Looking for ECM/ECU

I hope you don't mind a separate thread on this topic...

1994 Subaru Legacy Sedan. Auto transmission. 4DR. Front wheel drive.

Looking for an ECM/ECU (whatever you call it). This is the engine control computer that sits under the dash on the left side, to the left of the steering column. Here are some pics:

http://oi65.tinypic.com/lz6dz.jpg

http://oi67.tinypic.com/102mo87.jpg

http://oi68.tinypic.com/2hz0j1z.jpg

The only one I've found so far comes from a 1993 Legacy. How interchangeable are these? Do I need one from the exact same year/model/etc? Or are they compatible from year to year?

Anyone know of a source to find one of these? I'm still looking, but so far this specific model is rare, everything I'm finding is for a newer model or has different connector, different mounting bracket, etc.

Found one! Found several actually - sorry for taking up bandwidth here, it took a while before I finally found it.

http://www.carpart.com. Their search actually flagged several of these. A junkyard in Winston Oregon listed on there, $70 shipping to Arizona included. Don't know how long it's been on the shelf. What is weird is the serial number ( I'm assuming it's a serial number, maybe it's a sub model number?) is 3521, and mine is 3525. If that really is a serial number, then my car and the one they have must have been right behind each other in the assembly line.... go figure....

Good news!

You're not wasting bandwidth. Everything you share will stay here and may help someone else later.

If you are near the Prescott area there are a few of us Subaru nuts here that could perhaps help out in person. I for one would enjoy seeing your weird ignition problem in person to perhaps better understand.

I'm in Phoenix. Well, I have a Glendale mailing address because the Glendale post office services our neighborhood, but the city of Phoenix provides sewer/water/police etc. I have it torn apart right now, I unplugged a lot of stuff under the dash so I could get the ECU out.

This problem started maybe a month or so ago. The first symptom was that it would stumble slightly at idle. Since this engine is extremely smooth at idle, it was rather noticeable, but not really that bad. As time went on, it slowly got worse. Then one day I noticed that with the key on but engine off, the tach would twitch, jump up to 500 rpm or so and then back to zero. It would do this every couple of seconds. Then one day I noticed that not only would the tach jump, but the injectors would rattle as if they were being fired. With the engine off. And when the engine did run, it would miss severely. Finally it got to the point where I did not trust it, so I parked it. Now, with the key on, ever few seconds or so the tach twitchs, the injectors rattle and the check engine light flashes off and on sporadically - it's not flashing a code, it's too sporadic.

So I'm up to about 85% sure it's the ECU. If not, then I start tracing wiring - power, ground, crank sensor circuit. I suppose a bad/corroded connector somewhere might cause this, but I'm not looking forwards to tearing stuff apart to find it <sigh>....

If I unplug all of the sensors except the crank sensor, it still does it. If I just unplug the crank sensor, it stops. So I replaced the crank sensor, but that did not change anything. So my theory is that the amplifier/circuitry inside the ECU that picks up the crank sensor is bad and feeds spurious signals to the controller, causing it to fire the injectors and ignition. Not quite sure why/how that would cause the CEL to flash like it does

If you are near the Prescott area there are a few of us Subaru nuts here that could perhaps help out in person. I for one would enjoy seeing your weird ignition problem in person to perhaps better understand.

Subaru nuts is an understatement. I have 6 Subarus right now, trying to downsize to only about 3. Our group here in the quad cities is kind of unique. We all have different models, NA and turbo, and we're definitely not the Subaru turbo-bros in the valley where everyone is slow and NA is just slower.

I think if it was a wiring only problem it wouldn't be so rythmic, and it seems unlikely that it would do it in a completely motionless car at thermal equilibrium. I like your idea of an active noise filter or amplifier having a bad FET or capacitor that is electically punching through under quiescent current/voltage and then needs to charge back up before it does it again. That would give you a roughly rythmic glitch. Such a problem would easily not show with a visual inspection of the ECU. Infrared of the computer while it tries to run might show it, and desoldering any electrolytic caps and checking them in a dedicated tester might show it. If this car was the equivalent of a 1921 Rolls Royce you would probably take those steps to figure it out and fix it. Given it's a recent-ish commodity car the right way to go is put in a different ECU and see what happens.

tldr; you're likely close to your answer.

Here is an update for anyone following this thread.

1) Replacing ECU did not fix problem.

2) I have discovered that just about any mild surge to the electrical system - turn signals, honking horn, etc. - causes the fuel pump to cycle and injectors to chatter, injecting fuel into a non-running engine. It frequently does it by itself, but I can easily trigger it just by turning on turn signals for a second. I can start the engine and it runs, even drive it, but it's a bit rough.

I have unplugged most sensors under the hood, unplugged the fuel pump relay and put a test light across it so I can have a visual indicator of this happening.

Today I'm going to drop the ECU and check all power and ground connections, and then all inputs and outputs. Why oh why did they shove it up under the dash where you can hardly get to it? I'm going to remove driver seat so I can at least lay down and get comfortable while doing this. I'm tempted to pull the instrument console just to see if it gives me better access to the wiring.

I would put toward the top of the list to load test the grounds and power leads. Unless you already did and I've forgotten since you started...

Just did. Jury rigged a 5 amp load (three tail light bulbs lol) and verified it draws about five amps. Tested every ground and every supply pin, and all of them are less than .1 volts from battery or ground. Well within tolerances. Dang.

I think the ECU is bad. What else is left? The sensors are disconnected, and what is still connected is verified working with good wiring. I think these old ECUs just go bad eventually and I bought a bad one. I'm tempted to try yet another one, but that is a hundred dollar bill just to get one from another junkyard with no guarantee that it is good...<sigh>...

Just did. Jury rigged a 5 amp load (three tail light bulbs lol) and verified it draws about five amps. Tested every ground and every supply pin, and all of them are less than .1 volts from battery or ground. Well within tolerances. Dang.

 

I think the ECU is bad. What else is left? The sensors are disconnected, and what is still connected is verified working with good wiring. I think these old ECUs just go bad eventually and I bought a bad one. I'm tempted to try yet another one, but that is a hundred dollar bill just to get one from another junkyard with no guarantee that it is good...<sigh>...

I actually went in and replaced the electrolytics. Sadly did not change anything. I checked every wire and every sensor that connects to this thing.

Eventually I decided to stop throwing time and money a this car, and retired it. **sobs I've had that care for 20 years... **sobs... I still miss it. My wife has a 2016 Subaru. This old Legacy was a much better car.