[for3300]

I have an 83 chevy Silverado with a 75 c20 engine. I want to install some fusible links for the ignition system. Can you guys suggests which devices I should put them between? (e.g battery and alternator) also, do I need to have a hot wire going to the junction block? from what I can tell if looks like something to the left on the junction block is connected there(maybe blower motor, not sure). Trying to sort out what the guy before me did and straighten things out. I have no major mods on the truck, as a matter of fact I am trying to stay as basic and as close to the original setup as I can. Thx guys.

[75gmck25]

I have the stock wiring on my '75 GMC, and I think I can tell you where the fusible links are.

- Battery to large starter terminal is a standard battery cable with no fusible links.

- Large starter terminal to the 63 amp alternator BAT connector is a 10 gauge wire with a 14 gauge fusible link at the starter end. I have upgraded mine to 8 gauge because I now have a 94 amp alternator.

- Large starter terminal to the junction on the firewall is a 10 gauge wire with a fusible link at the starter. This is the main power to the junction.

- At the firewall junction there is a 12 gauge wire with a fusible link that runs straight down and connects into the truck fuse box and to the engine wiring.

- The wire that goes straight out of the junction to the passenger side is for the high speed blower relay, and it should have a 30 amp fuse holder inline. The blower on my truck seems to really take the full 30 amps, and this fuse holder gets very hot sometimes.

Some other considerations
- If the truck has an ammeter (standard in '75) there are also some extra wiring connections and fuses for the ammeter shunt. You don't need this for a newer truck with a voltmeter.
- The exact gauge of the wiring will vary based on the alternator size and the number of accessories. There were no power windows and locks in '75, so the alternator and wiring was probably smaller gauge.
- Many SI alternator wiring connectors use a jumper to connect the BAT terminal to the other 12 volt terminal (V-white) on the two-wire alternator plug. In the '75 wiring this connection to 12 volts for the 2-wire plug is farther back up in the wiring loom, but its still electrically the same. The brown wire on the two-wire connector goes to a switched ignition power source.

Bruce

[for3300]

Hey man thx for the detailed reply. Since you made that point about the not having power windows on a 75. I called Oreilly's Auto Parts to verify that they sell the same alternator for a 75 and an 83, because the truck ( the '83) has power windows and locks. Now I can get some fuseible links installed, I thank God nothing crazy electrically has happend so far and I can take some precautions by getting things straightened out now. Thx again man!

[75gmck25]

The decision about whether to upgrade to a bigger alternator is a personal choice. My 63 amp alternator was completely unable to keep up at idle when I had both lights and HVAC fan running, and it was not much better at highway speed.

A rebuilt 94 amp alternator was about the same price as a 63 amp, and all I had to do was find a mid-eighties 12si with a V-belt pulley to replace the old 10si. It directly replaces the 10si as a bolt-in. I did use a bigger charge wire, and the bolt to tighten it is now a metric threaded bolt, but that wasn't hard to change.

Bruce