main power to fuse block

[jg1977c20]

1977 c20 cheyenne 350 no a/c or power windows etc .


im trying to pinpoint wether i have 1 or 2 main power wires into the fuse box - every diagram shows 2 - but i see only 1 that has the alt bat feed spliced into it then terminates at the junction block on firewall

it also has a red wire that comes from the starter lug to the junction which i presume feeds the above wire going back to fuse box

the only other red wire coming from the fuse box goes to the batt side of the HEI ignition

theres about a 12 guage purple wire which would be the start solenoid signal wire



am i right that i only have one main power wire ?  im installing a 150 amp alt so i need to upgrade the charge wire and figured i may as well clean up the wiring a little and put everything to junction - that way i'll have the remote sense wire and accesory power in the same spot

[hatzie]

In 1977 you should have a fusible link at the starter that feeds the junction block stud on the firewall.  Circuit # 2J
The alternator has two circuit 2 wires BAT wire #2C and Sense wire 2G.  One cab circuit 2E to the firewall bulkhead passthrough plug.  All three of these ( 2C, 2G &, 2E) share one fusible link on the junction stud.

All circuit 2 wires in the engine compartment are red.

There are two sets of wiring diagrams.  The multi-year diagrams are from a Haynes type manual.  They're OK but they are incomplete and don't have good reliable specific detail...

My scans of the OEM GM wiring diagrams are in the tech section.  They are year specific. http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.msg252586#msg252586

[jg1977c20]

Your "The Man"  - i can't thank you enough - the haynes manuals are very vague , my manual shows 2J going from the starter in thru bulk head connector - clearly thats not right .

if i may probe your thoughts :

i am installing a 150 amp alt :

1. ) should i just run the new larger batt wire from the alt direct to the junction ,disconnecting the old wire at alt. of course .

2.) run the batt. wire from alt. to junction ,remove 2J from junction to starter and run a new larger cable from junction direct to battery - and if if this is the better choice i'm thinking i should also put the cab power and alt sense wire on their own fused links direct to the junction

[bd]

Wiring the alternator output post direct to battery positive through 4 AWG copper cable will result in less than 1% voltage loss across an estimated 3-foot run with a maximum alternator output of 150 amps.  Protect the new 4-ga charge cable with a 6" long, 8-gauge fusible link at its connection to the battery.  The #2 regulator (sensing) circuit can then connect directly to the alternator output post through a short, 18-ga jumper with no ill effects on battery charging.  There is no need to protect the regulator sensing lead with any type of fuse.  With this upgrade/modification, eliminate circuits 2C/2G running between the alternator and the firewall junction block.

Other beneficial modifications might include new 2-gauge or larger, copper battery cables; upgrading ckt 2J to 6-gauge between the starter and junction block (protected by a 10-gauge link at the starter); upgrading ckt 2E to 10-gauge between the junction block through the bulkhead connector, to its splice under the dash (protected by a 14-gauge link at the junction block); and separating the 12-ga headlamp circuit 2D from that same under-dash splice to reroute it out through an unused cavity in the bulkhead connector to the firewall junction block through its own dedicated 16-gauge link.

But, whether you choose to upgrade the power bus into the cab (circuits 2J/2E) and separate out headlamp circuit 2D, depends on what you have visualized for future current load placed on the wiring.