ECU Questions

[Jessekrs123]

I bought an ECU a while back from somebody on here in hopes of fixing an intermittent cold start issue. In the case you do not know, I have tested every circuit under the sun, tested all grounds, everything checked out fine, so I figure it must be the ECU.

Now here is my problem.

My original ECU, PN 22611 AB770F has had a cold starting issue. I have band-aided this issue by running a separate parallel circuit in which I can drop a resistor in, changing the coolant temp sensor resistance that the ECU sees, and the car will start.

The ECU that I switched out today, PN 22611 AA563 does not work on my car. The car runs, but it will not idle. I plugged in the green connectors for diagnostics and it is giving me code 3, which I do not think is even a valid error code.

I put the original ECU back in and it is back to normal now.

My question are as follows:
- What would a code 3 mean?
- Are there non-interchangeable first gen OBD I N/A Legacy ECUs?
- Does anybody know what ECUs are interchangeable?

Thanks for all your help guys, it is much appreciated.

[vrg3]

Your original ECU part number is a 94 non-turbo 5MT ECU. Is your car a 94? Your signature mentions a 93.

The ECU you tried swapping in came off a 90-91 non-turbo 5MT. It's not that surprising that it doesn't idle right; 90-91 manual non-turbo models had a different type of IAC valve, a different MAF sensor, and even slightly different injectors.

You should be able to use an ECU from any 92 non-turbo model or a 93-94 non-turbo manual model. All of those cars came with the same IAC valve, injectors, and MAF sensor as your car, and have an ECU that's supposed to work with a manual transmission. The part numbers you can use are:

22611AA93x - 92 non-turbo automatic and manual

22611AB06x - 93 non-turbo manual

22611AB77x - 94 non-turbo manual

The last digit doesn't matter -- hence the x. And the part number may or may not have an "F" after it; this also doesn't matter.

It's possible that an ECU from a 93 or 94 automatic non-turbo would work, but it's not specified to.

What do you mean when you say it gives you a "code 3?" Does it give you three short blinks, a long break, then three short blinks, then a long break, and so on? Because all actual codes are 2 digits -- long blinks (tens digit) followed by short blinks (ones digit) followed by a long break.

[vrg3]

Oh, and, did you try just replacing the coolant temperature sensor? Even if it tests good it may have an intermittent problem that could make it your culprit. These sensors do fail and are cheap and easy to replace.

[Jessekrs123]

Wow, thanks a lot vrg, you're always so helpful.

I have a good amount of spare coolant temp sensors on hand, new from the dealer. I've replaced them numerous times with no luck. At certain resistances (1020, 1080, 1180, 1280, 1380 ohms) the car loses spark for some reason. There is also a relation to no starting and engine cranking speed. For some reason if the car cranks slower on a real cold day, it will start no problem. If it cranks at a regular speed, the car starting is a shot in the dark. With those factors aside, if I drop a resistor in a floating parallel circuit, I can reduce the resistance to something below 1000 ohms and the car will start. (Anything under 1000 ohms, the car will regain spark) Once the car starts, the resistor comes out and the car runs off the true value of the sensor.

I know I am not the only one with this problem, the parts guy at the Subaru dealer said there are a handful of first gens with the same problem. I am trying to see if there is a possible relation between certain ECUs and this problem. I've gone through every part of every circuit involving spark/starting, and everything tested out fine, which leads me to believe it must be the ECU.

And yes, with the diag wires plugged in, I was getting a code 3. (Three short flashes, a pause, then three short flashes over and over)

I'll be in the market for a correct ECU... Thanks a lot vrg.

[vrg3]

I've never heard of this not-starting-at-certain-temperature-sensor-resistances problem. Sounds weird... The only explanation I could imagine is a bad ECU. So I guess you're on the right track if you're planning on replacing the ECU with a compatible one.

And I have no explanation for the "code 3" other than a malfunctioning ECU.

But it seems so unlikely that your luck would be so bad that you'd get two faulty ECUs of two completely different designs. I can't think of a reason that a Hitachi ECU would ever blink out the pattern you're seeing, even if it was connected to a JECS engine.

In your testing, have you been able to determine whether it's absence of fuel, spark, or both that makes it fail to start? Or are both present but somehow the fuel mixture is wrong? Does the engine simply crank, or does it kinda-sorta catch a little but not enough to start?

I'd be very curious to compare an ECU exhibiting this behavior with one of the same part number that works correctly -- in particular, I wonder if there's damage to a part of the ROM where a coolant temperature lookup table is stored. So weird though.

[Jessekrs123]

When the car does not start, there is only an absence of spark. I've gone through extremely extensive testing. When testing the car when it would not start, there was fuel, timing and compression (obviously) but no spark. Tested out power and ground to both the igniter and coil pack and they were both properly grounded and receiving 12v (with key on) Test for spark through the wires and nothing on any cylinder.

Before I ran the parallel circuit into the cabin, I would originally unplug the connector from the coolant temp sensor and insert a resistor directly into the connector. That would enable the car to start. Then one day while pulling the connector off, I broke off one of the wires and had to solder it back in. From then on I just ran 2 wires from each wire going into the coolant temp sensor and ran 2 female prongs into the cabin. From there I could monitor the resistance the coolant temp sensor was giving off, and insert a certain value resistor to reduce the overall resistance in a no-start issue. The car will start if the resistance is anything lower than 1000 ohms.

The original thread for this issue is here if you want to just look through it, its pretty in depth:
http://bbs.legacycentral.org/viewtopic.php?f=5&t=45777

Thanks for your help vrg.

[vrg3]

Curiouser and curiouser. There's no spark? That makes even less sense.

Happy to help however I can... Let us know if another ECU fixes it!

[x2hawt4ux]

ok i think i might know about the flashing #3 code.

if you read here: http://legacycentral.org/library/literature/codes.htm the paragraph above the chart says "Now the light can blink in two different ways. If everything checks out so far, it will blink to indicate a number 01, 02, 03, 04, 05, 06, 07, or 08. If a problem is already detected, it will blink according to the chart below." does this help with that? Did you try connecting the black plugs?

[rob]

It sounds like a marginal cam or crank sensor. They can cause strange starting behavior. I had a problem where my car would sometimes fail to start when hot. It was like it missed the first few engine revolutions and would flood out by the time it figured things out. Sometimes continuous cranking would get it going, but it would always compression start if I was smart enough to park on a hill. Replacing one or both (can't remember exactly) completely solved the problem. It is worth a shot to try.

-Rob

[Jessekrs123]

It sounds like a marginal cam or crank sensor. They can cause strange starting behavior. I had a problem where my car would sometimes fail to start when hot. It was like it missed the first few engine revolutions and would flood out by the time it figured things out. Sometimes continuous cranking would get it going, but it would always compression start if I was smart enough to park on a hill. Replacing one or both (can't remember exactly) completely solved the problem. It is worth a shot to try.

-Rob

Thanks rob, but this has been done. Lucky for me, I am good friends with the parts guys at Subaru who willfully handed me a brand new crank and cam sensor for no charge, despite them being ridiculously expensive. Both crank and cam sensors hardly have 4,000 miles on them with no build up on the sensors themselves.

Another thing to add to this: The cam sensors used in the later gen Legacys (the ones where the harness can be disconnected at the actual sensor instead of disconnecting it at the bracket under the throttle body) is fully compatible with the first generation Legacys.... Even though that isn't very relevant, just thought I'd let everyone know because it is a PITA to get an older style cam sensor and have to run the wires under the intake manifold and hook it up under the throttle body. The more you know

[subydaddy]

I had this problem as well. It is extremely frustrating. PM me if you still need an ECU, I'll have to check mine, I don't remember what year it's from, I believe a 93.

[Jessekrs123]

I had this problem as well. It is extremely frustrating. PM me if you still need an ECU, I'll have to check mine, I don't remember what year it's from, I believe a 93.

Subydaddy, just give me the numbers on the ECU, I would like to see if there is a connection between common part numbers and this issue.

I am 6,000% positive that it is in fact the ECU that is at fault. Upon many failed starts this morning, I inserted the wrong ECU, and the car fired up fine other than it wouldn't idle due to the fact the wrong ECU was being used. Let it warm up just a bit, plugged in the original ECU and it fired up just fine. Thank god I will be swapping engines out soon and kick this pain in the ass problem.