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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.
     
  11. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Thanks again Denis. Yeah I went ahead and shipped my ECU back to FAST.
     
  12. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Found the correct pink wire, ran it straight to the battery and now I’ve got a 14.5 reading. I want to clean up my battery area. Could I hook up the 2 red wires from the ECU that go to the battery to a power distribution block? And also could I tie that pink wire into one of the wires of my ignition switch?
     
  13. Denis

    Denis Member

    Joined:
    Dec 2, 2009
    Location:
    Mendocino County, Northern Calif
    Sounds like you found the solution to the voltage problem. The pink wire that goes to the ECU is normally tied to the ignition side of the ignition switch. Be careful here because the accessory side of the ignition switch turns off the 12V to accessories during cranking and the engine will not start if you connect to that side of the switch.

    If there is a voltage drop issue with the connection to the ignition switch you can use a relay with the coil connected between the ignition and ground and the contacts connected to the battery and pink wire going to the ECU. Personally, I would not worry about a little voltage drop as all the pink wire does is provide power to a relay that turns the ECU on and off and provide a voltage source that the ECU measures and shows as system voltage. If you do use a relay, one with 2A contacts would be adequate.

    With regard to the two red wires connected to the battery from the ECU, it is important to leave them connected directly to the battery as the battery functions as a filter effectively shorting out any electrical noise that exists on the two red wires. If the wires are connected to a power distribution block the impedance of the wire connecting between the block and the battery will reduce the filtering effect of the battery, even if you use a very heavy wire. Not something that should be done!
     
  14. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    All good to know. This FAST system sure is finicky, guess I'll forget that distribution block. Also, when I connected the pink wire to a 12V constant hot under the dash, I was only getting a 14 volt reading. When connected directly to the battery, I had a 14.5 volt reading. Guess its a distance thing since the battery is in the trunk?
     
  15. Pfingstl

    Pfingstl Member

    Joined:
    May 30, 2016
    Location:
    Denmark
    As I said earlier. You need a good 12V going to a really and then to the pink wire.

    You can use the old ign. plus to control the relay.

    Works for me with the battery behind the drivers seat.
     
  16. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Well I've fixed my problem unintentionally but hey I think it'll work. All I did was run the pink wire to a 3 terminal toggle switch. ECU to switch, another pink wire from the switch going straight to the battery, then a ground. Car idled for 10 minutes and the wire didn't get hot this time. That and I'm finally reading 14.3 volts on my laptop. No extra relay needed. I don't know why the same setup got the wire hot last time. Maybe had 2 wires crossed on the switch? Hell I'll take what I can get. Now, I hope the experts will tell me this is safe otherwise I'm sending the car to the crusher. Thoughts?
     
  17. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    There's a Tesla orbiting in space. It probably needs some company. Give Elon Musk a call.;)
     
  18. Pfingstl

    Pfingstl Member

    Joined:
    May 30, 2016
    Location:
    Denmark
    You have lost me somewhere between the first and last post.

    Did you ever try any of the suggested solutions to solve the problems or just some trial and error...

    As someone here in Denmark commented on Elon Musk sending a Tesla into space. “If I had a Tesla I would do the some.....”
     
  19. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Oh yeah, I tried it all. I’m putting an in line fuse between the switch and the battery. Man I sure do appreciate the help. Don’t think ole Elon would want his technology contaminated with my electrical skills.
     

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