HEI power wiring...
 

I have a 74' with HEI ignition. The factory HEI wiring is completely gone. I have it powered incorrectly and its melting the insulation on a tan wire Im using from the firewall bulkhead.

I would like to properly wire the Positive + power supply to the coil. Looking at the schematic (my interpretation) the + is a 20G Y (Yellow) labeled 20Y 7A which splices to 20Y 7B. The 20Y 7B looks like it attaches to the R terminal on the starter solenoid. Is this correct?

From 20Y 7A it also splices off to 20 P/B 3. What is P/B? is it pink or blue or purple or blue? This wire then appears to terminate at the firewall bulkhead engine harness terminal 3.

The schematic also calls for a 1.35 Ohm resistor for the V8 engine where is splices at 20P/B 3. Do I need the resistor?

Basically do I just need to run a wire from the + Terminal on the coil to the R Terminal on the Starter Solenoid, add in a splice wire back to the number 3 terminal on the firewall bulkhead engine harness?

Re: HEI power wiring...
 

This one is easier to read.

Re: HEI power wiring...
 

Pretty interesting, my 74 still had points ignition.

Re: HEI power wiring...
 

As did mine Al.
All the wiring is gone for the hei because it never existed.
All you need is a dedicated 12v power source for the hei. Either from a 12v spade on the fuse box or from the ignition switch.
The yellow wire and resistance wire are not needed for an hei.

Re: HEI power wiring...
 

1974 would be a points ignition system. If you are converting to HEI then you need to replace or route around the resistor wire and up grade to 12 ga wire. I usually will run a bypass wire from the bulkhead connector and where it plugs the original resistor wire in the bulkhead connector. The yellow wire that runs down to the starter is the R wire that supplies power to the coil in cranking. Early ignition switches did not allow power to the ignition system when cranking the engine.

Re: HEI power wiring...
 

My '74 was also points but I converted to HEI. I removed the factory distributor power wire from a ~76 parts harness I had and replaced the resistor wire my '74 had. Easy swap and looks factory since I used factory wiring.

Re: HEI power wiring...
 

Okay... this makes sense. I've owned the truck since I was 16/17 and have had the HEI wired this way since I got it. I just recently upgraded to a more powerful coil and maybe thats why its melting the insulation.

HEI Power & By-Pass Wiring...
 

" Early ignition switches did not allow power to the ignition system when cranking the engine. "

Not sure where you ever got that idea .

The entire point of the second terminal on the starter solenoid was to by pass the calibrated ignition wire from the key switch when cranking so the spark was stronger .

I add HEI systems to everything and my simple method is one of those tiny cube relays that's rated for 50 amperes, kicked by the low power feed (7 ~ 9 VDC) from the key switch .

Easy and doesn't hack up the factory harness and isn't glaringly obvious .

Re: HEI Power & By-Pass Wiring...
 

The OEM coil will draw 50 amps? I like the idea of running it this way. I need to add a fusible link and I plan on connecting it to the accessory lug on the factory busbar. Do you run a fuse? How big of a fuse is needed?

Ignition Power Requirements
 

No ;

Either way draws under 15 amperes .

Ignition systems are typically not fused for safety reasons, I don't -ever- fuse them as I don't think I know more than the Engineers .

If it worries you, set up a separate dedicated power circuit and add a circuit breaker .