73 GMC Truck Wiring Diagrams/Battery Cables

[bd]

Route and clamp the positive cable and primary starter wiring parallel to the oil pan, between the oil pan and motor mount bracket.  For example...




A starter heat shield will help shroud the solenoid and starter motor from direct, radiant exhaust heat...




An effective trim-to-fit fabric substitute for a rigid factory heat shield is available from Design Engineering (DEI).

Do you have a conventional (points and condenser) distributor, or has it been upgraded to HEI?

[Spool]

Your help is invaluable.

2 bolts taking out the starter. Check.
Anything to know when replacing it?

Running the B+ along the oil pan is a great idea. Thank you.
The heat shield is also very cool. I'll order when the next check comes in; Monday perhaps.

I'd have to take a closer look at the distributor, but my guess is stock at this moment. Will this change my configuration?

I'll be taking the starter out once I have all the parts, heat shield, 3/8ths connections for the new 2gauge wire...
But until then, I'm not able to trace the other wire sitting on the battery terminal behind my B+ on the solenoid.
It's black, any guess as to what it is, or maybe its part of the 3 or 4 wires electric taped together going to the distributor that is sitting too close to my header?
I was under the impression that the B+ was independent on that terimnal and the others where to come from the solenoid winding terminal.

[bd]

Original equipment conventional distributors and later model HEI are wired differently.  If it has conventional ignition, decide now whether you want to upgrade and we will address the wiring accordingly.  In addition, does the instrument cluster have original factory gauges with an ammeter (this also affects wiring configuration)?  If it has an ammeter, does the ammeter work?

[Spool]

I've attached a photo of the distributor. Delco-Remy but I'm not sure if it's original or HEI.
For now, I'd like to keep it as to avoid more expenses on an upgrade.

The instrument cluster does have original factory gauges the only gauge that works is the oil pressure/speedometer.
I have an separate temp gauge below that I monitor on hot days, but that's it.
The Ammeter has never worked.

Can you tell if it's stock or HEI?

[bd]

The good news is that the ignition has been upgraded to HEI!      You now have the added minor step of determining whether the wiring was upgraded to match it.  Disregarding the short, pink/black/brown three-wire jumper that connects the cap to the distributor housing, what is(are) the wire color(s) and gauge(s) that connect(s) the engine harness to the BAT terminal of the distributor cap (the BAT terminal is clearly labeled on the top, driver side of the cap)?


If you clean the wires connected to the starter solenoid, you should be able to coordinate the wire colors with the labels in the following image...



Is the wire labeled "YELLOW," in fact, yellow or is it black?
Unwrap the tape from the wire bundle about eight inches back from the starter.  Is the wire labeled "14-GA FUSIBLE LINK" spliced to a 12-gauge red wire?


Referring to the following image, what color is the wire that is circled?



Do you want to try to restore ammeter function if it is a simple matter of wiring?

[Spool]

Wow! Thank you for taking the time on this.

I will do this tomorrow. If I have as much trouble identifying the wires as I did taking the photos, I'll have to take the starter out to get a better look.

I've read stories of it being a bit of pain to get back in; is there anything to consider when reinstalling the starter? Anything that needs to be lined up a certain way?

[bd]

If I have as much trouble identifying the wires as I did taking the photos, I'll have to take the starter out to get a better look.

Disconnect the battery ground cable then use a compact mirror.

I've read stories of it being a bit of pain to get back in; is there anything to consider when reinstalling the starter? Anything that needs to be lined up a certain way?

Don't let it fall on your head!       If there are any shims between the starter and engine block, retain them for reinstallation.  Install and snug both starter bolts finger tight before tightening them down.

[Spool]

Cool, I'll try the compact mirror first.

If I absolutely have to remove it I will of course disconnect the battery.
I was thinking more of when I place it, do I need to place it in any particular way. As example/analogy... are there pins that need to align or does it just go right in?

[bd]

Starter installation is very forgiving.  The only alignment is on the mounting bolt holes - and shims if any were removed.

[Spool]

I was able to get in there without taking the starter out. It was fun; familiarizing myself with everything.

The wires are cooked behind the header/near the solenoid.
Unwrapping the electrical tape was more like braking it off in pieces. Very old and brittle.

I pulled it back all the way to the alternator to see if I could get a clean look on the wires, but they are
beyond legible; even after trying to clean off years of grease and glue.

The 5 main cables going from the starter (Not including the 2gauge+ from battery) run up past my alternator and back towards the distributor where it splits off.

There are:
2 Yellow (both of which are clipped) both small gauges, maybe 16 or 18. One of which was the one you had circled. It is indeed yellow when I cleaned it.
1 Yellow/Black?, maybe 14 gauge that runs to the solenoid.
1 Purple/Black?, maybe 8 or 10 gauge that also runs to the solenoid.
1 Red that runs to the Battery terminal.

I ended up re-taping them afterwards for now and thankfully it started up again for the time being.

This really helps me understand why my Starter works when it wants to while sometimes needing a little shake of the cables to get it going.

[bd]

^^^ Try using aerosol carb cleaner and a disposable rag to wipe the insulation clean.  The clipped and abandoned yellow wires are probably 20-gauge and originally part of the ignition primary wiring.  The wire labeled "yellow or black to solenoid" I suspect is actually 16-gauge black for the ammeter.  The purple wire is 12-gauge coming from the ignition switch to energize the starter solenoid.  The red is the 10-gauge B+ feed into the cab and to the alternator.




^^^ The yellow semi-translucent bulge in the red wire is the factory splice with a 6" long, 14-gauge fusible link that attaches to the B+ stud on the solenoid.  Plan on replacing that fusible link.  Arc damage and discoloration at the B+ stud on the solenoid suggests the battery cable was/is loose.  The previously mentioned black wire for the ammeter will reconnect to the B+ solenoid stud through a 6" long, 20-gauge fusible link along with the battery cable and the red 10-gauge B+ feed to the cab.




^^^ You will need to remove the tape from the harness all the way back to the firewall bulkhead connector.  I know, it's a PITA!




^^^ More than likely the 20-gauge yellow wires running between the starter, distributor and bulkhead connector will need to be stripped out and a new 12-gauge wire running between the bulkhead connector and distributor will need to be substituted.  This will ensure that the HEI system receives suitable primary power.


Please post pics of the firewall junction block (above the transmission dipstick) and the bulkhead connector (left of the steering column).  There remain questions concerning the ammeter circuit configuration and whether it has been altered.  Do you have a test light and VOM?

[blazer74]

Wow! Thank you for taking the time on this.

I will do this tomorrow. If I have as much trouble identifying the wires as I did taking the photos, I'll have to take the starter out to get a better look.

I've read stories of it being a bit of pain to get back in; is there anything to consider when reinstalling the starter? Anything that needs to be lined up a certain way?

Your getting some seriously good help.

[Spool]

Alright! Thank you again BD!
I'm growing in confidence over here between your help and making my way through the wires.

I've attached photos for easier communication but here's the break down of the 5 wires coming from the Starter.

2 Yellow wires that were clipped. (One is a smaller than the other)
-The smaller yellow goes all the way to the bottom left insert of the Bulkhead.
-The larger yellow goes to split. One to a pink wire going to the bulk and the other to a different yellow, going to the distributor cap. This split is in terrible condition as you'll see.
(This pink wire is actually labeled "Resistance Do Not Cut")

1 Red wire from Battery Terminal on Starter goes to my Junction box.

1 Purple wire goes to my bulkhead. Inserted in the bottom right.

1 Black wire goes to my bulkhead. Inserted in the top left. It also has something inline.

My bulkhead is filthy. Is there a way to clean it?
My wiring diagram is a significant help as I can't find anything labeled on the actual wires.

Side note (There was a small green clipped wire as well, it was harnessed with the single yellow wire going to the distributor cap.)
When I followed it back, it went to the bulkhead.)

Thanks again BD. I'm very grateful.
I'm thinking once these 5 wires are identified gauge wise, I can replace each one separately. What do you think?

[Spool]

In using the wiring diagram to help determine the gauges, would the Purple wire be in slot "2E" of the bulk connector?

I've attached a few schematics that have helped me wrap my head around this.

[bd]

Get on your mark!  Ready....  Set....  Using the factory circuit numbers so you can tie this discussion to your factory wiring manual:  the heavy red wire feeding power to the firewall junction block from the starter B+ post is 10-ga, circuit (ckt) 2D, labeled on the diagram as 10 R-2D.  Ckt 2D is protected by a 6" long, 14-ga fusible link at the B+ post.  This is important, because you will need to replace the fusible link mimicking the technique described in VileZambonie's technical thread, How to make a Fusible Link.  The factory fusible link has been compromised and cannot be trusted to be serviceable. 

Note - for the best results I recommend that you use non-insulated, plated copper electrical terminals.  All terminal ends and splices will need to be crimped and soldered using 60/40 rosin core solder.  In addition, ring-terminal ends and butt-splices will need to be insulated using dual-wall, adhesive lined, polyolefin (marine) heat shrink (such as Harbor Freight 67598 or similar).

The larger "clipped" yellow wire from the starter is 20-ga ckt 7B; the 3-way splice (yellow/yellow/pink) is "SPLICE 3-7" on the diagram, linking ckts 7A, 7B and 3; ckt 3 is a pink, 1.35-ohm resistance wire originally intended to power a conventional ignition system without burning the distributor points.  The entire 7A/7B/3 wire section needs to be stripped out and replaced with a 12-gauge (preferably pink) wire that runs from the bulkhead connector directly to the HEI "BAT" terminal - it will no longer run down to the starter.  You will unbolt and remove the bulkhead connector from the engine compartment side then remove and replace the complete distributor feed wire ckt 3, using new terminal ends.  The wires mating to ckt 3 inside the cab are already 12-gauge, so no further attention will be needed.

The 12-ga purple wire (ckt 6) energizes the solenoid during engine cranking and will be left alone as long as the terminal end at the solenoid is undamaged.  However, the 16-ga black wire sharing the solenoid "S" terminal with ckt 6 will be moved and reconnected to the solenoid B+ stud through a 6"-8" long, 20-ga fusible link; the black wire is ckt 105B, shunting a small amount of current through the dash ammeter.  The length of the fusible link has a direct impact on ammeter calibration.  The bulge in ckt 105B is an inline fuse holder for a SFE 4 amp fuse, installed to protect the ammeter.  More than likely you will need to replace the fuse.  However (don't you just hate "howevers?"), ckt 105B is only one-half of the ammeter circuit.  The mating half, ckt 106A, should connect to the firewall junction block through a redundant 4-amp fuse.  Now, you need to take inventory of the wire connections to the firewall junction block and enlighten me where they run and connect.

The "clipped" dark green wire that you discovered near the distributor is the temperature gauge sender lead ckt 35.  Extend ckt 35 to the temperature gauge sending unit using a dark green 18-ga wire.  The sender should be located in the left cylinder head, between spark plugs #1 and #3.  If the sender is missing, it is obsolete from all but a very few sources.  Read through Correct temperature sender and connector for 1973-1978 C/K trucks for more information.

The smaller of the two yellow wires at the solenoid remains in question - does the fuel gauge work?

I think I covered your last update.  Any questions?