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Old 10-18-2012, 12:38 AM   #1
wayne e benamati
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ignition coil resistor

where is the ignition coil resistor located on 1972 chevy truck with 350ci v8 ?
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Old 10-18-2012, 01:24 AM   #2
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Re: ignition coil resistor

Welcome to the forums. You'll enjoy it here.


It's a resistance wire that comes from the key switch through the firewall block by the master cylinder. The first picture is a view of it. It is the cloth covered one in the middle. It is whitish in color but when new it was orange, purple, and white.

The second picture shows how the wire runs to the starter or in some cases it runs to the coil. If it goes to the starter it connects to the "R"terminal opposite of the "S" terminal where the purple wire goes for the solenoid to start the engine. From there a yellow wire goes to the coil positive to provide a full 12 volts to the coil for starting. During normal running the ignition goes through the resistance wire to drop the coil voltage and prevent it from getting too hot and burning up. the resistance wire is shown by the red arrows.

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Old 10-18-2012, 12:46 PM   #3
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Re: ignition coil resistor

Quote:
Originally Posted by goodnok View Post
i did never know it before. thank you its cause me to get more knowledge, i perfactly unterstand how to wires runs starter
Thank you also and for all the readers of this thread and the forums who are considering a conversion to an HEI distributor. YOU MUST NOT USE THIS RESISTANCE WIRE FOR YOUR IGNITION. THE HEI DISTRIBUTOR HAS TO HAVE A FULL 12 VOLTS ANY TIME THE IGNITION IS ON, TO OPERATE PROPERLY.
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Old 10-18-2012, 10:07 PM   #4
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Re: ignition coil resistor

The stock GM resistor wire is rated the same as the porcelain resistors for GM, 1.80 ohms, +/- 1%. The wire is silver stranded, has a regular plastic insulation, under a second white plastic cover that makes the wire appear to be a cloth covered, as shown in the picture above. To get the correct resistance, this wire is 7-1/2 FEET long.

For the V8, the resistor wire travels from the firewall connector, over the top of the engine, down twords the starter motor, where if reverses inside the loom, back up to the coil.

For the later inline 6's, with distributor at the front of the engine, the wire starts in the same place, goes up and over the engine, then down to the engine wiring harness along the side of the engine, to the coil.

If this wire is damaged, NAPA has a part number for it. It can also be removed, and a regular copper stranded wire used in its place, as long as a porcelain resistor is used in its place, to the coil. NAPA p/n ICR13 is the correct part to use (this is the resistor that the 1955 through 1960's GM cars used before going to the resistor wire.

Easiest way to go full un-resisted wire for HEI, use the IGN or IGNITION terminal on the fuze box to the BATT terminal on the HEI, and tape the resistor wire off, feed it back into the over engine loom. This way, you have a clean wire to power up the HEI at full voltage, and the resistor wire still in place, and undamaged for future use as a coil power up wire if the HEI is ever removed and a point distributor re-installed.

Just FYI.
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Old 10-31-2012, 04:23 PM   #5
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Re: ignition coil resistor

Anybody know which wire it would be on the 305 V6 in my 67 GMC? Ordered a pertronix ignitor ii today and would like to be ready for when it arrives. I have included a picture of the fuse box if that helps.

Thank you for your help!
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Old 10-31-2012, 08:25 PM   #6
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Re: ignition coil resistor

Take this advice on the Ignitor, please....LEAVE THE RESISTOR IN PLACE, use the stock GM coil, good stock type resistance wires, and keep the plug gaps down, .032 to .035, NO LARGER.
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Old 10-31-2012, 09:10 PM   #7
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Re: ignition coil resistor

Thanks HEI451! Would you mind telling me why? I'm assuming the 305 V6 doesn't need that hot of a spark? I was planning using the Buick 3.8 HEI conversion mentioned on Jollys site, but I got the ignitor II for 50 so I couldn't pass up giving it a try.
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Old 11-01-2012, 10:38 AM   #8
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Re: ignition coil resistor

There are also much better ways to HEI that engine.

The PerTronix isn't an HEI, far, far from it, and every tiome they have tried to "improve" it, they have made it worse. One of the issues is the power transistor inside it, they don't like large currents and voltages. In other words, they fail, miserably when anything other than feeding them resisted voltage comes into play.

This also means they don't have the dwell extender drivers for higher performance coils that HEI has, and they don't do any more sdpark than a stock point system, on the point's first spark. They only do one thing, they don't experience point wear.

They also don't like large spark plug gaps, so that alone should tell you that they aren't a high output system. They were originally designed for industrial stuff, like fork lifts, and they haven't gotten any better with revision.

Think of them as a set of electronic points.

Since you got a deal on the unit, I'd use it, but, from my years of ignition system experience, I'd leave the resistor in place and functional with it.

Read my user name, I do those for a living, and did/still do point systems today. I won't use what you bought, but, no reason for you not to, just leave the rest of the system alone, don't get over hype'd into problems you really don't want, nor need.
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Old 11-01-2012, 11:45 AM   #9
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Cool Re: ignition coil resistor

Thank you for the information. I will contact you when I am ready to convert over to HEI I realize these are far from being HEI, just couldn't pass up the deal as all the rust on my truck is taking all my money haha.

Again, thank you for the information!
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Old 11-01-2012, 03:42 PM   #10
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Re: ignition coil resistor

A question on this subject...

I've seen a couple places where people say you can "replace" this resistance wire. When I got my truck, somebody already had, and they just cut it as close to the fuse block as possible, and butt-spliced regular copper wire to feed the HEI. It was spliced so close, the butt connector was up against the fuse block. I've run the truck that way and never noticed anything wrong.

Now that I know a little more about what was intended and how these trucks were put together, is this method of "replacing" the resistance wire acceptable for running an HEI? Or would you need to get a replacement terminal to crimp copper wire to, and install in the block? They are just Packard 56 male terminals, correct?

Thanks!
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Old 11-01-2012, 04:48 PM   #11
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Re: ignition coil resistor

What I recommend is to go take a look at the fuse box, see if there is a terminal there marked either IGN or IGNITION. If thee, test it for voltage with the ignition switch (key) in both start and run positions. Both have voltage, run a 14 gauge wire from that terminal out the cab, along the wiring loom, to the BAT terminal on the HEI. done. Then, you can take the resistor wire and tape it off, put it into the wiring loom, still workable, but now dormant.

If only in run, no issues, use the same setup from the fuse box to the BAT, run a second wire to the solenoid for start voltage, done.

The resistor wire in the firewall connector is very small gauge, and if you are replacing it, then, go to a dealer and get the right connector, wire up a complete new wire, don't splice into the stock resistor wire, not even an inch of it.
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Old 11-01-2012, 05:06 PM   #12
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Re: ignition coil resistor

Quote:
Originally Posted by HEI451 View Post
The resistor wire in the firewall connector is very small gauge, and if you are replacing it, then, go to a dealer and get the right connector, wire up a complete new wire, don't splice into the stock resistor wire, not even an inch of it.
10-4, thanks for your help! The original resistor wire is LONG gone (except that "inch" you speak of, ha ha), so this is probably what I will do. I've got all the spare connections on the fuse block inside operating relays for my fuel injection, tach, etc.

What gauge do you recommend? The wires under an HEI cap aren't very big, and on an external coil the positive and negative leads usually aren't much either.

Thanks!
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Old 11-01-2012, 11:50 PM   #13
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Re: ignition coil resistor

Everyone goes totally berzerko with the wire gauge for the large cap HEI, and it doesn't need to be that large. They preach using a 10 gauge wire, but, 12 gauge is just fine for one of those nightmares.

The problem is the coil itself, they just plain run hot because of their epoxy filling, and being under a cover, so, they make a lot of heat resistance.
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Old 11-09-2012, 07:42 PM   #14
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Re: ignition coil resistor

GM wiring harnesses for 6 cyl engines usually have a 1.8 Ohm ignition ballast resistor. V8 harnesses can have a resistor from 1.6 Ohm to 1.35 Ohm depending on year and where you're reading the info.

Secondary ignition spark is NOT produced from any "full 12 volts". If you only have 12 volts you need to start by fixing the alternator. The term "full 12 volts" makes me cringe. No matter what kind of ignition, it should have about 14 Volts coming to it from the ignition switch.

An ignition coil is a transformer with two windings, a primary and a secondary. The secondary winding has about 100 times more turns (winding) than the primary in most ignition coils. The ratio of turns primary to secondary is roughly the same as voltage ratio, primary to secondary.
12 volts on the primary, times 100 equals 1200 volts. Not enough to fire a spark plug in the engine. 14 X 100 = 1400 volts. Still not enough.

Connect 12V or 14V to the coil Primary and it produces a magnetic field around the coil wires called an Electro Magnetic Field or EMF. When you REMOVE the 12V or 14V, the magnetic field collapses and the REVERSE occurs. When a magnetic field collapses around a wire it causes current to flow in the wire. This is called Back EMF. When the magnetic field collapses across an ignition coil Primary winding it produces 200 - 300 Volts on the primary winding.

200V - 300V times the 100:1 ratio equals 20,000V - 30,000Volts on the secondary winding. Remember I did say roughly 100 times.

300Volts will cause arcing on the points and burn them up. That is why they have the ignition condenser. The condenser absorbs most of that 300 V to prevent arcing as the points are opening.

The ballast resistor is not the devil. The difference between 12 and 14 volts has very little effect on the primary voltage that actually produces the secondary voltage.
I have had the HEI in my truck running off the ballast resistor for about 15 years. The ballast resistor does drop 1.5-2.0 volts. The HEI doesn't care and neither do I. The truck runs great.

From HEI451
>>take a look at the fuse box, see if there is a terminal there marked either IGN or IGNITION. If thee, test it for voltage with the ignition switch (key) in both start and run positions. Both have voltage, run a 14 gauge wire from that terminal out the cab,<<

The ignition terminal on the ignition switch feeds both the fuse box and the firewall connector to the ballast wire. The wire from the Ign switch to the firewall connector is basically a straight shot with the fuse box connection tapping into it.
The drawing below shows more of the wiring on the inside of the firewall. The two halfs of the connector are mirror images. The 20W/OR/PPL second wire from top-right on engine side, connects to 12P second wire from top-left on dash side. The 12P goes to the Ign switch on the next page of the drawing.
Tapping into the fuse panel is taking the long route to the Ign and introduces extra crimps, connectors, solder joints or what ever.

Could not agree more on the 14 gauge. Stock HEI draws less than 2 Amps.

If you talk to Pertronix directly and get past the BS, they will tell you their unit will work just fine when wired through the ballast resistor. They will give you double-talk and keep reverting back to "it will work better without the resistor". Ask them how much better and you will get, "well it's just better".
As HEI451 says, the Pertronix (all of them) hate over-voltages. Not just over-voltage, Voltage spikes kill them instantly.

Sorry this image is so small. I tried to reduce the resolution and then make it wider, but I was unable to to do it.
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Old 12-03-2012, 07:17 PM   #15
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Re: ignition coil resistor

I have the 305 V6 that has not been started lately. The PO tied off the resistor wire and ran straight from the battery to the coil.
Before attempting to start I want to sort the wiring issue.

Still has points distrib although looks like new cap and points.

How do you "bench test" the resistor wire? I have the manual for the Suburban and the wiring diagram in it. But, they don't explain what you should see on the meter.

The ignition switch is gone ( I have a replacement from LMC). Not sure it is the right one but...

I have gleaned the following:
The resistor wire lowers the voltage.
Is that in start or run?? From 12+ down to 7 ish???
12 v in start for cranking??

If I start it hot wired as it is, what's going to go wrong??

The wiring diagram is helpful but I'm not sure I'm clear on what is going on.

I don't want to convert to HEI yet, as it has been run as it sits and I don't want to fight the wrong problem by changing anything. The wiring is a mess and it had no battery, so I have to sort the wiring out before adding fuel.

Any info will be a big help.

Thanks
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Old 12-03-2012, 10:26 PM   #16
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Re: ignition coil resistor

What year? different year trucks have the resistor wire in different locations.
The ballast resistor is measured with an Ohm meter. Most Chevy 6 cylinder ballast resistors have 1.8 Ohms resistance.

>>Is that in start or run?? From 12+ down to 7 ish???<<

That is a useless piece of information in start or run. The only thing for sure the 7 volts is telling you is that when the points close, they are shorting the 12 volt battery voltage to ground.
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Old 12-03-2012, 11:01 PM   #17
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Re: ignition coil resistor

1966 GMC Suburban 305-V6

That was a question, sorry. All I meant was that the resistor lowers the volts, but I'm not clear when that happens or how to tell if it is working properly, as the wiring is a mess. There is very little in writing that explains fully the resistor wire, especially when it comes to the GMC V6. Most answers are just put in an HEI. I intend to do that after I get it running and before making changes.

What should the meter read at the + side of the coil with the switch in the run position?
What should the meter read at the + side of the coil with the switch in the start position? (Cranking)

Thanks
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Old 12-04-2012, 12:35 AM   #18
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Re: ignition coil resistor

I hope these images show well.
The ballast resistor is only used in "Run" position of ignition switch.
In the cranking position, it is by-passed by the starter solenoid.

Coming out of the Engine Harness Connector there is a wire labeled, "20 W/O/PPL". White/Orange/Purple was the color of that cloth covered wire when it was new. Today it will be dirty brown and the outer cloth covering may even be gone. That wire is the ballast resistor wire.
The Yellow wire (20 Y) feeds the coil from the "R" terminal on the solenoid. In start, the solenoid internally shorts the "B" to the "R" and the 20Y is fed directly from the battery cable. In run the solenoid releases and the "R" terminal becomes just a junction to connect the 20 W/O/PPl to the 20Y. The current from the battery now takes the long route from the battery to the plug on the driver side of the firewall. From that plug it goes up to the ignition switch, then over to the passenger side "Engine Harness Connecter" where it plugs into the 20 W/O/PPl wire. Through that resistor wire to the 20Y and finall up to the coil.

Key off. Connect your Ohm meter which should be on its lowest scale, to each end of that 20 W/O/PPl wire and it should read close to 1.8 Ohms.

On its lowest scale, your meter may pick up static electricity. Before you start, touch your two meter leads together. Do not touch with your fingers. If you get a reading, write that number down for reference. Now measure the resistance of the ballast wire and subtract that reference number.
You should then have a fairly accurate measurement of the resistor wire. The cheaper the meter the harder it is to get an accurate measurement.
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Old 12-04-2012, 03:19 PM   #19
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Re: ignition coil resistor

Thanks for the info.
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Old 09-09-2016, 05:10 PM   #20
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Re: ignition coil resistor

any Ideas as how to wire in a external resistor on a 64 chevy panel with a 350 in it?
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Old 09-09-2016, 08:10 PM   #21
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Re: ignition coil resistor

Quote:
Originally Posted by ironman3452 View Post
any Ideas as how to wire in a external resistor on a 64 chevy panel with a 350 in it?
Very simple:
This is the resistor circled in red.

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run one wire from the ignition switch to the ballast resistor, either end doesn't matter which, then run another wire from the other end to the positive terminal on the ignition coil.

In order to provide 12 volts for starting you need to run a wire from the starter solenoid small post, on the opposite terminal from the starter solenoid engagement wire shown below, to the same post on the coil positive.


Attachment 1569087

That's all there is to it.


I forgot to add that if you are running an HEI distributor, you do not need the resistor nor should you have it in the ignition wire.
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Old 05-11-2019, 09:43 AM   #22
howell57
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Re: ignition coil resistor

Help! Which color wire is hot when on "RUN" ? I have a brown wire with white stripe that is hot when key is on RUN, but goes off when I turn key to START.
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Old 05-12-2019, 10:58 AM   #23
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Re: ignition coil resistor

What year is your truck? A points distributor/coil gets power from the resistor or resistor wire that has power in the on position, and gets power from the starter solenoid during start.
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Old 05-12-2019, 11:20 AM   #24
howell57
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Re: ignition coil resistor

Truck is a 72 C10 but it had an HEI system. It had at least six wires on the positive side of the starter. I have put a check light on every wire and turned the ignition key to every position to try and identify a constant hot when key is on RUN. I located one brown wire with a white strip that is on when key is in RUN position but goes out when key is put into START position.
With the L83, I only have two wires to hook up, one being a pink wire that needs constant 12 volts and a red wire that connects to the OEM purple wire that fires the starter. Easy fix is to route a constant 12 volt to a switch and just use the key START. I ordered such a switch. I'm an old SBC guy and this new GEN V engine is a whole new learning curve!!!
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Old 05-12-2019, 12:33 PM   #25
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Re: ignition coil resistor

To power an HEI just connect it to the firewall connector where the resistor wire normally connects. http://67-72chevytrucks.com/vboard/s...d.php?t=708975

You normally would have one big fat wire going from the positive battery terminal to the starter, and the purple wire from the firewall connector goes to the small inboard terminal on the starter. You don't need any other wires on the starter, just those 2.
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