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earth ground for chassis harness???


joshiroku

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hey guys i just got a 78 620 that im working on and i have a few things not working. i noticed near the battery was two wires coming out of the chassis harness over the passenger headlight. one large gauge wire and one smaller gauge wire both terminating to an eyelet. is this suppose to be a ground??  if so what was the proper location?? 

i checked the 78 wiring diagram and there is a body ground on the diagram but i was hoping someone had a pic or knew for sure where it is suppose to go.

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I am not as familiar with picky little details on the 620 electrical, but I know 521 trucks.   This info is for a 521.

The battery grounds to the engine.  There is a pigtail on the engine end of the negative battery cable, that goes to the alternator frame.  The alternator has a second ground wire that goes to the voltage regulator. 

620 info here.  Your truck may have had a internally regulated alternator, or not.  I do not know when that switch was made.

Back to 521 grounds.

There is a second black wire on the alternator frame, it goes to one of the voltage regulator bolts.  This ground wire connects cab sheet metal back to battery negative.  It is very important that this ground stays functioning.  If this ground goes away, the cab tries to ground through the throttle cable.  If your 620 has a plastic outer throttle cable with a metal inner cable, your truck needs the ground from the alternator to the cab sheet metal.

 

like all pickup trucks, you also have to make sure the truck frame, and bed are grounded.

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If in doubt, add grounds everywhere.  I typically wire cars from scratch with the battery grounded to both the engine and the chassis.  If the battery is up front, generally speaking you want to wire the headlight harness directly to ground as well to guarantee the brightest possible lights.  I don't recall the specifics of the 620's factory wiring either, but the alternator, headlights, and battery can safely all ground together.  

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The ground system, or lack thereof on these trucks is sad.  When I got my '76 it had one ground from the battery to the fender.  I added a strap from the battery to the frame, one from the body to the frame, and one from the motor to the frame.  If and when you decide to do a motor swap on this truck, these grounds will become crucial.  It's pretty cheap insurance to make sure your electrical system works properly.

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The most important ground connection is the battery to the engine.  This is where the highest current flows.  The next most important ground connection is from the alternator back to the negative battery terminal.  This is where all the charging current flows, and all the electrical load of the electrical stuff on the truck.  If the vehicle has an external alternator voltage regulator, it is very important that the battery, alternator, and voltage regulator have a very good ground connection to each other.

The engine has to ground to the frame.

The engine has to ground to cab sheet metal.

The engine has to ground to the bed.

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One big happy family of grounds, all holding hands.  Kum Ba Yah.  

EFI systems get picky - they want all grounds attaching at one point on the engine to reduce feedback issues with RFI/EMI noise, so you may want to keep that in mind as you add straps.  Voltage pulsing between ground straps creates signals that you don't want interrupting sensor function.  

 

The other issue - look at ground straps as a sewer pipe.  Plan for a BIG party.   Electricity is like water.  Don't run a 1" sewer pipe, or you're asking for trouble.   :poop:  :poop:  :poop:   Are you ever going to drop a 4" bomb in the toilet?  Probably not, but that's the size of your sewer pipe.  Overkill your grounds for good reason.  

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It has a crimped connector.  Any time you have two different metals connected electrically, and they get wet, one metal corrodes, and then the electrical connection goes away.

The bright and shiny braid looks nice when new, but when it gets dirty, it is hard to clean.  I prefer a insulated wire, with a smooth covering, that can be wiped off.  If a bare wire touches a terminal that is hot, instant short circuit.

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hey guys i just got a 78 620 that im working on and i have a few things not working. i noticed near the battery was two wires coming out of the chassis harness over the passenger headlight. one large gauge wire and one smaller gauge wire both terminating to an eyelet. is this suppose to be a ground??  if so what was the proper location?? 

i checked the 78 wiring diagram and there is a body ground on the diagram but i was hoping someone had a pic or knew for sure where it is suppose to go.

On my 620 the two ground wires that go to the single eyelet by passenger headlight goes to the top radiator mounting bolt.

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Grounding the frame isn't that much good unless using it to transmit the ground to say the back of the vehicle. The frame is well isolated from the body by rubber cab and box mounts and brake and coolant hoses. Even the engine transmission drive shaft and differential are isolated by rubber mounts from the frame. If grounding the frame ground it to the body at several places and to the block. Saw a ground from the one side of a rubber engine isolater to the other, a rather elegant solution. All electrical parts are mounted to the body sheet metal. 

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hey guys i just got a 78 620 that im working on and i have a few things not working. i noticed near the battery was two wires coming out of the chassis harness over the passenger headlight. one large gauge wire and one smaller gauge wire both terminating to an eyelet. is this suppose to be a ground??  if so what was the proper location?? 

i checked the 78 wiring diagram and there is a body ground on the diagram but i was hoping someone had a pic or knew for sure where it is suppose to go.

 

Those are the factory harness grounds, and on the core support between the battery and radiator is the factory spot for them.

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There is a double wire ground on the rad support just to the inside of the pass side high beam. This is true on the '78 and the '73 so probably all 620s. Headlights need a good ground is probably why.

 

There is a black ground wire to the alternator just below it on the isolated engine and the large battery negative cable to the head just behind the fuel pump. That appears to be it. Just one ground to the body on the rad support. All the interior electrics and all exterior marker, turn, taillights are ground return to the battery by a single large Black wire. The horn and interior lamp switch grounds to the body. (could have sworn there was a ground from harness to the steering column?

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  • 2 weeks later...
  • 9 months later...

Could a bad ground cause an alternator to read low on a voltmeter?

Voltmeter read 11.90 from the alternator, 12.0 from battery while running, defrost on, no lights.

The alt grounds to the engine correct? If so those bolts need to be clean and tight, yes?

Idk why i have such a hard time visualizing the electrical stuff.

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  • 1 year later...

I've just discovered I have a big ground problem:

  • Ruined 2 accelerator cables
  • Totally burned the VR black wire which also broke off at the harness.

This is how I have things connected:

  • Neg Bat Cable to Fuel Pump mounting bolt
  • Braided Strap from Neg Bat Term to Radiator Mounting Bolt

Observations of my 74 wiring:

  1. Small VR black wire, same size s the other VR wires
  2. Large Alt Black wire to Loom
  3. Large Loom ground to Headlight Relay sheet metal mounting screw into the radiator support just to the right of the radiator.
  4. Large Loom ground from Net Bat Term

The only small wire is from the VR.  Where does it terminate?  or is there a loom internal splice to the large black wires?

 

Further regarding the VR: 

 

For this wire to fry, if the circuit is as shown in the manual, another VR wire would have to fry since it would be in the circuit loop with the big amperage.  However, I believe the ground wire is also connected to the VR chassis in which case if the body ground lug was not making contact, the current would make its way up the small black to the VR chassis which is screwed to the body.

 

Is it possible the this ground lug is 3. above?  I checked mine and I could rotate the lug under the screw head with my finger.  Using a small sheet metal screw for a major ground lug is just wrong.

 

Some electrical system weirdnesses:

  • Dash lights very dim
  • Turn signals cause headlights to dim
  • Every once in a while, charging up a hill winding engine out, engine would shut off without any warning.  By the time I pulled off the road, engine worked just fine.
  • The monkeys that cobbled this truck up got into the wiring around the battery.
  • ... might be missing another....

I would have though the braided strap from Bat Neg Term to Rad Sup would have been sufficient.

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If in doubt, add grounds everywhere.  I typically wire cars from scratch with the battery grounded to both the engine and the chassis.  If the battery is up front, generally speaking you want to wire the headlight harness directly to ground as well to guarantee the brightest possible lights.  I don't recall the specifics of the 620's factory wiring either, but the alternator, headlights, and battery can safely all ground together.  

I make sure there is a direct wire from the negative batery terminal to the engine, frame and body.  If this is a pickup I run a baterry ground to the cab and the bed.

 

Scroll down and you can download the wiring diagram for your 620

 

http://www.davidcmurphy.com/olddat/620tech.htm

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I've just discovered I have a big ground problem:

Rui

how I have things connected:

  • Neg Bat Cable to Fuel Pump mounting bolt
  •  

 

w58-rebuilt-oem-cylinder-head.png

 

 

See that threaded raised boss just to the left of the fuel pump studs. That's where the ground strap should go. The stud will work but the pump is isolated from the head with a 1/4" plastic spacer and two gaskets. The lug on the battery cable will make much better contact directly on the head casting.

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Thanks for the gound cable location, Mike.

 

I saw some tubes about good grounds for sound systems and the ultimate body ground is the seat belt bolt.

 

I DO NOT like the Loom ground ring connector on the Headlight Relay mounting screw.  It is a small sheet metal screw into base thickness sheet metal. 

 

Also, the VR "B" and Loom "B" should not share that connector.  If that screw goes open, then the ground path is no longer through that screw, but through the VR.  The Loom and VR should have separate grounds points.

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UPDATE:  Finally a little time to work on this. 

 

The ground boss on the head has a broken bolt in it - that is why they used the fuel pump bolt.  Don't like to booger drill and tap - like to extract properly and preserve boss threads. 

  • Note:  Seem to recall if you arc weld to the broken bolt, weld will not stick t the alum - so just build it up and by that time, it would come out easily.

 

I turned the headlights on (low beam) and measured 8 to 26ma at various locations:  15ma at the throttle cable firewall nut and 26ma from the broken "B" VR loom wire.

 

I did find a problem with the large "B" from the Neg Bat Term to the very large (>1/4in) spade connector in the loom:  the ring connector @ the NBT is corroding and the wire itself seems like a smaller gauge than in the loom.  Don't know if that is intentional but don't see why since there is a fuseable link on the Pos Term.

 

MODIFICATIONS:

 

Secondary Body Ground:  There is a heavy spot-welded firewall bracket for the hydraulic lines that actually has a hole in it for at least a 1/4 in bolt.  That is going to be my second body to battery (keeping strap from NBT to radial support bolt

 

Alternator Grounds:  1) to VR mounting screw and 2) to the NBT.

____________________

 

NOTE:  The throttle cable is a ground path which will always have some current flowing through it.  To be sure there is negligible current through it, the alternative parallel ground paths must be much larger gauge and/or much smaller resistance. 

 

Edited by Cardinal Grammeter
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