Any tuning needed if ECU gets replaced?

[Vanguard]

Vehicle: 1992 Subaru Legacy L AWD wagon non-turbo

If I replace the engine computer (ECU), do I need to do any timing or other
tuning due to the replacement? That is, is the ECU tweaked after
installation to match the current state of the vehicle? Due to other
problems, and if it looks necessary, I'm wondering what else would be needed
if the ECU gets replaced. I found a salvage yard that has an ECU supposedly
for my car model and only wants $30 for it. Seems pretty cheap, even for a
used one, so maybe I want to get it in the chance that eventually it turns
out that the ECU needs to get replaced (there are problems which point to
the ECU possibly being defective).

 

[Rick Courtright]

If I replace the engine computer (ECU), do I need to do any timing or other
tuning due to the replacement? That is, is the ECU tweaked after

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

 

[Vanguard]

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 (http://www.ecutek.com/tuning/oemcompare/). I
almost wonder whether it matters if the ECU is listed as for LHD or RHD.

 

[Cam Penner]

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)?

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.)

 

[Rick Courtright]

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.

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

 

[Vanguard]

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).

 

[Cam Penner]

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 you were probably too close to the problem to see
that.

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.

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."

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
Bingo!

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)?

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.

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).

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.

 

[Vanguard]

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.

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.

 

[Vanguard]

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.

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.

 

[CompUser]

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.

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.

 

[Rick Courtright]

*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

 

[CompUser]

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!

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.

 

[Rick Courtright]

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