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Thread: batteries going dead...

  1. #1

    Default batteries going dead...

    Been having a hard start situation for a week or so, sometimes I go out to the truck and it turns over fast, then other times it turns over slow... 2 days ago I pulled both batt's out and charged them, load tested them and they tested good, so I put them back in the truck and later that day I put voltmeter on the batts and its chargeing @ 13.6 volts, but today I go to leave work and its starting to be sluggish again...it went dead before and got a jump and cranked over fast, shut it off 30 mins later and try to restart and I get a couple rev's out of it and thats about it...soo what to replace?
    1995 K2500 HD (F) vin - 201000 miles, 4.10 diff, turbomaster "fresh rebuilt" motor, Marine injectors, #9 resistor, straight piped, stacks, custom paint, 4" lift, 35's

  2. #2
    Join Date
    Feb 2007
    Posts
    2,962

    Default

    Test the charge voltage when the batteries are a bit low. The charge voltage should be 13.8 volts with the engine revs above idle. If one of the diodes has burnt out in the alternator, it won't be charging adequately.
    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 #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, new engine, 0 miles. 3.73 diff, 16 mpg
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, since 1992.

  3. #3
    Join Date
    Jun 2008
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    Burnet durnet Tx
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    911

    Default ...once you...

    ...start truk alternator should "full field"{charge wide open} to at least 14.2v...then as batteries recover from starting engine should pull back to anything over 13.02...beast runs @13.9-14.2v constant...alt.runs 12v to operate truk AND accessories{read BIG sterio and a/c,wipers,etc.}everything over 12v goes to recharge the batt...I don't care what you were told or what you "heard",an alternator WILL NOT CHARGE A LOW BATTERY...it will only maintain it...you cannot reliably test an alternator on vehicle with a low battery...
    ..93 3500HD 200h 4L80E 4.88..11150lbs allatime..88 c.c. dually..6.2..auto w/doug nash 2-sp..camo paint...58 A120 4by IH{old vfd brush trk} 71ford p/u 79 shovel {{{ hooorraaayyy the bus is gone}}} 93 2500 parts truk {gone to that great chrusher in the sky 01 burban{wifes'.."no you can't use mine"}

  4. #4

    Default

    always love your post truckdoc...just got a new alt in today and going to drop it in tomorrow and see what happens...I'll let yall know either way...thanks for the info...
    1995 K2500 HD (F) vin - 201000 miles, 4.10 diff, turbomaster "fresh rebuilt" motor, Marine injectors, #9 resistor, straight piped, stacks, custom paint, 4" lift, 35's

  5. #5
    Join Date
    Feb 2007
    Posts
    2,962

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    An alternator will charge a battery from completely flat to fully charged, that's it's function. The regulator is designed to maintain the alternator voltage at approximately 13.8 volts constant. If it doesn't, it has a fault, or the alternator does.
    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 #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, new engine, 0 miles. 3.73 diff, 16 mpg
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, since 1992.

  6. #6
    Join Date
    Jun 2008
    Location
    Burnet durnet Tx
    Posts
    911

    Default ...at@1500rpm...

    ...or even at idle,with an output of 13.8 you have a 1.8v trickle charge{1st 12v needed to run vehicle} that's with everything OFF...if you can let it run 15to 24hrs,yes an alternator will charge a flat battery...but it's still quicker to use a charger...and you still can't test an alternator correctly with a weak or lo battery...jd
    ..93 3500HD 200h 4L80E 4.88..11150lbs allatime..88 c.c. dually..6.2..auto w/doug nash 2-sp..camo paint...58 A120 4by IH{old vfd brush trk} 71ford p/u 79 shovel {{{ hooorraaayyy the bus is gone}}} 93 2500 parts truk {gone to that great chrusher in the sky 01 burban{wifes'.."no you can't use mine"}

  7. #7
    Join Date
    Feb 2007
    Posts
    2,962

    Default

    1.8 volt trickle charge? Rubbish! Put an ammeter in the circuit, the charge rate for flat batteries will be around 100 amps, depending on the alternator capacity, compare that with a battery charger. Have you never had an ammeter on the boat or vehicle, I have. My boat with an 80 amp alternator would charge the flat 110 amp hour battery in an hour and a half.
    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 #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, new engine, 0 miles. 3.73 diff, 16 mpg
    1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, since 1992.

  8. #8
    Join Date
    Jul 2009
    Posts
    362

    Default

    Ok. so what was the out come ?

    My truck is doing the same thing go to start its dead flat 0 nothing . . . charge it up truck fires right runs drive it around park it come back to it and its dead again . . .

    This last time we unhooked my batteries, I haven't been back to see if it'll be dead yet, trying to figure out weather its on the battery side or the truck side . . .


    Mike
    1995 GMC K2500 "Turbine":
    6.5-T (F), 4L80E, 4.10 gears, LT7.50-16D tires, 140K miles.

    Mods/Upgrades:
    1. D-tech PMD #9 Resister bumper mounted.

    2. Air-box Deletion / Open air intake.

    3. CDR Deletion / Valve cover breather.

    4. Open exhaust.

    5. 3500HD trans cooler.

    6. Mechanical waste gate conversion.

    7. Gauges: Engine Oil temp, Boost, Trans temp.

    Mike

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