73 GMC Truck Wiring Diagrams/Battery Cables

[Spool]

Okay. If you read my last response; I deleted the panic I had in not being able to get the truck to start after straining the poor connections these past two days and familiarized myself with everything you said. I'll be doing this tomorrow morning after going to Autozone.

For starters.
A fusible link is nothing more than a wire 2 gauges smaller than the main wire for safety?

I'm a bit confused on the ckt's though.
Am I interpreting the wire diagram correctly when placing them side by side as shown in my photo attached or is it upside down making my second photo correct?

Also, what are the terminals for the Bulk Connector. As you can see, mine is full of crud. Can I clean this?
I just took the bolt out of the center and the two under the dash to see the nest of wires.
Am I to run the wire through the firewall and connect from inside the cab or connect right to the bulk?

Lastly, just to confirm tomorrow's actions.

From the Battery Terminal which is 3/8th's in size.
(1) 14gauge @ 6'' fusible link > 10g Red wire to Junction.

(1) 20gauge @ 8'' fusible link > 16g Black wire > SFE 4 AMP Fuse > ckt105 in Bulkhead. {I've noticed the wire diagram states 18gauge. Should I go 18?}

From the Solenoid Terminal which is 1/4'' in size.
(1) 12gauge > ckt6 in Bulkhead.

From the Distributor Cap to Bulkhead.
(1) 12gauge preferably pink > ckt3 {Is the 12gauge also 1.35 OMH Res?}

Thank you again BD.
I'll be tackling the gauges and junction box after I get the truck started again.
In the meantime I need to learn to solder and pick up some stuff in the morning.

[bd]

Amazing. Thank you sir. This certainly is enough for me to get busy.
One thing I've always wondered, which I'd like to confirm. The junction box gets it's power from the B+ off the starter?  Correct

Sadly, I've shaken this starter wires more than they had wished to be shaken as they held on for dear life and now
I can't start the truck. Battery is fine, so I know it's the terrible connections on the starter.

So I need a priority list.

Tomorrow morning, I'll be going to Autozone to grab a few terminal ends.
I know the Battery Terminal is 3/8ths but have to grab the nut to measure the solenoid terminal.  #8 ring
And wire wise I'll need...
12 gauge for the purple end.  by #8 ring
16 gauge for the black end.  18-ga, remember you will install a 20-ga fusible link - The black wire is not part of the crank circuit, so can be ignored for the interim.
2 gauge for the B+  by 3/8" ring

With that, I'll replace the Battery cable itself which I've been waiting to do until I had all the parts.
And do a quick fix on the burnt out wire ends attached to the starter.

I gotta get the truck running for work now.
And then study and take my time to do the job right this upcoming weekend or next week.

Do you think the priorities are straight?

Don't forget the solder and marine heat shrink.  BTW - most parts houses will try to sell you PVC heat shrink - sadly, they don't know the difference and will tell you it's all the same, or "That's what everybody uses."  Ignore anyone who tells you that and question any other "assurances" they may claim.  Don't waste your money on PVC shrink tubing.

Pick up a small assortment of 18-ga, 16-14-ga, and 12-10-ga ring terminals and butt splices.  Don't forget the clamps to secure the new battery cable along the oil pan, away from the exhaust and motor mount.  For any other needed materials, review this entire thread.

[Spool]

I've broken the mojo of our correspondence by editing my last post.

I'm trying to determine if I need any connectors for the Bulkhead.

[Irish_Alley]

where did you get that wiring diagram?

[Spool]

where did you get that wiring diagram?

Hey Irish. That wiring diagram is from the 1973 GMC Truck Wiring Diagrams book. X-7305.

[bd]

A fusible link is nothing more than a wire 2 gauges smaller than the main wire for safety?

Not correct!  Fusible wire is manufactured from a special alloy conductor that is jacketed by special high-temperature, non-flammable insulation.  It is a flexible, high current, slow-blow fuse that provides primary circuit protection - meaning that ofttimes there is supplemental fast acting circuit protection in the form of fuses or circuit breakers downstream from the fusible link.  The fusible link is analogous to the main circuit breaker in a building that feeds subordinate breakers protecting the individual circuits.  Collectively, fusible links provide the primary electrical protection for the vehicle.

The formula for correctly sizing a fusible link is to use link that is four (4) wire gauges smaller than the wire it is intended to protect.  Industry wide and even across industries, fusible links are cut to 6" length, regardless of gauge.  Thus, 10-ga wire is protected by a 6" long, 14-ga fusible link; 12-ga wire is protected by a 6" long, 16-ga fusible link; 14-ga wire is protected by a 6" long, 18-gauge fusible link; 16-ga wire is protected by a 6" long, 20-ga fusible link and so on.  Rarely, there exist special circumstances that slightly modify the given formulae for both gauge selection and length.

Am I interpreting the wire diagram correctly when placing them side by side as shown in my photo attached or is it upside down making my second photo correct?

^^^^ This image is correct.  The schematic illustrates the connectors from the terminal side.

Also, what are the terminals for the Bulk[head] Connector. As you can see, mine is full of crud. Can I clean this?
I just took the bolt out of the center and the two under the dash to see the nest of wires.
Am I to run the wire through the firewall and connect from inside the cab or connect right to the bulk?

The terminals needed either will be male or female, depending on which side of the bulkhead connector you are servicing.  To replace ckt 3 between the bulkhead and distributor requires Packard Series 56 male (for bulkhead) and female (for distributor) terminals matched to the gauge of the wire, or 12-gauge terminals (following images).  Twelve gauge terminals of this type may be difficult to procure, locally.  You will also need a Series 56 connector to mate with the distributor.  In addition, you will need a good pair of wire crimpers.  To remove Series 56 male terminals, squeeze the spade end of the terminal across its edges, forcing the terminal to narrow, then pull straight out on the wire.

 


The "crud" encasing the connector is hardened antioxidant/sealer encrusted with dust.  It is unsightly, but harmless.  I recommend leaving it in place, except where it could interfere with electrical continuity.  To remove the sealer, douse the connector repeatedly with carburetor spray and use a nylon bristle brush.  It will make a mess and stain any clothing it contacts.  Repack the connector with fresh antioxidant paste during reassembly to ensure moisture rejection.

To disassemble the bulkhead connector, remove only the center bolt on the engine compartment side.  Backing the bolt out should force the connector apart without prying.

From the Battery Terminal which is 3/8th's in size.
(1) 14gauge @ 6'' fusible link > 10g Red wire to Junction.  CORRECT

(1) 20gauge @ 8'' fusible link > 16g Black wire > SFE 4 AMP Fuse > ckt105 in Bulkhead. {I've noticed the wire diagram states 18gauge. Should I go 18?}  YOU ARE NOT REPLACING THE INLINE FUSE HOLDER INTO THE BULKHEAD CONNECTOR, MERELY ADDING A 20-GAUGE FUSIBLE LINK TO THE END OF THE 16-GA BLACK AT THE STARTER B+ SOLENOID STUD.  OTHERWISE, YOUR STATEMENT IS CORRECT.

From the Solenoid Terminal which is 1/4'' in size.  THE "S" TERMINAL IS NOT 1/4" - 1/4" IS TOO LARGE!  IT IS #8 OR #10 MACHINE THREAD.
(1) 12gauge > ckt6 in Bulkhead.  AGAIN, THERE IS NO NEED TO REPLACE THE ENTIRE WIRE.

From the Distributor Cap to Bulkhead.
(1) 12gauge preferably pink > ckt3 {Is the 12gauge also 1.35 OHM Res?}  THE ORIGINAL FACTORY PINK IS A RESISTANCE WIRE.  THE SUBSTITUTE YOU ARE FABRICATING IS NOT.

[Spool]

Thanks BD.

So for the interim due to the resources I have in the moment...

Is it okay to pull the split 3-7, and patch the pink from the bulk connector into the 12gauge yellow from the distributor cap for now?

[bd]

You can clip away the yellow that comes from the starter, tape the splice and run it for the interim.  It just limits the power available to the ignition system.

[Spool]

Okay. Should I splice the Pink from the Bulk directly into the Yellow off the distributor cap for the interim as well?

Also, Autozone has nothing desirable which means I'll be ordering mainly from online and will have to wait for some parts.

I have zinc ring terminals for now and a 14 gauge fusible link 9'' in length which will allow me to repair/replace the 10gauge Red B+ to the Junction today.

However, they didn't have a 20gauge fusible link for my Black wire to the solenoid, which is in certain need of repair.
Nor did they have 20gauge wire at all. For the interim would you go straight to the solenoid or try and finesse and electric tape the connection as is?

[bd]

 




^^^^  Splice 3-7 already exists.  Why cut it out only to resplice it?  Just clip off the extraneous small yellow run down to the starter and tape the splice to insulate it until you have the logistics sorted to replace the complete wire (ckt 3) into the bulkhead.  If the engine has been running that way, why reinvent the hack only to replace it later?  Or, am I missing something?

Until you receive the 20-gauge fusible link, just leave the black wire disconnected.  Tape it up out of the way.  Leaving it disconnected until the ammeter circuit is reestablished should have no significant effect.

[Spool]

Copy that on the black wire awaiting the 20gauge fusible link.

I must have miss-read the initial direction about the 3-7 Splice.
Originally, I thought I was to remove the the splice due to the condition it's in.

Cutting the yellow to the starter is easy and done.
But I was under the impression that I could take ckt 3 straight into the distributor cap as a clean connection.
Because as of now it goes bulk ckt3>pink>poor splice to think yellow gauge>poor butt connection to thicker 12 gauge yellow into distributor cap.

 The picture I attached isn't great, but it almost appears as if it's female spade.
So my thought was, if so than just cut the pink straight into the distributor cap with a female spade
Or, if it's in fact a Series 56, then just butt connect the pink to the yellow directly (for now) as to not have the poor existing splice and poor butt connection that currently exist.

Good idea/Bad idea/too novice to try and come up with workarounds?

[blazer74]

.

[bd]

I was under the impression that I could take ckt 3 straight into the distributor cap as a clean connection.
Because as of now it goes bulk ckt3>pink>poor splice to think yellow gauge>poor butt connection to thicker 12 gauge yellow into distributor cap.

So my thought was, if so than just cut the pink straight into the distributor cap with a female spade

For a temporary repair, you can do exactly as you're thinking if in your judgement it's a prudent response, assuming of course the pink wire (ckt 3) is long enough.  Just bear in mind that the existing pink wire is a resistance wire that will consume (waste) approximately 7 volts - as in decrease the voltage supplied to the distributor by 7 volts.  This means the HEI system will attempt to function on one-half of the voltage is was designed to use.  Try running on one leg.  It may work over the short course, but is less than optimum.  Over the long run, the engine may experience random misfire.

As you've observed, the terminal in the cap is standard 1/4" spade.  The advantage in using the OE Series 56 terminal over the long term is that it will maintain a constant pressure connection regardless of temperature, while the OE connector will lock into the distributor cap and prevent the B+ lead from inadvertently being dislodged.

There is a caveat regarding the bulkhead connector.  The Series 56 male terminal is captive to the style of terminal retention designed into the connector housing.  No other style of terminal is a suitable replacement.

[Spool]

Alright! Got the truck running with the resources available.
For now, I had to crimp and tape as I need to research/learn/purchase soldering gear and heat shrink gun.

I've attached photos of the work completed. Starts better than it did before.
Turns out the reason the truck stopped running last night was the ring terminal of the B+ from the battery snapped off.

So next up is ordering a 20gauge fusible link and a SFE 4 AMP fuse for the Black wire on ckt105
-https://www.summitracing.com/parts/pco-5557pt?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-pico-wiring&gclid=Cj0KCQjwnubLBRC_ARIsAASsNNkWULzMj06nChsiM3z8fbXbQMWorGnSzofGLq3_z2Xiac2jMyzv6C0aAvvCEALw_wcB
-https://www.summitracing.com/parts/bss-sfe4
Do you think those two items will do the trick?

I'm about to order the heatshrink wrap you recommended.
I took notice to the heatshrink gun on the same page; I'll be ordering that as well. $13 isn't bad at all.

I'm gonna grab some Thermal Sleeves for the wires to the starter as they are only an inch away from my headers.
Do you think this would be a wise choice?
https://www.summitracing.com/parts/tay-2584

Soldering wise, VileZambonie's post was very informative.
I'm new to this, anything to consider in a soldering iron? Or is a $20 soldering iron on Summit going to do the trick?

With all that. I'll complete the Black wire on ckt105 to my Battery Terminal on the Starter when the parts arrive.
And then move on to the 3-7 Splice. (Which I ended up leaving and keeping as you suggested. I'm not sure I fully understand it's function as of yet, but do want to ensure
the distributor gets all the power needed like you had mentioned.)

BD.... Thank you again, so very much for all your help on this. I'm very grateful and just want you to know that I appreciate your time and knowledge.
Thank you again.

[bd]

Repairing ckt 105 and replacing the 4-amp fuse is only one-half of the ammeter circuit.  You still need to inventory the wires connecting to the junction block and report - one of the wires should be ammeter ckt 106 protected by a redundant 4-amp fuse.  In other words, there is more to do to restore ammeter function.

The thermal sleeve you linked is probably too small for the B+ cable and associated starter wiring.  Consider using a larger size, say 3/4" - 1" inside diameter.  In addition, consider rotating the battery cable ~180° so the cable exits the solenoid pointing away from the exhaust.  Don't forget to secure the cable to prevent burning and chafing.

A decent soldering gun for general electrical repair is the Weller 8200, 100/140w.

And then move on to the 3-7 Splice. (Which I ended up leaving and keeping as you suggested. I'm not sure I fully understand it's function as of yet, but do want to ensure
the distributor gets all the power needed like you had mentioned.)

What don't you grasp?