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  1. #1
    Carb and Points!
    Join Date
    Jun 2024

    First post


    I recently got a 1989 Chevy 1-ton van chassis with 350 tbi and a TH-400 3-speed. ECM is 1228747 with code AKWC. I also bought an OBD1-to-usb cable and downloaded TunerPro RT. I'm currently working a couple of the diagnostic trees in the Emission Control Manual. Also, I changed tire/wheel sizes to get away from 16.5" rims and am considering an aftermarket overdrive unit.

    As I start into this, I have a few initial questions:
    1. How can I best avoid doing anything fatal to the ECM. What actions are best avoided during initial scans?
    2. What ways might the new tire circumference negatively impact ECM performance, given that it now reads an inaccurate VSS signal?
    3. Where can one find the best tutorial for TunerPro RT?

    Thanks in advance for any help. I look forward to collaborating here!
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  2. #2

  3. #3
    Carb and Points!
    Join Date
    Jun 2024
    cheers learning a lot from the posts. I have a Chev G30 RV with 5.7L TBI 4L80-E 16197427 ECU. Did find $0D asm which looked like it matched 7427. not sure of cpu maybe 68332 Motorola. Looking for a assembler so we can see if we can build full image. Does anyone have a schematic of the 7427 board, I have pin outs of the connectors and 1995 wiring diagram as well as vacuum line diagram. Downloaded TunerPro RT, trying to figure out what xdf ads files are needed to match the ECU. I built up a OBDC 1 cable to USB, hope to be able to get some data off to better understand how things are running. Did learn today most new IAC servos are shit. But you can adjust the gap by turning the pintel or using different thickness gaskets. Or drill the plug on the TBI to expose the #20 idle adjustment screw. Goal of course is to solve error 35 issue. You need to do disconnect battery for 30 sec, then do the IAC reset procedure. That sorta worked, SES light went out, but after driving 20 miles, when stopped at long light SES light fluttered on, but went out soon as we started driving. Suspect idle is little off still, will hook up vac gauge to see if a leak exists, check timing as well. In Drive it idles at 730rpm when at operating temperature. Cold idle in Park is 650, warm 850. As long as SES is out, runs much better than before. I'm sure I missed something. Your comments would be greatly appreciated. The engine and transmission have 25K on them, its a goodwrench rebuilt power train. J

  4. #4
    Carb and Points!
    Join Date
    Jun 2024

    No DRAC that I've been able to find. There's a mechanical speedometer cable from the transmission up to the back of the instrument cluster which houses an analog speedometer with manual odometer up to 99,999.9 and manual trip up to 999.9. When things are cold, I can see the speedometer needle softly bounce in time with a light noise from the cable. So I'd say it's mechanical all the way. If there is a VSS or VSS buffer, they must be somewhere up behind the instrument cluster, which I haven't yet attempted to remove.

    As an aside, calculations from an extended trip during the last month suggest that the odometer over-indicates by roughly 3.8%, the speedometer appears to over-indicate by about 7%. If the ECM is getting a speed signal from a VSS driven by either of these sources, I'm curious to know if that has a meaningful impact on how the engine is being commanded to operate.

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