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ECU Backwards Compatibility?


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#1 PonchoCatalina

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Posted 04 January 2007 - 11:43 AM

I'm beginning to think the ECU on my '91 Loyale has a problem (at least that's what the FSM flow charts seem to lead to). I am going to re-test everything to verify, but should I need to replace it, will an older SPFI ECU work? My local salvage yard has an '88 or '89 wagon that's SPFI, but has an automatic transmission (mine's a 5-speed). To further complicate the issue, I have a 5-speed D/R transmission out of an '87 hatchback that I want to transplant into the Loyale sometime in the future. I know there is a problem with the neutral switch if you use an '87 transmission with a '91 ECU, but if I get an older ECU, I can maybe kill two birds with one stone and not have to worry about that issue. So, whaddaya think? Do I track down an exact matching-numbers '91 Loyale ECU, or go with the older one? I appreciate your thoughts.
-Ian

#2 calebz

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Posted 04 January 2007 - 11:48 AM

Compare part numbers. If they match, you're good to go.

#3 grossgary

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Posted 04 January 2007 - 12:13 PM

search around and get one that's the same part number. the problem is pricing, it might be hard to find exactly the same part number for a reasonable price, if that's the case then i'd start experimenting. and post in the Parts Wanted Forum, someone may have the ECU you need.

i wouldn't worry about the auto/manual issue, that won't end up being ECU specific issues. the reverse light deal is always an issue, but has nothing to do with the ECU. on the XT6 the ECU's are completely interchangeable across FWD, AWD auto and manual.

ECU's have a billion different part numbers, but don't all have to match. they do need to be close though, but it's annoying figuring out which ones are compatible by part number. you will be very limited by part number, subaru likes to changes the numbers if the color of one screw is different. i would imagine an SPFI ECU will work. you could post here once you locate one and ask if there are possible EGR or other differences, but i think you'll be fine, but that's guesswork too as i don't have any specific SPFI ECU experience.

for what it's worth an XT (4 cylinder) can run on an XT6 (6 cylinder) ECU...not very well but it does drive!

#4 GeneralDisorder

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Posted 04 January 2007 - 02:32 PM

Yes, the older ECU will work fine. They are nearly identical, with the only differences being the manual Loyale ECU has a clutch switch and the older one does not. An auto ECU will work too, but you may get a strange code about the park switch malfunctioning (or you may not).

Why do you think the ECU is bad? I've never seen one fail. And if it runs then it's likely not bad.

As far as the neutral switch - well that's going to be interesting. You'll have to play it by ear as the 87 tranny may or may not have the switch. You may have to use the clutch switch on your loyale pedal assembly to simulate the neutral switch so the ECU doesn't complain about not seeing it.

GD

#5 PonchoCatalina

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Posted 04 January 2007 - 03:39 PM

Thanks, GD, I had gathered that there was compatibility between SPFI ECU's, but I wasn't certain. My ECU may be fine and the vehicle does run, but I've got a code 23 and 24 (MAF and IAC, respectively) flashing and I can't seem to figure out why. I've performed testing per the diagnostic flowcharts in the FSM for both, and I wind up at "replace control unit." Voltages and resistances are within the flowchart's given normal ranges, grounds are good, and I have continuity to the ECU at each wire involved. As I said, I will re-run the tests completely to verify my readings, but it's starting to look like maybe an ECU problem.



Yes, the older ECU will work fine. They are nearly identical, with the only differences being the manual Loyale ECU has a clutch switch and the older one does not. An auto ECU will work too, but you may get a strange code about the park switch malfunctioning (or you may not).

Why do you think the ECU is bad? I've never seen one fail. And if it runs then it's likely not bad.

As far as the neutral switch - well that's going to be interesting. You'll have to play it by ear as the 87 tranny may or may not have the switch. You may have to use the clutch switch on your loyale pedal assembly to simulate the neutral switch so the ECU doesn't complain about not seeing it.

GD



#6 grossgary

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Posted 04 January 2007 - 04:02 PM

i don't know if this SPFI has the hot wire style MAF or not, i think it does? if the car runs bad, hesitating and backfiring, remove the MAF and look at the hair thin wires. if one is broken, this will be your problem. if one wire breaks an XT6 will still run, but badly, i would assume other models would perform similarly. while you're in there, have a can of MAF sensor cleaner handy and if the wires are good, go ahead and clean them anyway.

as for the idle air control, on the XT6, the IAC is always remedied by a simple cleaning. drench it internally with brake cleaner and it'll free right up. i've never seen one actually fail...i'm sure it's happened, but it's very uncommon.

#7 PonchoCatalina

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Posted 04 January 2007 - 04:16 PM

i don't know if this SPFI has the hot wire style MAF or not, i think it does? if the car runs bad, hesitating and backfiring, remove the MAF and look at the hair thin wires. if one is broken, this will be your problem. if one wire breaks an XT6 will still run, but badly, i would assume other models would perform similarly. while you're in there, have a can of MAF sensor cleaner handy and if the wires are good, go ahead and clean them anyway.

as for the idle air control, on the XT6, the IAC is always remedied by a simple cleaning. drench it internally with brake cleaner and it'll free right up. i've never seen one actually fail...i'm sure it's happened, but it's very uncommon.



Yep - ithe MAF is hot-wire style and the wires are intact. In fact, the MAF was installed new just before I bought the car back in September, and the car has only been driven a handful of miles due to a head gasket problem (now rectified). As far as the IAC, it might need a cleaning, but I don't think that would cause an immediate error code. Somehow the ECU is not communicating correctly with the IAC and MAF as soon as the ignition is turned to "on" and these codes will not clear until the cause is remedied.

#8 grossgary

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Posted 04 January 2007 - 04:38 PM

check the engine harness and plugs into the ECU. if they've ever been worked on before, it's possible a pin (or two) were bent when reinserting the plugs. unlikely, but it has happened before and is difficult to nail down.

have you checked the pinouts at the ECU for continuity from these two sensors?

#9 GeneralDisorder

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Posted 04 January 2007 - 06:24 PM

Sounds like a wireing issue to me. Check the plugs between the manifold harness and the main harness. Verify the continuity of the wires to the MAF and IAC. Clear your codes, and do a full D-Check.

The IAC is just a resistance value to the ECU - it's just a solenoid with two wires.

The MAF needs voltage, so make sure it's got voltage at the sensor, and then verify the other wires show the correct resistance and that it changes when you blow air over the element.

GD

#10 PonchoCatalina

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Posted 04 January 2007 - 10:06 PM

Sounds like a wireing issue to me. Check the plugs between the manifold harness and the main harness. Verify the continuity of the wires to the MAF and IAC. Clear your codes, and do a full D-Check.

The IAC is just a resistance value to the ECU - it's just a solenoid with two wires.

The MAF needs voltage, so make sure it's got voltage at the sensor, and then verify the other wires show the correct resistance and that it changes when you blow air over the element.

GD


I agree, it does seem like a wiring issue. I expected to find a bad connection, corrosion, broken wire, or something along those lines, but I found everything in order. All three wires at the MAF have continuity all the way from the sensor to the ECU and all the values check out, including voltage increasing when air is blown through the MAF. Likewise, the IAC has continuity all the way from it's white wire at the connector to pin 45 on the ECU (the other wire goes to ground and it checks out too). I think I may be in the same situation as azsubaru in this post: http://www.ultimates...ight=loyale ecu Coincidentally he also had a '91 Loyale with the same problem, at least as far as the IAC is concerned. Post #4 in that thread describes exactly the same thing I'm finding. Also, the codes won't seem to clear without fixing the problems, and D-check won't work because plugging the green connectors together kills the engine. What a mess! Thanks again for all the ideas - I'm kind of a wits end.

#11 GeneralDisorder

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Posted 04 January 2007 - 10:48 PM

You should be plugging the green connectors together before you start the engine. Do you mean it doesn't start with them plugged in? If you can't get it to run in D-Check then there's really no telling that the issue *really* is. U-Check isn't very accurate, and the FIRST step in verifying any codes is to clear them, and confirm them with D-Check.

I would say you have a sensor problem. Try a different MAF just in case, and perhaps adjust the IAC screw open a bit further. D-Check turns off the timing advance, and it sounds like you have a lean condition that makes it die....

GD

#12 PonchoCatalina

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Posted 04 January 2007 - 10:57 PM

You should be plugging the green connectors together before you start the engine. Do you mean it doesn't start with them plugged in? If you can't get it to run in D-Check then there's really no telling that the issue *really* is. U-Check isn't very accurate, and the FIRST step in verifying any codes is to clear them, and confirm them with D-Check.

I would say you have a sensor problem. Try a different MAF just in case, and perhaps adjust the IAC screw open a bit further. D-Check turns off the timing advance, and it sounds like you have a lean condition that makes it die....

GD


Actually, I plugged in the green connectors with the engine running and it died, but then I realized that was wrong so I unplugged them, turned off the car, then connected them, and attempted to restart. It wouldn't restart until I disconnected them, then it fired right up. The errors are showing in U-check mode. I'll try clearing codes again, and also try a different MAF and adjusting the IAC screw. As for clearing the codes, I tried the "connect white AND green connectors" approach - was that correct? Thanks for all the help and tips!!

#13 GeneralDisorder

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Posted 04 January 2007 - 11:12 PM

"connect white AND green connectors" approach - was that correct? Thanks for all the help and tips!!


Clearing memory is the same procedure as D-Check just with both sets connected..... Warm engine, Ignition on, depress pedal fully, release to halfway for min. 2 seconds, release fully, start and drive till you either get a code, or the all-clear blinking CEL.

You have to run the full D-Check for the codes to go away. The alternative is to leave the battery disconnected for a while and the ECU will lose it's memory.

Your first, and most important goal is to make it run in D-Check and fix the codes in the order they are reported. Once you accomplish that, the ECU will be able to help you. The codes from u-check are useless as the diagnostic routines are very simple and not always capable of giving correct information if there are multiple failures.

GD




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