SBC Rewiring Question (1 Viewer)

[EWheeler]

Have searched and read and understand what will work (lots of Coolerman posts and RWBeringer threads/posts) , but I don't see anyone having done what I am thinking.

SBC in a 2/80 FJ40 with HEI distributor. Cleaning up some odd PO wiring and in complete honestly, I screwed with something that didn't need screwing with and ended up making a running truck not run. They had installed a toggle switch on the dash that provided 12V power to the HEI coil from some rinky dink fuse block that was piggybacked onto the OE fuse block. I removed that mess before determining what effect it's removal would have on the truck, having assumed incorrectly it was all non-essential.

Harness tracing & figuring out why I didn't have 12V+ to the coil took me all the way to pulling the main dash harness out and while I was in there, I ditched the OE Emissions Computer and associated wiring, the OE Carb Cooling relay and associated wiring, and also ditched the OE non used AC wiring and plugs that were hidden in the main harness.

This brings me to my question - since I am this deep in the harness, I am wondering if I can ditch the Yazaki Ignition resistor wire that is now accessible and then use the large gauge BY ignition ON wire to POWER my HEI coil? I understand that this is not possible with the resistor wire in place due to the reduced voltage of that circuit with the resistor wire in place. I have also read about the need to provide a better (hotter?) power supply to the coil during starting (unsure if this also applies to HEI?) - but I can do this easily with a parallel circuit from my existing 4 post ford starter relay.

Thoughts on the below circuit? I understand I can achieve full battery power to the HEI coil at all times with a dedicated relay triggered by the BY wire from the harness WITHOUT removing the pink Yazaki wire. (Cruiser Wiring - https://globalsoftware-inc.com/coolerman/fj40/wiringmainframe.htm) I don't see the need to do this if I remove the Yazaki wire.....

 

[pbgbottle]

you must have a wire around with 12v ignition on you can use I don’t think I would use that big fat juicy wire. I’m pretty sure I read the HEI can run with as little as 6 volts. I can’t quite remember what I actually did but I know I have some flimsy little wire coming from the harness and provides 12v to the coil. Been like that for 20-25 years. I’d have to go look at the color. But I didn’t do anything fancy. I did more fancy stuff to power up my starter with fat juicy thick wires and a relay. Others will have to chime in as I only commented to get this thread going for ya. Haha cheers. Pretty sure any 12v switched power will work. I also had to run a dummy light from my alternator as my truck wouldn’t shut off the power would back flow and keep it running. Cheers.

 

[EWheeler]

you must have a wire around with 12v ignition on you can use I don’t think I would use that big fat juicy wire. I’m pretty sure I read the HEI can run with as little as 6 volts. I can’t quite remember what I actually did but I know I have some flimsy little wire coming from the harness and provides 12v to the coil. Been like that for 20-25 years. I’d have to go look at the color. But I didn’t do anything fancy. I did more fancy stuff to power up my starter with fat juicy thick wires and a relay. Others will have to chime in as I only commented to get this thread going for ya. Haha cheers. Pretty sure any 12v switched power will work. I also had to run a dummy light from my alternator as my truck wouldn’t shut off the power would back flow and keep it running. Cheers.

HA!! The dummy light, that is my next question. Its there, on the dash. Glad I didn't remove that sucker as well. Currently reading MAD electrical tech articles to figure out if it's wired correctly because I don't think it is..... The dummy light signal wire at the alternator has continuity with my ignition START wiring (black/white stripe) that sends the activation signal from ignition switch to the starter solenoid. That seems incorrect.

I bought the truck without knowing it ran, because if it didn't, it was still a good deal! Seller said it ran fine but needed a carb rebuild. He has someone rebuild the carb (Weber built Carter ABF) and then he couldn't figure out how to re-connect the throttle cable and gave up.


To answer your question, the only wire with 12V ignition on is the tiny little Black/yellow stripe wire that originally went to the ignitor:

https://globalsoftware-inc.com/coolerman/fj40/schematics/FJ40/1980FJ40/Haynes1980FJ40Page1.jpg

If I didn't already have the main harness out, I would use that to trigger a relay. I still could...

 

[pb4ugo]

When I 1st did my carb'd sbc back in the 80's, I found a keyed on 12 volt circuit and powered the hei. The alt wiring was altered and still used the ammeter. About 15yrs ago I pulled the harness and replaced/reworked it after it leaked smoke. I found mad electric too. I think on the back of the fuse panel, it has 2 spades, 1 for key on, and 1 hot all the time. I think I tapped into key on for the hei. My alt charge wire goes to the batt. The white wire at the batt feeds the fuse box. The ammeter is bypassed cause it wont work anyways. I run an idiot light. Yrs ago I never ran a light, the sensing terminal wasn't hooked up. I'd have to rev the engine a little to activate the alt.

On my 2nd 40 is basically wired the same way. Except the b/y goes to the hei. I added 2 wires from stk. 1 for the idiot light and the other was the alt charge wire to the batt. The rest is all stk. I never pulled the harness. I use the stk gauges in the cluster with the 40's stk sensors and color coded wiring.

 

[EWheeler]

MAD electric is saying HEI wants as much voltage as possible:

Catalog

I think I will ultimately end up installing a relay to give battery/alternator voltage to the HEI, but for now, will use BY for HEI just to get the truck running. There is some other harness/wiring cleanup associated with the SBC that I want to do, but will require moving the battery back to the passenger side from it current location on drivers side front fender well.

Dummy light gets ACCY power from LR (light blue/red stripe) wire at the fuse block in drivers footwell. Need to investigate further why I am getting continuity to the BW START wiring from the ignition switch at terminal 1 on the chevy alternator......