How many amps does an HEI distributor draw?

[NorCalC20LWB]

Got the truck back from paint finally, and am about to start a complete rewire with the AAW Classic Update kit. Would like to wire in a couple of on/off switches for anti theft, one being on the hot feed to the distributor. Any idea how many amps the switch would need to be rated for? Did a search on here and came up with mixed results.

[Andy4639]

If it where me I would tie a relay into it and only have the switch as the trigger. This way you have very little amperage running though the switch.

Google not me.

Because an HEI can draw as much or more than 25 Amps in the instant when the module first switches on the coil, a damaged ignition switch may not be able to supply the full amperage required by a HEI. HEI ignitions are Amperage hungry. 12V / . 4 Ohm (coil resistance) = 30 Amps


Actually, an HEI draws only 2- to 3-amps, compared to the approximate 6-amps an external oil-filled coil draws. The lower amperage draw results in a much cooler running HEI coil.

[tim_mc]

I built my anti-theft switch by installing a 20A switch to ground out the HEI tach lead. I've tested and demonstrated it many times and had no problems with the HEI module. When the HEI tach lead is grounded it will crank, but will not fire.

[NorCalC20LWB]

Andy, your reply sums up what was causing confusion from doing a search, I gather that HEI does not use many amps normally, but can spike. Great tip about using a relay, not quite sure what that is buy will research.

Tim, I had no idea grounding the tach wire would kill the engine, thanks for that idea.

As usual, great information, thank you.

Brian

[toolboxchev]

I will chime in here. Been having lots of electrical problems due to a suspect heater blower only motor.

Had my system over that the guys shop that rebuilds distributors, he advised me to put in a 7.5 amp fuse. I have stepped it down to 5 due to the electrical system going nuts.

Hei modules do not like high voltage situations, it will shorten their lifespan. Pulled from Crankshaft Coalition.

[rockyrivermark]

Originally Posted by NorCalC20LWB

Got the truck back from paint finally, and am about to start a complete rewire with the AAW Classic Update kit. Would like to wire in a couple of on/off switches for anti theft, one being on the hot feed to the distributor. Any idea how many amps the switch would need to be rated for? Did a search on here and came up with mixed results.

I did a similar thing in my truck 3 years ago and works great. Very simple to install. I mounted 4 - three position switches under dash my steering wheel.
So it’s like a combination of those 4 switches with 3 positions in order to start the truck. The switches I used are on in the up or down position and off in the middle position. So after I park I put all 4 in middle position.
Switch 1- goes to horn in both on postions. I was going to ad a timer to this but never did.
Switch 2 is nuetral safety switch off.
Switch. 3 brings power in and when in right position takes power to the 4th switch
Switch 4 sends power to the HEI.
That along with power locks on $75 alarm system
And fake lowjac detail are what I use

[franken]

"12V / . 4 Ohm (coil resistance) = 30 Amps" from post #2 isn't accurate or meaningful in the ignition circuit. The voltage to the coil is interrupted right before each cylinder fires, and inductance is what creates the high voltage to make spark. Thus the current through the coil is much less than what's presented above.
While the switching action of the HEI is closed, a magnetic field is generated in the coil, which creates impedance (think the inductive equivalent of resistance), which is much higher than the DC resistance of the coil.
That said, the HEI is fed w/ at least a 12AWG wire so yes, an HEI wants some current.

[BlouDon]

Originally Posted by franken

"12V / . 4 Ohm (coil resistance) = 30 Amps" from post #2 isn't accurate or meaningful in the ignition circuit. The voltage to the coil is interrupted right before each cylinder fires, and inductance is what creates the high voltage to make spark. Thus the current through the coil is much less than what's presented above.
While the switching action of the HEI is closed, a magnetic field is generated in the coil, which creates impedance (think the inductive equivalent of resistance), which is much higher than the DC resistance of the coil.
That said, the HEI is fed w/ at least a 12AWG wire so yes, an HEI wants some current.

Close! Below I changed your text to be even closer.

Originally Posted by franken

"12V / . 4 Ohm (coil resistance) = 30 Amps" from post #2 isn't accurate or meaningful in the ignition circuit. The CURRENT to the coil is interrupted right before each cylinder fires, and STORED MAGNETIC FIELD is what creates the high voltage to make spark. Thus the current through the coil is much less than what's presented above.
While the switching action of the HEI is closed, a magnetic field is generated in the coil, which is used to generate a very high voltage on the secondary winding of the coil which creates the spark.

That said, the HEI is fed w/ at least a 12AWG wire, to avoid voltage drop on the 12V feed to the coil so yes, an HEI wants some current.

The HEI system will NEVER draw more than about 8 amps. When the coil is switched on (like when the points close) the current is 0 (ZERO ) Amps - because of the inductance of the coil primary winding. The current then starts to ramp up from 0 to 6 - 8 Amps during about 0.0025seconds (2.5 mS), and when the time for the spark arrives, the current is switched off dropping back to 0 Amps (like when points would open) and that's when the spark occurs. The AVERAGE current is therefore very, very small.

Inductors (coil primary circuit) are like humans - they don't like change, and try to force the current to continue flowing and this causes a rapidly rising voltage (up to around 350V) over the coil primary winding . This rapidly rising voltage now becomes the "input" to the "transformer" (coil secondary circuit) in the coil and the output of this transformer is presented to the HT center terminal of the coil.

[BlouDon]

12V / . 4 Ohm (coil resistance) = 30 Amps

This calculation is mathematically and in electronic terms correct.

However, whilst the coil resistance may be 0.4Ohm, it is the coil primary winding's INDUCTANCE combined with the HEI ignition module's intelligence that will see to it that this current (30A) is never reached. Eg, if the engine is stationary, the HEI will not switch the coil on, so unlike with points, the coil will not be fried.

If, instead of the HEI module, the dissy had points, and the engine was stationary with points closed, indeed current would attempt to reach 30A but wires are likely to start releasing their smoke long very quickly. This is why many points systems have "ballast resistors" who's purpose it is to limit the amount of current that can flow through the coil when the points are closed.

[franken]

I see the previous two posts have gone into splitting hairs and let's make this harder to understand mode. Let's try no to bugger up the OP's thread further...
The DC calculation of current isn't applicable to an inductive load, which the HEI is. 'Nuff said.
See what SC has to say about HEI current. http://www.superchevy.com/how-to/eng...-distributors/

[rockyrivermark]

Omg. These posts are too much but certainly entertaining.
Bottom line
Use thicker wire.
Mark

[NorCalC20LWB]

There is some great info here, and I appreciate everybody taking the time to add their comments. That being said, I can be as pedantic as the next guy, but I'm having a hard time understanding all the specifics here (hey, be nice, I was only a business major with a minor in partying).

I guess it boils down to: Is it ok to interrupt the power wire going to an HEI distributor with a simple on/off switch? And if so, what should the amp rating of the switch be? From the answers given above that seems to be a not so simple question, with a lot of "it depends" caveats thrown in. Ahhhh, and I thought these old trucks were simple......

[AussieinNC]

Automotive Wire Amperage Capacity Chart
Recommended Length and Amperage for Automotive Wire while maintaining a 2% or less voltage drop at 12 volts
Automotive Wire Size 5 Amps 10 Amps 15 Amps 20 Amps 25 Amps 30 Amps

14 Gauge Wire (AWG) 18.4 ft 9.2 ft 6.1 ft 4.6 ft 3.7 ft 3.1 ft
12 Gauge Wire (AWG) 29.4 ft 14.7 ft 9.8 ft 7.4 ft 5.9 ft 4.9 ft
10 Gauge Wire (AWG) 46.8 ft 23.4 ft 15.6 ft 11.7 ft 9.4 ft 7.8 ft
8 Gauge Wire (AWG) 74.4 ft 37.2 ft 24.8 ft 18.6 ft 14.9 ft 12.4 ft

Essentially use either 14 g or 12 g wire to run the 12v feed to the HEI

[Stocker]

Originally Posted by NorCalC20LWB

I guess it boils down to: Is it ok to interrupt the power wire going to an HEI distributor with a simple on/off switch? And if so, what should the amp rating of the switch be?

Yes, it's OK. A few years or so back, my HEI wire somehow fell off the Unfused Ign lug (iirc). The truck would not start until it was re-installed. Electrically, that's no different than opening a switch.

As to the amp rating on the switch, after all the above discussion, I wouldn't hazard a guess. If it were me, I'd rummage through my stuff and find a stout switch, probably from a 120v appliance.

[MidLifer]

I guess I’m with the first commenter above. To be safe, wire up a relay. Use the toggle switch to trigger it.

Here’s a very easy to understand article about how to do it:
https://itstillruns.com/wire-automot...y-4912752.html

[Steeveedee]

The difference in price for the various toggle switches is so small that it makes no difference. Go with a switch that can handle 12VDC and 30A and fuggeddaboutit.

[AussieinNC]

Originally Posted by Steeveedee

The difference in price for the various toggle switches is so small that it makes no difference. Go with a switch that can handle 12VDC and 30A and fuggeddaboutit.

"fuggeddaboutit"....WOW !

I thought "thingamajig" "doohickey" and "watchamacallit"....were the best...but "fuggeddaboutit" takes the cake

[Steeveedee]

Originally Posted by AussieinNC

"fuggeddaboutit"....WOW !

I thought "thingamajig" "doohickey" and "watchamacallit"....were the best...but "fuggeddaboutit" takes the cake

You're on the east coast and never heard that pronunciation of "forget about it"? You may as well add "cravistat" and "doomaflotchie" to your list.

Oh, and "hoozagizzee"

[BlouDon]

Originally Posted by NorCalC20LWB

what should the amp rating of the switch be?

10A switch will do fine.