When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Frankly, all devices draw an initial surge, but it is tiny and limited to a few milliseconds in non-motors
That is not the case with certain resistive loads, incandencent loads for example pose high inrush currents...perhaps not a matter in a car but for the sake of accuracy....
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by plaidside
You beat me to it. The GM HEI primary resistance, IIRC, has .5 ohms resistance and using Ohms law that has a potential to flow 24 amps.
The circuitry in the HEI module will automatically limit the coil primary current to 5.6-5.8 amps by switching the power transistor from saturation mode into linear (resistive) mode. The coil is an inductor, which means the current won't go to the moon instantaneously, and the current monitoring circuitry has plenty of time to stop the primary current from exceeding its calibrated current level (no current spikes).
(Of course, everything goes out the window if you have a defective module, or the TACH line gets shorted to ground.)
I think this is being way over thought. I have seen rigged wiring feeding Hei's for what seems like forever and no issues except a crappy connector with intermitten issues. Many with 18 gauge crap or smaller. This morning I oscoped my factory Hei unit to show what really is happening.
True, but let me provide some more “theory” to the mix. The reason engineers spec 12 gauge wire is to ensure as much of the 12v as possible is available at the HEI unit and not dropped across the feed wire. We are talking very small resistances (or inductive reactance where voltage leads current by 90 degrees or a quarter wavelength) and every ohm of wire resistance matters. In other words, don’t look at this as a current strategy, but as a voltage strategy.
The Test I show is on a 16 ga input. The oscope picture shows the top red line as voltage (channel B) with voltage table represented on the right. Amprage is represented in blue (channel A) with the current draw table on the left. Max current is 6 amps. Reving stays the same. Under a load it will increase based on afr, timing and running compression, but I didnt go that far in testing.
Using a fast hand held multimeter you can see the contrast in current draw compared to the oscope. As it shows 1.5 amps. Not very accuate.
The small voltage drop shows my harness has a small issue of 1 volt but in general it is hobbish to chase it.
All of this proves you could replace the harness ignition feed from the ignition switch to the HEI (pink wire) with a 12ga wire and the current flow through the ignition switch would be minor and easily within spec...however, only if your ignition switch provides 12v on that lead when in the START position which most do not.
Bottom line, 12ga wire is not for current flow, but for voltage preservation. Greg still needs a reliable 12v source that is maintained while cranking, so the relay is a good solution.
Dumb question....doesnt the ignition switch NEED to provide 12V to the pink lead in both the START and RUN position in order to provide power to the stock coil (through the resistor) in order to start the motor when cranking?? I haven't put a voltmeter on it yet to verify but assumed that was the case.
Unfortunately, no. If you inspect the ballast resistor you will see the ignition switch connection on the bottom of the resistor and two connections on the top. One of those goes to the ignition, the other to the I terminal on the starter. When the starter is engaged, it provides 12v during cranking to the top of the ballast resistor effectively bypassing it to provide the full 12v during cranking. The ignition switch then typically drops power to the Ignition outputs causing all devices powered by the Ignition to drop out ensuring all power is available to the coil.
Ok, so I've got the ballast resistor with the pink wire coming in from the bottom and going out the top...
Then at the coil, I have a pink wire from the ballast resistor (right) and a lighter pink? from the starter (left) both going to the positive side of the coil.
If I'm going to run a relay just to be on the safe side as the integrity of my harness is still somewhat questionable in my mind, which of these wires should be triggering the relay in order to ensure 12V both while cranking and in run?
Got it, thanks....am I correct in assuming that I need to bypass the ballast resistor on the firewall or leave as is?
If you are determined to modify the wiring with a relay, you might as well remove the ballast completely. It is not used with HEI. Use the pink wire that goes to the ignition switch to trigger your relay. Depending on where you mount the relay, you can either connect the two pink wires together on the SAME end of the ballast resistor (thus bypassing it) and then connect the pink that previously went from the ballast to the coil, to trigger the relay, OR you can remove it completely and connect the relay trigger to the pink wire coming out of the dash, OR you can just make it easy and connect the pink wire to the 12v connection on the HEI and forget about the relay. That's what I'd do. Your car, your choice. Either way you decide, don't leave the ballast resistor connected in the circuit. You can leave it in the car for looks, but one end must be disconnected from ANY wires so that it is not in the circuit.
In summary, what I would do is move one of the pink wires to the other end of the ballast so that they are both connected under the same screw on the same end, then I would disconnect the pink on the + coil screw that came from the ballast and connect it to my HEI. Leave the pink wire that goes from the + on the coil to the starter connected as is. I have seen it done many times without issue and the wires are big enough too carry the current needed. Remember the other pink wire to the starter has been carrying full 12v to the coil since the car was built, and it's the same size as the one to the ignition.
If you don't feel comfortable with that, you have plenty of other advice in this thread to follow.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by GordonR
I think this is being way over thought. I have seen rigged wiring feeding Hei's for what seems like forever and no issues except a crappy connector with intermitten issues. Many with 18 gauge crap or smaller. This morning I oscoped my factory Hei unit to show what really is happening.
Your setup to measure the primary current has my attention. What are you using?
Thanks.
You beat me to it. The GM HEI primary resistance, IIRC, has .5 ohms resistance and using Ohms law that has a potential to flow 24 amps.
