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14.5 at battery 10.7 at ECM

Discussion in 'FAST Support Forum' started by J-440, Apr 7, 2018.

  1. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Had this problem awhile back and took the advice of installing 2/0 welding cable from the alternator to the battery. Problem still persists when the car is idling. With everything on, the car still idles fine and nothing seems to change. Haven't driven it yet though. With the smaller wire previously installed, I had a few issues, but not anymore. Just my ECM is reading low on the volts. Multimeter at the battery and the alternator post is 14.5. With everything on it's 14.3.
    I'm getting a solid 12 volt reading from the pink wire going to the ECM. Should I be concerned? Thanks again.
     
  2. Denis

    Denis Member

    Joined:
    Dec 2, 2009
    Location:
    Mendocino County, Northern Calif
    If you are getting 14.5 volts at the battery and alternator and only 12 volts at the pink wire (which should come off of the ignition switch) that amounts to a 2.5 volt loss between the battery and ignition switch. From my point of view that is excessive I would expect perhaps a half to one volt loss between the battery and the ignition switch. This still does not explain the ECU reading of 10.7 volts however, my XFI voltage readings are always lower than what I see at the ignition switch.

    One thing that would be an easy fix would be to use the ignition switch feed going to the pink wire to operate a relay and connect a wire from the battery to the relay and use the output of the relay to feed 12V to pink wire. This should give you an additional 2.5 volts headroom at the ECU.
     
  3. Pfingstl

    Pfingstl Member

    Joined:
    May 30, 2016
    Location:
    Denmark
    I use the solution with a 12V feed to a relay.

    I actually made this solution prior to the FAST install because an upgrade to HEI distributor with MSD module with rev. limiter needed more power than the original ignition wire could deliver.
     
  4. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Sweet. Gonna try that this weekend. Thanks!!!
     
  5. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Well I wired up a 30amp relay the way you said and there is no change. Do I need a higher amp relay or do I need to look at something else?
     
  6. Pfingstl

    Pfingstl Member

    Joined:
    May 30, 2016
    Location:
    Denmark
    Assuming the wire going from the battery to the relay and on to the pink is ok it sounds strange.

    To check for ecu Problems try to connect the pink wire directly to the battery and start the engine.
     
  7. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    You've had numerous previous issues that have had to be corrected - ongoing since 2013. I wouldn't be surprised if all those previous issues have created cumulative damage to your XFI ECU. You should contact FAST Tech. Your ECU may likely be in need of warranty repair service.
     
  8. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, I ran the ECU wire straight to the battery with no change. I tried switching the ECU wire and the battery wire to that relay and I heard a "click" from the relay and still no change. Switched them back how they were supposed to be, same results.
     
  9. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Oh yeah I agree. My wiring is definitely questionable sometimes.
     
  10. Denis

    Denis Member

    Joined:
    Dec 2, 2009
    Location:
    Mendocino County, Northern Calif
    A 10 amp relay would have done the job so relay size is not an issue. By connecting the pink wire directly to the battery the ECU should be seeing your battery voltage of 14.5V with the engine running. Or assuming the ECU is reading low you still should have increased the voltage at the ECU by 2.5V, by connecting directly to the battery. If it is still reading 10.7 volts I think the only conclusion it that there is an issue with the ECU, as AA suggested above.
     

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