4EAT TCU part number search

[wagonjunkie]

G'Day,

First post from me...but a long time "lurker". Bloody good site this is.
I believe in doing as much research as I can to avoid wasting anyone's time.

I have an Australian delivered 1993 Legacy AWD wagon, EJ22E, with a slightly troublesome 4EAT.
My car is the updated "series 2" shape (not sure if the USA received the same updated models during the life-cycle?)...and it has approximately 157,000 kms on it (about 98,000 miles).
The car is our daily driver - it's not a performance project or modified in anyway....except for new suspension.

Anyway, my quest to fix my torque bind issues has lead to the TCU. It has a fried capacitor.

Does anyone know much about the TCU part number differences?
(I know this has been asked many times...I have searched and searched and never really found a definitive guide...just lots of speculation and "maybes")

My cars' TCU # is: 31711 AC301.
I know I can likely use a 31711 AC302 TCU, because this was a known revision/update to the same TCU (??)

So far I have access (wrecking yard) to two other TCU's: 31711 AB335.........&........31711 AB572
I have a gut feeling that the 'AB' lettering is significantly earlier than my car (ie '90-92?) and is also for the Turbo model....can anyone confirm/deny this?

Has anyone ever seen a reliable chart that would explain the TCU numbers with any logical sequence or model correlation?

Can I assume that any TCU # ending in AC3## would probably be good enough?
I don't mind if the replacement TCU has slightly different driving characteristics, just as long as it controls the Duty C properly and shifts the forward gears predictably/normally.

Any help offered would be much appreciated.

[wtdash]

HI,
Welcome to the LC BBS site.

Yes, our '92-'94 was called 'post-facelift' vs. the '90-'91 'pre-facelift'.

Sorry, I can't direct you to any specific info on the part numbers, but the TCU from any '90-'94 should work.

The Torque Bind (TB) can have multiple causes, and it sounds like you've identified the cause, but have you tried the FWD fuse? It's by the passenger's side strut tower - driver's side Down Under? It'll help determine if it's electrical or mechanical.

GL,
TD

[wagonjunkie]

Thanks for your reply.

Over the last 6-8months I have tried periods with the fuse in & periods with the fuse out.
But either way it's the same - aggressive TB.
The dash light for 'FWD" illuminates every time the fuse is in. (I believe this is relevant for some symptoms...?)
...but it doesn't make any difference to the mechanical outcome.

After driving around for months with the rear section of the tail shaft removed, I finally got around to replacing the duty c solenoid.
Visually, everything in the transfer housing looked very good. The clutches & drive plates were straight & clean. The drum(s) didn't appear to have any abnormal wear or signs of fretting or binding. The backside of the final driven plate (forward end) had a circular wear groove from the opposing bearing face, but it didn't appear, to me, like it was abnormal. The clutches had good "meat" on them. There were "no signs of a struggle.." y'know.
Three or four fluid changes this year have always delivered perfect looking tranny oil too.

The first few test drives with the new solenoid were kinda patchy. The TB would disappear for maybe 20-40 miles....and then return again. When it came back the last time it was permanent...and severe.

At this point I removed the TCU & took the cover off. I discovered World War Three inside. One of the larger capacitors had overheated and 'popped'....and another smaller one showed signs of heat stress/discolouration.
There is a burn/scorch mark on the underside of the steel cover and the board itself is burned around the base of the capacitor. (If there was an easy way to attached or copy/paste photos to these posts I would attach the photos...??)
The strangest thing is that it appears like some one has been in there before. There is a large, hand-scribbled 'X' with a circle around it, next to the failed capacitor. Methinks someone else repaired the TCU under similar circumstances....but I'll never know for sure.

I'm waiting on a used TCU with similar numbers to be shipped (see my first post). Apparently it was removed from a working vehicle with no issues.
I think I will get the local electronics shop to replace the capacitor on my TCU board & do their best to test the whole unit. They admitted on the phone they have never repaired anything of this nature though. I'll keep it as last-resort spare....or maybe for troubleshooting.

My concern now is that the car, through an unknown electrical fault or earth problem, will proceed to fry the new TCU just like it did to the original one....

I feel like a private detective on the trail of the previous owners "cold case"

[Legacy777]

Welcome to the BBS.

As for the TCU's, I don't have any specific info on your TCU numbers, but can tell you from looking through the parts book at the AC numbered TCU's appeared to be for the 93-94 models in North America. Everywhere else were one year behind, 92-93. Also, the numbering didn't always make a lot of sense, but as long as the last number is an incrimental change, it'll just be a revision change. If the second last number changes, that may be a different TCU.

Here are the part #'s for the North American Legacy TCU's, they are all listed under item 1. You can see how the number changes based on the different models and years.

http://www.main.experiencetherave.com/s ... umbers.pdf


Also, to post pics, you need to have them hosted on a website like google pics, dropbox, imageshack, photobucket, etc.

[wagonjunkie]

Hey thanks a lot for the file, Leg777.

It looks to me like, ROUGHLY speaking, 'AA' series was '90-'91, 'AB' series was roughly '91-'92...and 'AC' was '93-'94....
...but as for the subtle differences between numbers....well, there's prolly 6 Japanese technicians/designers who know what they mean and they're all retired or six feet under!

A new (used) TCU just arrived about an hour ago, so I'm gonna put it in the car as soon as I have time and see how it drives.

Many thanks for you help so far....

[Legacy777]

You're welcome.

Let us know how the new TCU works

[wagonjunkie]

OK, the replacement TCU is now in the car. (whoever decided the location for the ECU & TCU was a good idea is a sadistic maniac. What's wrong with the front kick-panels?).

I've only driven about half a gas tank but so far, so good. No TB.
And for the first time in almost a year, the POWER light is not trying to give me a seizure at start-up.
So now I think I just need more miles to make sure it's all going to stay this way.

The transmission harness & connectors checked OK with my multimeter before I installed the TCU.
The duty c coil tested at around 15.5 ohms....and the book says 9-15...??

The old TCU output voltage was approx. 12.5 with no throttle, and 12.5 with full throttle (weird?)
....but I guess it's not a completely accurate test because there is no speed sensor inputs during a static test..?
...and plain voltage is not strictly the same as duty cycle...?

The new TCU gave approx. 12V with no throttle, and 5V with full throttle.
Does anyone know if that range (12 - 5 V) is OK?
The book states that the output voltage, at full throttle, should be no more than 0.5V..??

Many thanks again.
SB

[Legacy777]

Glad to hear it sounds like the new TCU is working.

When you did the last test, was the gear selector in the "1st gear" position? If not, try it in 1st gear and retest.

[wagonjunkie]

See if these links work for the photos of the TCU failure.

[Legacy777]

yeah that capacitor got fried!

[wagonjunkie]

was the gear selector in the "1st gear" position? If not, try it in 1st gear and retest.

Actually no, I only tested with the shifter at 'D'.
Ahhh....I remember reading something about preset duty cycle levels for each gear. I think 1st & reverse always receive maximum "AWD" yeah?

Good idea, I'll check it out....

[Legacy777]

Yeah, the AWD won't be as agressive in D as it will be in 1st, so the voltage you saw is probably good for a reading in D.