1972 GT grounds?

Discussion in 'Ranchero Tech Help' started by FordR500GT, Oct 10, 2019 at 1:43 AM.

  1. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    So I’ve been fighting a problem with this 72 GT. It doesn’t crank very strongly, and kills the batteries (usually fully charged) quite quickly. I’ve replaced all of the ignition components from battery all the way down to the starter. I can only assume it’s a problem with a ground somewhere. Does anyone have a wiring diagram that can show me all of the correct engine and body grounds? The one ground going from the block to the firewall is intact, otherwise I don’t see any others.
     
  2. burninbush

    burninbush In Maximum Overdrive

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    This isn't that hard to figure out ... to crank the car you need a full battery; the negative cable goes to ground somewhere on the motor block, and the other goes from battery to starter relay and then to the starter itself. Cables are cheap, if you can't tell by looking at it, replace it. Ignition has no part at all in your problem as stated.

    You didn't mention the starter relay, that should be inside your fixup. That unit needs a good ground also, from it's case to the body metal. It's worth mentioning that you need a motor-2-body ground connection, usually a small braided cable from the rear part of the head onto the firewall (and be sure both ends have a really good connection.)

    A battery that runs down is a problem, there is a leak somewhere. If you have a voltmeter, set it to measure current and see how much goes from a (disconnected) battery cable to the post where the cable used to be connected. Can be either positive or negative.
     
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  3. andrewok1

    andrewok1 In Maximum Overdrive GOLD MEMBER SILVER MEMBER

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    ground battery to block firewall to block andbattery to frame, it can be a starter even rebuilds are bad a handful of times , in todays world

     
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  4. 1979 RANCHERO GT

    1979 RANCHERO GT In Overdrive

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    I had an alternator that would suck a battery dead in 20 minutes and it was only three months old and not from the national parts chain stores. It took three alternators before I found a good one, no problems since.
     
  5. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    The factory grounding is as follows: passenger front corner of block, to frame, then to battery negative. Also, there are two ground paths attached to the back of the cylinder heads, harness ground wire to back of driver side head, and and braided wire strap from firewall to back of passenger side head. The one everyone typically misses is the frame ground, especially if they have the original factory cable, and don't clean it up. Now, having said that, you can run a separate new ground cable from the lower starter bolt ( you'll need one a 1/4" longer), along the block skirt and under the mount, to the negative terminal. That will give you an extra ground path in addition to the factory ground path.
     
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  6. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    Thank you, Andy. This is what I needed.
     
  7. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    Ok yes I worded that wrong. I was not referring to ignition system itself. Cables are brand new, I believe 2 gauge. Solenoid was replaced as well, grounds from there and from battery to block are all intact. Battery fully charged.
     
  8. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    As far as the parts that I have replaced, all are brand new, none are rebuilt or remanufactured.
    I thought about that as well, so I tried it on the other 72. Cranks just fine.
     
  9. burninbush

    burninbush In Maximum Overdrive

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    Woof! There isn't much left at this point ... have you replaced the starter relay? That's also in the circuit, must pass the full starter current.

    It sounds like something is hugely pulling current when it's not supposed to do that -- turned off. Just for ducks, try disconnecting the alternator (at the starter relay), presumably leaving only the starter and the relay in the circuit; does it crank better now? If yes, then you have to find whatever is pulling that much current when it should be off. Might be useful to disconnect items at the fuse box.

    A voltmeter (that measures current) would directly answer that question -- see above.

    Have you tested the new alternator? Even new they can fail. Autozone will do that for free if you bring it in.
     
  10. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    That is a good point, even if you charge up the battery fresh, when you crank, even if there's a partial short, it'll take away from the starter circuit. And, a quick test...put your voltmeter on the battery, then energize the solenoid at the trigger wire, rather than turn the key on. The battery voltage during cranking should be 10.5 to 11.5 volts, indicating no other draws. Anything between 9 and 10.5, something's bad from the box. Below 9, you either have a bad cell, or a parallel 'sneak' short.
     
  11. andrewok1

    andrewok1 In Maximum Overdrive GOLD MEMBER SILVER MEMBER

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    AS 79 GT said alternator, pull battery power from the alternator, tape of cable so you dont short to ground , then crank it
    i recall having a Ford Fairlane was doing the same thing
    ANDY
     

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