Results 1 to 13 of 13

Thread: ECM and fuel pump relay control

  1. #1
    Fuel Injected!
    Join Date
    Jul 2012
    Location
    Louisiana
    Age
    62
    Posts
    42

    ECM and fuel pump relay control

    Okay, one more trip to the well.... (this is what you guys get for being so helpful).

    I finished stripping my harness and my (hopefully) last issue of confusion is with the 2 fuel pump relay connections at the ecm.

    Pin A1 is listed on the 7747 wiring diagram as the "Fuel Pump Relay Control," which based on the diagram on top of the relay does exactly what the diagram says.

    What has me stumped is the "fuel pump input" at B2; on my harness the wire taps into the wire from the switched output of the relay to the fuel pump, and as near as I can tell this wouldn't do anything other than tell the ecm that the fuel pump has power. Is that right? If so, why? More to the point, do I need or want the B2 fuel pump input circuit on a carb to TBI swap?

    Also, when the relay is not energized the fuel pump input circuit is switched to another relay output (it would be pin 87a on a Hella/Bosch relay) that is shown as "fuel pump test" on the wiring diagram. What the heck is that?
    Last edited by belaw; 07-30-2012 at 12:59 AM.

  2. #2
    Fuel Injected!
    Join Date
    Jan 2012
    Location
    Roy WA
    Posts
    272
    Ooo let me try to answer something for once!

    The ECM watches fuel pump voltage and I believe uses that to "correct" for the amount of fuel injected at that voltage (which I assume theoretically corresponds to pressure).
    "Frankentruck" is a "1985" K5, TPI 350 '165/6E (Vortec Heads, SDPC2000 base, roller cam, headers), 32 Spline SM465, 205 w/VSS, 10b/14SF, 3.42's and 33" tires. Soon to be boxed frame, '87 sheetmetal, and TPI w/ '727

  3. #3
    RIP EagleMark's Avatar
    Join Date
    Feb 2011
    Location
    North Idaho
    Age
    63
    Posts
    10,477


    The fuel pump test wire is for just that. Hook power to it and run pump to test or prime.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  4. #4
    Super Moderator
    Join Date
    Mar 2011
    Location
    Camden, MI
    Age
    35
    Posts
    3,026
    i find no flaws here. :)

    generally, if you're watching both battery voltage and fuel pump voltage via the datastream, fuel pump voltage will almost always appear smoother. could be due to capacitor filtering inside the ECM for each channel's A/D circuit itself being different or just that the fuel pump tends to get a cleaner source of power, but i have yet to see a vehicle where fuel pump voltage looked noisier than battery voltage. GM may or may not have realized this as well and chose to use fuel pump voltage rather than battery voltage for injector corrections.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  5. #5
    Fuel Injected!
    Join Date
    Jan 2012
    Location
    Roy WA
    Posts
    272
    Heading off on a tangent...is it normal to see a ~-1V difference at the fuel pump vs. battery voltage in the datastream?

    Years back when I met with Mark Mansur and he was checking the data on my truck, he made mention that the difference in voltage (between batt and fuel pump) needed to be corrected. But what you say makes sense, perhaps I need to see what is actually getting to the pump voltage-wise?
    "Frankentruck" is a "1985" K5, TPI 350 '165/6E (Vortec Heads, SDPC2000 base, roller cam, headers), 32 Spline SM465, 205 w/VSS, 10b/14SF, 3.42's and 33" tires. Soon to be boxed frame, '87 sheetmetal, and TPI w/ '727

  6. #6
    Super Moderator
    Join Date
    Mar 2011
    Location
    Camden, MI
    Age
    35
    Posts
    3,026
    mine are generally within about .3 volts of each other.... with the fuel pump voltage almost always being higher. there are exceptions, like when a large electrical draw is kicked on (fans, lots of brake lights(8 on a 88-96 grand prix), A/C etc). then all bets are off, since battery voltage will always dip, yet fuel pump voltage may stay close to where it's at, sometimes it will also drop. may depend on which load causes it.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  7. #7
    RIP EagleMark's Avatar
    Join Date
    Feb 2011
    Location
    North Idaho
    Age
    63
    Posts
    10,477
    Quote Originally Posted by RobertISaar View Post
    i find no flaws here. :)
    That's not as funny as "I approve this message!" or was it "I approve this purchase!"

    Your lucky to have one voltage in data on1227747, but that incoming line is important.

    As for the voltage drop, yeah on a conversion but it shouldn't be. Injector voltage compensation tables will adjust for some. Probably wire gauge to pump from relay, or just lower alternator charging of the conversion vehicle, or in older truck conversions the ground back off pump to body or frame. In conversions I run the ground back to block.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  8. #8
    Fuel Injected!
    Join Date
    Dec 2012
    Age
    46
    Posts
    68
    I am currently at a wall with my setup. While data logging my bat volts are 13-14v but my fuel pump voltage is 5-6v. The car runs like crap when this happens. I traced the tan/white wire from the relay to the ECM and it is getting 13-14v but it appears it is loosing or miscommunicating the voltage in the ECM. I think it is time to try another ECM to rule out an internal clitch.
    Jason
    1987 Silverado V-10 TBI
    Goodwrench 350 260hp/350tq motor
    3.08 gear 400 trans
    Moates Tuning Equip
    TunerPro Software
    Level, Plumb, and Square

  9. #9
    It sounds like a bad ecm or something is wrong with the $6e adx

  10. #10
    RIP EagleMark's Avatar
    Join Date
    Feb 2011
    Location
    North Idaho
    Age
    63
    Posts
    10,477
    Or it could really be 5-6 volts at fuel pump because or wiring or ground issue?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  11. #11
    Isn't The voltage reading on a 1227165 ecm at B2 ?

  12. #12
    Fuel Injected!
    Join Date
    Dec 2012
    Age
    46
    Posts
    68
    im going to try another ecm this week just to see since that's the easiest
    Jason
    1987 Silverado V-10 TBI
    Goodwrench 350 260hp/350tq motor
    3.08 gear 400 trans
    Moates Tuning Equip
    TunerPro Software
    Level, Plumb, and Square

  13. #13
    Fuel Injected! JP86SS's Avatar
    Join Date
    Aug 2011
    Location
    Browns Town (Cavs too)
    Posts
    59
    In $8D it is selectable which of the two voltages is used for injector compensation.
    86 Monte, 406, Hyd Roller, 700R4 beefed, G3-APU1 and NVSRAM 730, S_AUJP

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •