctly! Now I definitely have doubts about the TCM! Good call johnceggleston. I am going to check the voltage from the TCM to the solenoid after this. The diags say it's suppose to be 1.5v-4v with no throttle, and 1v with throttle. I feel I may find it is 0v, thus showing that the TCM blew out a transistor.


If I'm really lucky, I will be able to see the bad transistor and replace it. Anyone know what the TCM looks like under the silver case? Is it worth pulling apart? johnceggleston, you sound like you may have looked at one before on the inside, considering you said you've seen scorch marks? I'm not sure how user serviceable they are, but will keep this thread updated. Thanks for the help everyone!

Edited by cegli

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Ugggh, I am now officially going crazy. The voltage to the dropping resistor is perfect with the accelerator pressed, and depressed. The voltage to the solenoid measures perfectly from the tcm.


The resistance of the whole loop of for the solenoid is perfect, the ground is also correct (<0.03ohms).


So basically, the solenoid, tcm and grounds measure correctly in the full loop. That leaves me with two possibilities?


1. The TCM is broken and is detecting the circuit incorrectly, causing it to shift into safe mode (only gears 1 and 3) for no reason.


2. The solenoid is actually broken, but it's resistance is still fine. Can this even happen?


3. The TCM is putting out the wrong code and something else is broken. I guess I should measure all the other solenoids again too...


Anyone have any ideas? I'm losing my mind. Wish I had a TCU to swap in quick. Anyone in Vancouver, BC have a 99-04 subaru that I can do a quick swap with?

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Fixed it! I just went to a junk yard, grabbed another TCU for $50.00 from almost the same exact car (99 legacy 30th anniversary, mine is a brighton se). Cleared all check engine codes, and plugged the new one in.


That was it, good to go from there! Just took a 45 minute drive with no issues, smooth shifting. Haven't taken it on the highway yet, but so far so good.


So my advice if you get an "AT Oil Temp" code would be to double check the solenoids resistance from the TCU plugs. If it is within range, try replacing the TCU first. It's way cheaper and it's only 2 screws and 2 bolts away!


Edit: As a side note, I did pull apart the original TCU, which is quite easy. It is relatively user serviceable if you know your electronics. It looks like a standard double sided PCB, with a big row of transistors, and the main chip in the center, etc. All of mine looked good, but if you blew out a transistor, would be easy to replace if you knew your way around a soldering iron.

Edited by cegli

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