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Thread: MPG vs Alternator

  1. #1
    Join Date
    Nov 2010
    Location
    Nashville
    Posts
    54

    Default MPG vs Alternator

    A question for the braintrust. I think I have a gasser alternator so that as I understand it the wrong RPM is generated and so the tach is not correct. If that is the case then will the ECM also see the incorrect signal and will it initiate a richer fuel mixture to the Injection Pump causing less MPG?
    95 K2500 Suburban, orig owner, 101000 miles,F vin, Banks Stinger system in 2000 (EGT & Boost gages,air intake,K&N,chip), CAT reqd here, 3.73 posi, 4L80E w/mercon V semisynthetic, Flight Systems remote PMD (#9 res), new IP @78,000 under warranty,SSD cooling sys w/ Dmax fan upgrade @95600 and all new hoses. New main pulley & Fluidamper @99200. PMDCable extension to bumper @ 101000. Walbro FR pump w/prefilter. FloPro Crossover. Diamond Eye 4" exhaust and high flow CAT

  2. #2
    Join Date
    Feb 2007
    Location
    Baja California Sur Mexico
    Posts
    2,804

    Default

    Alternator signal is for the tach only. Diesels don't have rich or lean mixture. Each intake stroke, the cylinder fills with air, and the amount of fuel injected, determines the revs/power.
    Grant H.
    1995 6.5 turbo GMC K2500 extra cab 4x4 pickup, PMD under bumper, mechanical wastegate, 228,000 miles, rebuilt engine, 4.10 diff, 16 mpg
    1994 6.5 turbo GMC K2500 4x4 suburban "F" VIN #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, Positrac 3.73 diff, 17 mpg highway.
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, 21 years.

  3. #3
    Join Date
    Nov 2007
    Location
    nor-cal
    Posts
    75

    Default

    what type of mileage are you talking about here?
    Kevin K. Nor-Cal Redneck
    '98 Chevy K3500 Dually Cowboy Cadilliac,VIN "F" L65 6.5TD, , 2" body lift, FSD Heat Sync, FLIGHT SYSTEMS upgraded D-TECH PMD,#9 pmd resistor, 6' extension harness, home-made redneck intake,3" stright stainles exaust with faux cat, upgraded and rebuilt 4l80eod,4.11 posi, 13-15 mpg at 55-60mph. Other goodies, Cobra 25-LX Cb radio with 8 foot whip, Rawson-Koenig Steel Tool box.

  4. #4
    Join Date
    Oct 2010
    Location
    Scugog Island Ontario Canada
    Posts
    203

    Default

    The ECM gets its rpm figure from the crank or optical sensor so alternator is not a factor.

  5. #5
    Join Date
    Nov 2010
    Location
    Nashville
    Posts
    54

    Default

    Thanks for replies. To answer question on mileage-running 75 w/AC and towing only a 1200 lbs trailer I am getting 13.9 mpg. I know I can get better by slowing down but tryng to eliminate any issues if possible. I also wonder about IP timing. I have not had timing checked and have no codes but wonder if static timing at the max limit would make a difference?
    95 K2500 Suburban, orig owner, 101000 miles,F vin, Banks Stinger system in 2000 (EGT & Boost gages,air intake,K&N,chip), CAT reqd here, 3.73 posi, 4L80E w/mercon V semisynthetic, Flight Systems remote PMD (#9 res), new IP @78,000 under warranty,SSD cooling sys w/ Dmax fan upgrade @95600 and all new hoses. New main pulley & Fluidamper @99200. PMDCable extension to bumper @ 101000. Walbro FR pump w/prefilter. FloPro Crossover. Diamond Eye 4" exhaust and high flow CAT

  6. #6
    Join Date
    Feb 2007
    Location
    Baja California Sur Mexico
    Posts
    2,804

    Default

    If you are towing a travel trailer, then at 75 mph, your mpg is fine.
    Grant H.
    1995 6.5 turbo GMC K2500 extra cab 4x4 pickup, PMD under bumper, mechanical wastegate, 228,000 miles, rebuilt engine, 4.10 diff, 16 mpg
    1994 6.5 turbo GMC K2500 4x4 suburban "F" VIN #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, Positrac 3.73 diff, 17 mpg highway.
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, 21 years.

  7. #7
    Join Date
    Nov 2007
    Location
    nor-cal
    Posts
    75

    Default

    i concur with grant, for one thing these are v8's and second 75mph on any diesel unless you have a six speed manual to help with the gearing is taxing the motor a bit and it has to burn a ton more fuel to keep up
    Kevin K. Nor-Cal Redneck
    '98 Chevy K3500 Dually Cowboy Cadilliac,VIN "F" L65 6.5TD, , 2" body lift, FSD Heat Sync, FLIGHT SYSTEMS upgraded D-TECH PMD,#9 pmd resistor, 6' extension harness, home-made redneck intake,3" stright stainles exaust with faux cat, upgraded and rebuilt 4l80eod,4.11 posi, 13-15 mpg at 55-60mph. Other goodies, Cobra 25-LX Cb radio with 8 foot whip, Rawson-Koenig Steel Tool box.

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