Any tuning needed if ECU gets replaced?

Hi,

My understanding of the ECU is it has a "default" state it returns to each time it's disconnected from power for a while (like the cheap and dirty disconnect your battery for 10-30 minutes method to clear error codes.) From there, it "learns" the state of the engine from all the sensor inputs. So I doubt you'd have to do any particular tuning--it should compensate for what it "reads" from your engine.

I know ECUs can go bad, but it's far less likely that the computer's bad than something else downstream. I'd look seriously at the O2 sensor we're talking about in your other thread as being "potential culprit No.

1." But... you could go ahead and try resetting your current ECU by disconnecting the battery cable before going further, since it's "free" and only takes a few minutes.

Best of luck!

Rick

Actually, as part of the procedure to read the error codes, you clear the ECU and check the codes later. So I figured that resetting and old ECU would be equivalent to putting in a new one: they both start from scratch.

It's getting tougher to find an ECU for my 13-year old car so if the salvage yard has the right one and it only costs $30 then I'll probably get it. Even if it turns out to be the O2 sensor (which I won't be able to determine until a couple months have gone by to notice the over-rich problem doesn't reappear because it occurs sporadically), I'm only out $30. The problem is that the online search at car-part.com comes up with the prompt page showing:

1- Ignition Cont, (center of firewall), w/o turbo; AT 2- Ignition Cont, (center of firewall), w/turbo 3- Ignition Cont, (center of firewall), w/o turbo; MT 4- Elec Cont Unit (ECU), (behind left side of dash), w/turbo, thru 9/91 5- Elec Cont Unit (ECU), (behind left side of dash), w/turbo, from 10/91 6- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; LHD 7- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; RHD; thru 8/91 8- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; RHD; from 9/91

I have the ECU (under the dash) so it would be one of the selections 4 to 8. It is a non-turbo, so it's down to selections 6 to 8. It is a 1992 so selection 7 is out, leaving me with:

6- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; LHD 8- Elec Cont Unit (ECU), (behind left side of dash), w/o turbo; RHD; from 9/91

Okay, so how do I tell if I've got a left-hand or right-hand drive? If I pick the LHD item (#6), there are a couple in-town salvage yards where I can get the ECU for $100. If I pick the RHD item (#8), there is an in-state salvage yard about 100 miles away (so I'd have them ship it) that only wants $30. At $30, I'd just get it and see later if it fixes the problem after first trying out other causes (O2 sensor, knock sensor, EGR valve, PCV). At $100, I would wait until it was the last stab at fixing the problem.

Presumably I am looking for "something" to be on either the left or right side of the engine to determine if it is a LHD or RHD model. What would be that "something" that I look for (that stands out like a sore thumb so dummy me can easily identify my engine type)?

What is interesting is if you put in the "wrong" ECU that you might actually improve your horsepower

I almost wonder whether it matters if the ECU is listed as for LHD or RHD.

Well, I would guess that the diagnostic procedure is as follows:

1) Remove the hood so you can see the engine and various moving parts of it from the drivers seat. Alternatively you can train a helper to let you know what is going on with hand signals. You MUST be sitting in the drivers seat. 2) Turn the ignition to that state one click before on, where the electrical system starts to power up, but before the starter turns over. 3) While the ignition is at this state, make sure the helper (if you have one) is standing clear, and look at your steering wheel. If it's on the left hand side of the car, you have LHD. If it's on the right hand side of the car, you have RHD.

(Sorry, couldn't resist.) :)

Hi,

You've got me confused now...

If you clear the ECU, you lose all stored codes. At least those that can be read "flashing light" style. I wonder if that's why you're not showing much? I always look for any "flashing light" codes BEFORE resetting my ECU. Then anything NEW will show up next time.

LHD vs RHD?

Ok, Cam already pulled your leg on that one, but he is right! Unless you've got one of the few right hand drive Subies that were generally sold as postal delivery vehicles, you've got a left hand drive car in the US or Canada. In Japan, the UK, maybe Oz and New Zealand (??), I'd be giving you bad advice!

Good luck!

Rick

Okay, so LHD and RHD has to do with the steering wheel. Geez, that's way too easy. That seemed so obvious and yet totally unrelated to the ECU that I figured it wasn't what the terms meant. Why the hell would the engine computer care whether the driver (who might be the only occupant but there might be others) is sitting on the left or right side? Or whether the steering column, pump, and rod are on which side? I figured it had to do something with which front wheel was the driving wheel (and the other the slave or differentialized wheel). I can't see the ECU would give a gnat's fart about where was the occupants or the steering wheel.

Since the ECU is the *engine* computer, I figured LHD and RHD had something to do with the engine, not which side of the car was the steering wheel. The position of the steering wheel actually has some actual effect on the fuel injection system? If so, please explain. Maybe I've left the rear defogger on, too, and that affected the fuel injection system. ;->

There's only one difference then that I could see between a ECU that was designated as for LHD or RHD: the mounting of the ECU. In other words, if the ECU's position changes as to where it is mounted then maybe it has a different bracket or mounting requirements. The ECU would be computer-logically the same for both LHD and RHD. If the mounting were the only difference, why is there such a large difference in pricing for a LHD ECU ($30) and a RHD ECU ($100)?

Since my ECU is under the dash on the driver side, and since that is on the left (in the USA for the model that I bought), it must be a LHD unit. Well, that helps since I can get one for just $30 (plus shipping).

I figured you were probably too close to the problem to see that. :)

It apparently doesn't. I followed the link you provided, and it said that the ECU was electronically equivalent, but had a different mounting orientation for RHD applications:

"The physical mountings of RHD and LHD units are a mirror image of each other. Other than the mounting differences, and ae78x is functionally equivalent to an ae80x. I.e. for the same value of 'x', the circuit board, software, maps and pin connections are the same. That we have found so far, any version of either RHD or LHD ECU will work in any car, with varying performance results."

Bingo!

Availability would be my guess. But aren't those numbers backwards from the way you first gave them? In your previous post, I think you said the RHD one was $30.

So, either the RHD is scarce - and as such commands a premium at $100 - or demand for the RHD one is so low that the wrecker just wants to get rid of it for $30. :)

Either way, on a 13 yo car - where you only SUSPECT the ECU, I'd get the cheaper one. You can put it in and duct tape it up to diagnose if that is the problem with less money outlayed. If it IS the problem you can either mount it creatively, or go get the $100 one and keep the other as a spare.

Yep, you're right. I got the costs swapped in my last post. Wishful thinking. The LHD costs more probably because there is more demand. Hmm, wonder what it would take to mount a RHD unit in the LHD location. If it just doesn't screw down to the body, I could just use some perforated metal strapping strip to hold it down. Hopefully the box size has the same dimensions to avoid clearance issues.

Thanks for the help.

I've tried reading the code after the "check engine" light came one. I then went through the procedure to clear the ECU, wait for the "check engine" light to come on again (could be immediate, could be a few days), and then check the error codes but it still reported no errors (except once - except I'll have to redo the procedure since I'm sure about one in six attempts showing an error code).

I figure if the ECU is turning on the check engine light but then reports no error codes that it must be "confused" (i.e., defective). If it turns on the light, it should report an error. If it turns on the light but says there is no error, I figure the ECU is screwed up.

In article , vanguard snipped-for-privacy@gawabNIX.com says...>

There's a couple of ways (at least) that error codes are stored in the ECU; not certain if this is a Subi trait or generic. I've run across postings where the CEL was thrown but ECU interrogation showed "no codes"...the term "historic codes" comes to mind. Under some conditions, these "other" CEL codes are readable

*at the time that the CEL is lit*...guess it depends on the OBDII device capabilities, to a degree.

Hi,

Since Vanguard's Subie's a '92, isn't it pre-OBDII? If so, there could be any number of proprietary things going on, which ISTR was one of the main reasons behind having a "standard" wasn't it? But, since my '90 stores all the codes it's thrown since the last reset, and his doesn't seem to be doing so, the idea of possibly having a bad ECU is seeming much more real!

Rick

Dunno...CRS. Understand the ECU idea, but I'd expect....MORE than an intermittent/occasional effect.

Wonder how dirty and corroded his grounds are, on a 92 Subaru..hmmm.

Indeed! I gathered that was the point of the TSB concerning cleaning connectors and protecting them with dielectric grease some years back: Subaru determined water would get into the connectors and cause corrosion, followed by bad voltage inputs to the ECU. I know on my car, there are STILL a couple of connectors that are problematic even after that treatment. Something will throw a code, the CEL goes on, I read the code, and it's a safe bet it's one of them... clean 'em and we're good to go again.

IME, by the time a car's 10 yrs old or so, it's NOT a waste of time to go thru the whole electrical system under the hood, cleaning contacts and all that. ESPECIALLY grounds as you suggest!

Rick