Jump to content

74 620, ground wire is burnt from the fuel tank sending unit


Recommended Posts

Hi All,

       I just brought a project 74 620 truck a couple days ago. I have a burnt ground wire from the fuel tank sending all the way to where the wire end at the ground on the altenator. I also have a burnt ground wire(black) from the voltage regulator. Does anybody know what may have caused this? Thanks in advance. :)

Link to comment
  • Replies 20
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Possibly someone reversed the battery cables. Jump started it with the cables reversed.


A big problem here is the melted wire could easily have cut into multiple other wires on several harnesses making trouble shooting of other problems very difficult.


.. and welcome to Ratsun.

  • Like 1
Link to comment

The burnt ground wire is connect at the altenator and runs back to to the fuel tank, and then run all the way back to the tail lights. Is this how it comes stock? Also the problem seems to start at the fuel tank because both the yellow and black are burnt to a crisp. The yellow wire is ok after about 10 inches, but the black wire(ground) is burnt all the way to the altenator.

Link to comment

Everything to the rear (fuel tank sender and all lighting) is grounded by the one black wire. None of this would draw enough power (normally) to melt the wires. If the power side were to touch ground, (a short) it would blow one of the several fuses as this is what fuses do, protect the wires. 

Link to comment

I would check all the other grounds.

This is what is on a 521.  Negative battery cable is attached to the cylinder head, by the fuel pump.  At this connection there is a pigtail that goes down to the alternator frame.  There is a second ground wire from the alternator frame that goes to a bolt that holds the voltage regulator to the inner fender.  This alternator to voltage regulator bolt grounds cab sheet metal.  If this ground is missing, the cab grounds through the throttle cable, and melts the outer casing.

The engine has to ground to the trucks frame.  I am not sure how this was done on a new 521, I add a short jumper around the rubber of a motor mount.

The bed has to ground to the trucks frame.   Again, a short jumper around a bed rubber insulator.


You cannot use bed or cab mounting bolts to supply a ground.  When they get wet, if there is not a separate ground, the current flowing through the bolts will erode, or electrically consume the bolt, by electrolysis.


The gas tank should ground to the frame or bed, to the part of the truck it is mounted to.   The black wire for the fuel sender is not there to ground the gas tank, it is there to handle the small current draw of the fuel tank sender only.


On a 521, there is a separate wiring harness on each tail light.  This harness attached to the tail light frame, that is connected to the bed.  The 521 tail lights work better if you ground them directly to the trucks frame.

Link to comment

Hello Ratsuns,

      So I replaced all the melted ground wire, gounded the engine, cab, and bed to the frame. But now when I try to start the truck the fuse that on the pigtail connected to the postive battery cable pops.Only when I try to start the engine tho. Do I have a bad ignition switch? Please help... :confused:

Link to comment

There is not a fuse in the starter cables.  The positive battery cable should go directly to a solenoid on the starter.  The negative battery cable goes to a bolt by the fuel pump.


Does the fuse pop when the key is in the run position, or only when you try to crank the engine?


The following is based on my knowledge of a 521 truck. 

The ignition switch makes two connections in the crank position, in addition to keeping the ignition system connected.  One connection in the crank position of the ignition switch supplies power to the starter solenoid only.  This makes the engine crank.  The second connection made when the key is in the crank position is a bypass of the ballast resistor.    If this, or the small wire going to the starter solenoid are grounded, that could pop the main fuse when trying to crank the engine.

Link to comment

How are you replacing it every time?  Has it been replaced with an ordinary fuse?????


The fusible link is rated at about 20 amps but will carry up to 200 amps for up to 5 seconds before melting. In a sense it is a delay or slow blow fuse that can sustain more than the rated current flow for a short period without blowing. There should be nothing 'on' during start that would draw that much power.

Link to comment

Yes, that shouldn't be happening. It doesn't do this in the start position only?


If only on the start position, then this also powers the Blue/yellow signal to the starter solenoid and to the Brown/Red wire to the coil + terminal. If you have enough fusible links... Unplug the starter solenoid and the Brown/Red wire to the coil. Now turn to start and nothing should blow. If the link still blows then one or maybe both of these wires are somehow grounded. Maybe pinched?


If the link does not blow, reattach the coil wire. If the link blows then there is a problem at the coil


If the link does not blow reconnect the starter. Link blows... something wrong with the starter solenoid.

  • Like 1
Link to comment

Thank you Mike and Daniel, using jumper wire to bypass the the ignition swicth, I was able to find that the bad wire is the Black- white, Coming from the fuse box to the ignition switch and out to the coil. I also did what Mike suggested by unplugging the black-red and black-white wires from the coil. Tried to crank the engine and the fusible link didnt pop, so I knew it had to be the wires to the coil that was grounded(short). I replaced both wires to the coil and now the engine starts and runs good! :thumbup:  Now Im back to fixing my first issue, taillight not working and now I think my alt wont hold a charge. :confused:

  • Like 1
Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.