74 FJ40 MAF HEI wiring question (1 Viewer)

Joined
Mar 20, 2016
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Lakeport, CA
Hello, I have a 74 FJ40 with an F155, desmogged, that I installed a MAF "lightning strike" HEI (mechanical advance) in place of the OEM vacuum retard dizzy.

This is what I've done, I have no doubt you will find I've not done it correctly, if I had I wouldn't be asking for help getting it right.

I've left the OEM coil/ballast resistor/igniter installed so far in case I have to go back to the old Dizzy. When I first installed it, I learned the "one wire hook up" was not correct, because I needed switched 12v power, and 12v starter power because the switched power cuts out while the starter is cranking, meaning no spark. So I took power from the +12v post on the coil where I found cranking volts, and power from the black/yellow wire near my starter.

FYI, I have a gear reduction starter that only has a black/white wire to it (plus battery lead and ground).

My 40 ran well and started easier than it had; until recently that is. Ive been having trouble starting it where its cranked and cranked before firing, and yesterday it wouldn't start at all. I knew I had fuel and air, so I tested spark and got none. I then tested for switched power, which I still have, but my cranking power is gone. I no longer have cranking volts available from the + post on the coil.

When I wire the HEI direct to the battery and crank it over it starts instantly, so I'm confident this is the problem.

The MAF directions said to use the original Dizzy wire, the DUI directions say I need alternator voltage (not sure how to get that).

Can someone help me get my wiring straight? Where is the appropriate place to pull power from for a one wire HEI that gives me cranking and running volts?

Once I get that figured out, can I just disconnect the old coil/ballast resistor/igniter and take all that out?

Thank you!
 
Joined
Feb 4, 2014
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Las Vegas
 
I don't know HEI, but I installed a DUI and I believe the setup is very similar.

In my '78, I removed the coil and igniter and taped off the Yazaki resistor wire on the large black-yellow wire, removed from coil (has wire instead of a ballast resistor).

For the DUI connections, I pulled 'unresisted' ignition power (call it "A") off the small Black-Yellow wire, removed from coil.

I wired "A" to a new 30amp relay (86)... to trigger the relay.

I wired 10ga FUSED 12v from the battery-positive to the battery side of the relay (30).

I wired a ground to the relay - 85

Then, I wired a 10ga power lead (87) to the DUI

I also wired my tach to the tach terminal in the DUI.

Ignition triggers (86) the relay... relay pulls the full battery voltage (30 to 87) to the DUI... with the relay, you aren't chancing overloading old wire with another power source.

Note: I previously pulled 12v from my coil+ to my electric fuel pumps and found, under extreme heat situations, that my coil+ only provided 5+ volts.

I don't know if that's normal or indicative if other problems... I just know it wasn't a reliable source for pumps that required/expected 12volts.

HTH

IMG_7557.PNG
 
Joined
Mar 20, 2016
Messages
38
Location
Lakeport, CA
HTH, thank you for this info. I'd considered doing something just like this. Yours is a great solution to ensuring you get +12 volts to the dizzy.

From your small black/yellow wire off of the coil +, do you get cranking volts and running volts?

My thinking was that if I did the relay approach, the relay would activate when i turned the ignition on, but would deactivate once I began cranking, then reactivate once the key returns to the "on" position. I use a relay wired to the ignition for my accessories and it does just that. I must be missing something here. I will disconnect the coil and test again for cranking/ignition voltage at that wire to see if that changes anything.
 
Joined
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Messages
7,146
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HTH=Hope that helps ;) I go by Danny.

Yes, the small 'by' wire is static voltage... As long as the ignition switch is NOT turned OFF... on my '78.
 

Coolerman

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Jan 5, 2004
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6,666
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Paint Lick, KY
 
 
 
pngunme, that is by far the best way to wire up these HEI ignitions. They require a lot of amps to run.

Jacob Bridges: There should be TWO Black/Yellow wires near the igniter. If you use the LARGER 12ga BY wire, you can feed the HEI directly without a relay. However the relay will take all that load off the already overloaded key switch. The smaller 18ga BY wire was for a filter cap but would also easily power the relay. These two wires are tied together inside the harness BTW.

I am curious about one thing though: The BY wire that originally fed your igniter should stay hot as long as the key is on or in start position. If yours is not, I would suspect you may have a key switch issue.

FYI You may be confusing the old starters that had an extra terminal that went to the coil + (bypassing the ballast resistor) to get the full +12V to the coil + when starting. That was a Black/White wire. It is referred to as a Ballast Bypass System. It only had power when the key was in the start position. Once the truck started, power was removed from that wire, and now the coil got it's power from the BY yellow wire that went through a ballast resistor. You want to make sure to remove that ballast resistor if you use the large BY wire for direct power to the HEI, or your HEI system will only have about 9 volts to it and will not function at it best potential!
 
Joined
Mar 20, 2016
Messages
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Location
Lakeport, CA
Thank you for all of the input, I’ll try to bring some resolution to this thread. As suggested here, I ended up installing a separate relay wired directly to the battery, and onto the HEI unit. The relay is triggered by constant/static 12 volts that I found on my GM ignition switch. Other than that power source, I could not find any source of power that had voltage during cranking other than the starter wire.

The relay solution works phenomenally! Sometimes I would crank and crank to get my cruiser to start. With the current setup, it starts faster than I can blink. Such an improvement!

Now I’ve got to figure out why I’ve got an unifentified fluid seeping from the bolts connecting my intake and exhaust manifolds... but I’ll start a new thread for that. Thank you again!
 

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