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My '70 C/10 - The Rat Bastard... [Won't Idle.]

y5mgisi said:
Im wondering how the relay will jump it up from 7.5v to 12v?
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Doesn't. A relay allows a lower voltage/current to control a larger one. The relay has thicker/shorter wire going straight to the battery, rather than meandering from the battery to the fusebox back to the dizzy.

The shorter/thicker is the key (or so my girlfriend says ... ba dum cha) so it carries more juice than the harness did. Bit surprised the brand new harness was delivering such a low voltage, but we'll think happy thoughts. (Mike, I swear the HEI on the crewcab ran at like 8V on the original resistor wire ... didn't run *well*, of course, and was much happier with a nice new 10AWG hot wire.)

Now you got me thinking I should check the voltage on the Blazer...

-- A
 
It takes 7.5v minimum to activate the magnet in the relay.
Which connects the battery power to the load, i.e., the distributor.

All a relay is, is a magnetic switch that connects main input power (bat), to a load.
The activation voltage just flips the switch.

If I understand it correctly.
 
TheBeast_88K5 said:
It takes 7.5v minimum to activate the magnet in the relay.
Which connects the battery power to the load, i.e., the distributor.

All a relay is, is a magnetic switch that connects main input power (bat), to a load.
The activation voltage just flips the switch.

If I understand it correctly.
Click to expand...

You're on target. (Mostly -- to be reeeeally accurate, the coil in the relay takes slightly more voltage to kick in than it does to hold it, so the "pick up" voltage is a smidge higher than the "drop out" voltage, but that's nit picking.)

Did I mention I was gonna be an electrical engineer when I grew up? Didn't happen, either the EE nor the growing up, but I can still talk like one sometimes :D

-- A
 
I guess I'mjust questioning the relays ability to carry 12v if you are eexperiencing such a big voltage loss through the ignition switch.
 
What gauge wire are running from battery, to ignition switch, to fuse box, to the coil?
 
y5mgisi said:
I guess I'mjust questioning the relays ability to carry 12v if you are eexperiencing such a big voltage loss through the ignition switch.
Click to expand...

Akin to the honey badger, a relay doesn't care what voltage it switches. Could be 12VDC, could be 240VAC.

In this case it allows a sort-of-12VDC on the coil to switch a much larger wire to the battery which hopefully carries really-12VDC. The two are entirely separate circuits, if that makes sense.

I'm still wondering if 10.8VDC wouldn't be fine for the HEI as I swear mine ran much lower, so Mike, I worry about your coil and/or connections in the dizzy. But try the relay thing first as you have the setup for it before you go thinking brand new parts are bad.

-- A
 
The purple wire coming from the i terminal on the starter is what I used when I did HEI. Works fine. No relay needed.
 
TheBeast_88K5 said:
Thanks for responding, guys. :D

NorCal_Chris came by... We think we solved it.

The "12v Ign hot", that ran the MSD, now the HEI "BAT",
was dropping the power to 10.8v during cranking.
My msd wouldn't fire is voltage was below 11 volts. But my hei would fire down to 10 volts. I have the msd hooked directly to the battery now with the power on switch hooked into the ign spot on the fuse block. It works much better now. Even with that it still has problems if the power is under 11 volts. Battery has to be fully charged for the msd box to fire on my truck. I haven't tried starter voltage drop to see how much of a difference it gets. I had to put a 800 cca battery in to make sure it has enough power to crank and fire at the same time.
Click to expand...
Click to expand...
 
So where are the burn out pictures with the sound of a 8000 rpm BBC? Every time see this thread bumped a get excited only to be let down, again.
 
Let's hope that's it. HEI is fussy with input voltage. There's a reason they sell those voltage step up kits for guys running high compression that don't want cd ign. It steps the input voltage up to a consistent 14 or 15v and burns way hotter.
 
that has to be a resistor wire in that harness.. it's the most feasible way for it to be doing that...

was this harness made for early 70's vehicles, or was it a modern universal?

that's very odd...
 
ryoken said:
that has to be a resistor wire in that harness.. it's the most feasible way for it to be doing that...

was this harness made for early 70's vehicles, or was it a modern universal?

that's very odd...
Click to expand...

Its "GM Universal", designed for a 90's pickup.
Gonna place a call to EZ-Wiring, for the 450th time, and ask their opinion.
 
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