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Swapping ecm`s

Discussion in 'The Garage' started by Canadianbuilder, Dec 29, 2005.

  1. Canadianbuilder

    Canadianbuilder Registered Member

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    I put a 305 tbi (don`t laugh its what I had laying around)th400/205 combo in the 89 suburban I`m building.The burb was a 2 wheel drive with a 350 tbi in it originally.Is it necessary to swap in an ecm from a 305 truck?and if so why?.I haven`t had a chance to actually drive my burb yet but the 305 starts and runs good.The idle is somewhat high on it though, probably 2000 rpm when the engine is cold and drops to about 800 after a few minutes when it warms up.The temperature here has been around 28* f.
    I have the original ecm for the engine which came out of an 88 chev extended cab 4x4 with manual transmission.Will this ecm plug into the suburban harness?Thanks for any info you guys can give me.
     
  2. mouse

    mouse 1/2 ton status

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    The 305 ecm will bolt right in. The computer should both be 7747, but the chip inside will be programmed differently. If you have both, run the 305 version for better performance.
     
  3. blackblazer717

    blackblazer717 1/2 ton status

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    ya i say if you have the 305 computer, install it be safe
    and that high idle sounds a little large i wonder if it will be better with the correct computer
    LUKE
     
  4. jac6695

    jac6695 1/2 ton status

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    If the 305 ECM is from the manual tranny truck, it will run the engine just fine with the exception of two issues I can think of:
    1. the idle will be about 100 RPM higher then required for the TH400 (so keep the foot on the brake at a light).
    2. you will need to disconnect the original GM kick down relay (mounted on the firewall near the fuel pump relay). The ECM will energize the relay at the same time as it would light the "upshift" light in the manual tranny truck. With the kick down relay disconnected, you will have no automatic kick down and will have to do it manually. Make sense? PM me if not (I may not check back here).

    Edit: just noticed that you can't send PM's. I did a similar swap years ago. I used a TBI 454 from a 4 speed truck, and mated it to a TH400/Doubler. I went through the trouble of wiring in a kick down relay for the TH400 into the TBI harness (all of the circuits are in the right place, but the ECM controls it differently- found out the hard way). The higher idle worked well for my big block and Doubler- I could let it idle over many obstacles
     
    Last edited: Dec 30, 2005
  5. Canadianbuilder

    Canadianbuilder Registered Member

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    Thanks

    Thanks for the advice gentlemen.I will probably swap the 305 ecm into the burb and see if there is any noticeable difference in the way the engine runs.It would be nice if it lowered the rpm of the high idle when the engine is cold but I`m not sure I would be that lucky.The burb had a 700r4 in it when it was a 2 wheel drive and I haven`t installed an electric kickdown for the 400 yet.
    I was wondering if not having the 700r4 wiring harness connected to anything will have an effect on the ecm?Any ideas?
    I`m kinda thinking this is going to be the last fuel injected truck I build for a long time.I`m not really happy with the complex wiring and multitude of sensors and the guessing if this is compatible with that etc. lol.
    Although I guess its just a matter of getting used to it.
     
  6. jac6695

    jac6695 1/2 ton status

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    Oh, so the 2 wheel drive Burb was originally 350/700R4 combo? What did you do for a speed sensor? You might need to add one. Check with JagsThatRun (www.jtr.com I think). They have a VSS that will screw on to your speedometer connector on the 205. You might also have some emissions issues if all of the parts of the system aren't there. If your local laws don't require the emissions stuff, I would suggest having a chip burned to eliminate it, and some other unused parts of the system not used.

    Don't be intimidated by TBI. It isn't much more complex then a good carb system, and parts are readily available at most parts stores. Keep learning about the system, and pretty soon you will be moving up in the GM fuel injection world.
     
  7. Canadianbuilder

    Canadianbuilder Registered Member

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    On the 89 burb`s the speed sensor is built into the speedo and is cable driven like the older trucks.We don`t really have too much emission testing here yet.I think it might be common down in Toronto where they have air quality problems.I have never had to get any vehicles of my own smog tested,but up here they do smog test commercial vehicles(transport trucks)every year.I was watching on the news tonight though and apparently the law requires vehicles 5 years old and newer to be tested.I personally don`t know anyone that has had to have their personal vehicle tested though.They were saying though that starting in 2007 they want to start testing vehicles up to 20 years old(1988 and newer)so we`ll see what happens then.
    I have dual exhaust on my truck and no cats on it and when I am ready to put it on the road guaranteed they won`t care about not having the cats when I take it for a safety check.
     

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