76gmc grain truck shorting out battery

Milk · Apr 4, 2020

Hi folks,
Got another problem with my GMC grain truck that maybe someone on this forum can help me diagnose.She has a dead short somewhere in the electrical system. When you hook up the battery cables, she sparks heavily and runs down the battery within a minute. I replaced the starter yesterday thinking that was causing the direct shorting out and the silinoid was very bad, loose, broken switch post, but the problem is not fixed. The truck ran fine this morning for about 45 minutes hauling gravel, then just died on me . Now truck is dead in the water! Any suggestions would be greatly appropriated! Thanks in advance!

Calrod · Apr 4, 2020

Dead in the water like it ran the battery
down again?
the best solution for a drain on the
battery is process of elimination. you
changed the starter so disconect the hot
lead on the alternator. check and see if
you still get an arc at the battery
cable. keep checking large consumers like
lights,electric motors,fans ect.

Milk · Apr 4, 2020

Yes, battery is dead again! Second battery try! I will try unhooking the alternator. Thanks!

DeltaRed · Apr 4, 2020

Same thing last week with a neighbor's 4630.The light switch. It was melted and shorting to ground.

JK-NY · Apr 4, 2020

Try checking any relays under dashboard or on firewall to see if they are really hot, then pull. Check that circuit, swap in an equivalent relay etc. Look over battery cables real good .Then Unhook everything you can easily, (heater, alternator etc.). At some point you will find it. Post back if you want more advice .

JMOR · Apr 4, 2020

Current that kills a battery in 60 seconds ought to be making some smoke! And smell.

Janicholson · Apr 4, 2020

To avoid flames and sparks you could use a head light wired between the non grounded post, and the
terminal. it will light up bright. Then first disconnect the alternator main output wire. Light
stays on, not that. Then disconnect the big terminal leading to the starter motor, including other
wires that night be there. light stays on? that lead is shorted between the battery and the starter.
Light off, the starter might be jammed on, or stuck. Could be!! disconnect the solenoid wire from
the Key. Attach big wire -- no light, starter motor still must be stuck, and being energized by
key. Jim

Bob · Apr 4, 2020

(quoted from post at 13:38:07 04/04/20) Yes, battery is dead again! Second battery try! I will try unhooking the alternator. Thanks!

GOOD plan.

Also, Jim's suggestion to connect a lamp between the cable and battery before going "short hunting" is a GREAT suggestion.

If the short remains with the alternator disconnected, check the red wire that goes on the top starter solenoid terminal, it passes upwards near the back of the manifold and it's insulation can get burned or damaged by the heat from the manifold, ESPECIALLY if there's an exhaust leak in that area.

Check wiring forward from the firewall to the alternator.

Michael Soldan · Apr 4, 2020

A bad alternator will short out a battery in 3 seconds !

used red MN · Apr 4, 2020

Michael has a good point but I agree with JMOR any short that will kill a battery in minutes has to be in the positive heavy main battery cable. Is there a short tube that bolts on the bell housing bolts that the positive cable runs through? Is it shorting out in there? I don?t remember if heavy GM trucks had these but some pickups and or cars did. Anywhere else along the cable that it has a holder clamp around it that may have failed insulation that is shorting. Make sure you double check you connection at the starter so you are sure nothing is shorting there. Is the positive cable rubbing through along the frame or by the exhaust manifold? There is another possibility but it generally would not cause heavy sparking at the battery terminal, this is the possibility of an intermittent connection of a fuseable link. I believe those are located by the starter or on a connection stud on the firewall. If one of those was failing it would seem as though your battery was dead because all power would be lost.

used red MN · Apr 4, 2020

Now that I reread Bob?s post I think I remember the short metal tube I am talking about only contained the hot feed to the rest of the vehicle that the fuseable links were on near the starter and the solenoid wires, not the positive cable. The tube helped protect those wires from the right hand exhaust manifold heat. Give me a break it has been 35 or so years plus since I crawled around and under that vintage of vehicles.

M w J · Apr 4, 2020

Does it have electric shift 2 speed that someone bypassed the fuse on?

Milk · Apr 4, 2020

Found the problem guys! It was the heavy cable from the starter that goes back to about the middle of the truck. The cable was hitting the exhaust pipe, muffler and was melted through the insulation down to the cable! Problem solved, Thanks guys!!

Bob · Apr 4, 2020

(quoted from post at 16:50:20 04/04/20) Found the problem guys! It was the heavy cable from the starter that goes back to about the middle of the truck. The cable was hitting the exhaust pipe, muffler and was melted through the insulation down to the cable! Problem solved, Thanks guys!!
<img src="https://www.yesterdaystractors.com/cvphotos/cvphoto15.jpg">

Was that cable still in use, or left over from an electric/hydraulic "power pack", or some other no longer used aftermarket equipment?

Calrod · Apr 4, 2020

good deal zip tie that stuff its good to hear a 76 chevy still working hard.

SVcummins · Apr 5, 2020

If it?s sucking juice that fast something should be extremely
hot ! Be careful cuz it will burn you bad

76gmc grain truck shorting out battery

Milk

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Calrod

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Milk

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DeltaRed

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JK-NY

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JMOR

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Janicholson

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Bob

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Michael Soldan

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used red MN

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used red MN

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M w J

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Calrod

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