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Need help with ECU

Old 03-02-09, 03:00 PM
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Need help with ECU

As with most race cars, mine is pieced from several other cars. I started with a '88 GXL, stripped it down, removed all non-esential equipment (air pump, A/C, heater core, radio, etc.) I have a '91 donor car that I'm going to use the pedal assymbly, gauges, and computer for the manual transmission I got out of another '88.

My question is, what kind of problems can I expect to run into from the S5 ECU runnning a S4 motor, and how will not having some of the original systems effect other aspects related to the ECU? I have been studying this thing, but since I'm not very experianced in this matter, nor are there any resources near that can help, I've been teaching myself as I go. Any help would greatly help.

Also, what other tips can anyone give as far as tuning with what comes stock, and removing emissions, etc..
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Old 03-02-09, 03:17 PM
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It's been years since I researched this, but my first thoughts are that the wiring harness from an S4 won't match up with an S5 ECU. You'll be missing the electrical connectors required for an S5 ECU to talk to the oil injector pump for sure. Since it's an S4 motor, that's no big deal, except the ECU is expecting to talk to the pump and if it can't, it puts you into limp home mode. Again, it's been several years since I messed around learning about this, perhaps someone has found a way to get the ECU to think the oil pump is there, I don't know.

Depending on the year of the S4, you may be running different impediance injectors.

You'd be better off finding the appropriate ECU for your year engine.
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Old 03-02-09, 03:31 PM
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thats basically right. the s5 ecu needs to run the s5 engine harness, and it needs more stuff hooked up to make it happy, like the metering pump. also the s5 ecu does not plug in under the dash to the s4 car. you can swap harnesses or rewire it .

its been done, its not super hard, but it is kind of long and painstaking

running the s5 ecu on the s4 engine is ok
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Old 03-02-09, 03:36 PM
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you really want the ECU, front cover, throttlebody, and engine harness to be from the same series if you want to save yourself a lot of headaches
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Old 03-03-09, 10:54 AM
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That's about what I figured, only after I bought the '91 did I discover the major changes between that one and my '88. The price was right on both cars, under $500 each, so I figured if I needed to do some extra work, or send the ECU off to get programmed, that cost would be worth it, since I got such a deal. Addititonally, since my labor is free, I don't mind putting my time into it.
--The killer is, the '91 I got had a chevy small block conversion, but whoever did it, really did a hack-job, and just started cutting up the wiring harness. so even if I swapped harnesses, still no go. The factory wiring harness in the engine compartment has been cut out.
--So either run the S5 ECU, and get a S5 motor and stock up on butt connectors, OR replace/remap the S4 ECU to be compatible with the manual trans. and other componets that have been deleted.

--Anybody have any companies or other sources I could research?
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Old 03-03-09, 04:32 PM
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Originally Posted by PHouston View Post
OR replace/remap the S4 ECU to be compatible with the manual trans. and other componets that have been deleted.
I'm confused. You have an S4 motor and S4 wiring, just hook up an S4 ECU and you are good to go. Nothing has to be remapped or replaced, this is a race car as you said. The ECU will be fine.

The most difficult thing you could do is try and use a hacked up harness or try and start mixing this stuff.
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Old 03-03-09, 04:58 PM
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it would be simpler to get the s4 stuff working, and this would get you out on track sooner. the s4 ecu's are the same auto and stick so no remapping needed.

save the s5 ecu for season 2? just a thought
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Old 03-04-09, 10:23 AM
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I was under the impression that the red-line limit is lower for the auto. And, Since I don't have tons of money, I'll be using the stock AAS shocks, for now, just manually securing the valves in the "sport setting". With all the sensors and stuff gone, I would expect the ECU to start getting confused and going into "limp home mode" Which I do not know if it would affect engine management systems, but I have seen wierder things.

I'm also concerned with the security system that the GXL came with. With those sensors or computer gone, could the system disable the ignition componets. I know the system is capable of doing it, if the computer thought the car was being stolen.

Even if the trans doesn't matter to the ECU, I know the emissions equipment will.
--Note-- I thank all of you for the feedback, it is a big help to me, as this is my first race car build, and my first rotary period.
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Old 03-05-09, 11:56 AM
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Honestly, not really. I've got all those damn smog vacuum valve acutator connectors ziptied in a bundle under the UIM, smog pump is blocked off, 6 ports wired open.

I've run into no issues with the ECU shutting down the ignition due to it thinking it was being stolen with the security system removed, and my car was a GXL.

Remember this is late 80s technology, it's not *that* smart.

As for the rev limiter, either auto or manual you'll want that removed anyway.

(here's a pic as proof of the smog stuff being gone, mind you, these are S4 electronics with an S5 engine/LIM/UIM. )



PaulC
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