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  1. #1

    Default new battery but barely cranks

    I put the battery in the back and have a new battery, but the engine barely cranks over. It was turning over with an old battery but barely so I went and got a 800cc super start premium 34/78prmj battery thinking that was the problem but it now barely moves with the new battery.

    The motor is a new 347 that I got a few minutes of break-in in before it died (megasquirt tuning)

    I have the ground bolted to the quad shock mount and both cylinder heads have ground straps to the firewall cut off switch bolts are tights
    solenoid it tight and starter is tight

    So is there anything obvious I could be missing

    Thanks

    Todd

  2. #2

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    I am thinking ground. Shouldn't the ground from the battery go to block, timing cover location?

  3. #3

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    Quote Originally Posted by RazorbackGT View Post
    I am thinking ground. Shouldn't the ground from the battery go to block, timing cover location?
    Yes; the ground from the battery is attached to the front of the passenger side of the block. When the starter motor is engaged its a VERY high current draw and your grounds (wire size) using the body points are not adequate to pass that much current and causes a voltage drop which results in too low of a voltage to adequately crank the starter motor.
    Last edited by JacksAO; 09-02-2018 at 09:12 AM.

  4. #4

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    Since I have put the battery in the back should I run a ground wire full length to battery block or can I go frame to block?
    which would give me battery to frame and frame to block

  5. #5
    FEP Senior Member droopie85gt's Avatar
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    Quote Originally Posted by toddf View Post
    Since I have put the battery in the back should I run a ground wire full length to battery block or can I go frame to block?
    which would give me battery to frame and frame to block
    People will say either. First question is what size cables did you run? It should be at least 00 on the power AND ground. IF you ground it to the body, you need to grind it down to bare metal and use a serrated washer. The protect the connection with some electric or lithium grease.
    Then in the front again, a 00 or a BIG copper braided cable to a place on the block and frame that are ground to bare metal.

    Just remember, 12v electrical travels in a circuit or think of it as a loop. If you have a 4 gauge power feed and 12 gauge ground wire, you only have the current capacity of the 12 gauge wire.
    1985 GT, Sunroof, 5 Lug, Rear Discs, 01 Graphite Bullets, 88 forged piston shortblock, 2.02/1.60 Alum heads, Weiand Stealth, Holley C950 TBI, BBK Long tubes

  6. #6
    FEP Power Member Ourobos's Avatar
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    Ground. You need 0/1 gauge cables, especially the ground. I hate trunk mounted batteries for this reason.
    1986 CHP SSP Coupe

  7. #7

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    I did use 0 size and just went from block to frame in front on driver side would going from block to frame on passenger help?

  8. #8
    FEP Power Member Ourobos's Avatar
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    Like more grounds in the rear near the battery would help more. If your mounting points are bare metal or dirty, clean that quad shock mount up first. Then add another would be my suggestion
    1986 CHP SSP Coupe

  9. #9

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    is your timing too far advanced?
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
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  10. #10
    FEP Member brianj's Avatar
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    Is this a newer gear reduced starter or the old, large starter? As asked above, have you set the static timing in the time you had it running? I'm not sure if megasquirt controls timing, but it's something to look at. I'm assuming here ypu have checked al, of your connections on the ground and power side.
    1983 Mustang G.T. No-option stripper- I like strippers.
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  11. #11

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    did you do a voltage drop test along the various positive wire points
    1986 GT T Top- stock except for magnaflow cat back
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    2013 GT- usual bolt ons

  12. #12

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    The added ground worked
    as far as the timing goes, I don't know yet with the megasquirt I think I am close,, but it is so erratic I can't get a good number

  13. #13

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    Why your at it, it's a good idea to run motor to body ground strap also, if not already done so...

  14. #14
    FEP Super Member erratic50's Avatar
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    If it barely cranks, test the starter. Also since it’s fresh pull the plugs and check for water in the holes - a head gasket failure would make it near impossible to crank.

    Also - process of elimination - what does it do with normal length cables and the battery in front?

  15. #15
    FEP Power Member Ourobos's Avatar
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    Quote Originally Posted by erratic50 View Post
    If it barely cranks, test the starter. Also since it’s fresh pull the plugs and check for water in the holes - a head gasket failure would make it near impossible to crank.

    Also - process of elimination - what does it do with normal length cables and the battery in front?
    He added an additional ground in the rear and solved it per post # 12
    1986 CHP SSP Coupe

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