Ignition coil discussion

[Foxhound]

I was going to write up a thread about possibly modifying an ignition coil, but upon further research it's not that simple. Is it ever? The coils that came with the truck were out of spec so I figured: I have ngk sparks and plugs, why not a couple ngk coils too, right? Wrong...or maybe not. The NGK coils weren't in spec with the service manuals ohm range of 1.04-1.27. They were even different among themselves, one being closer to 1.6 and the other 1.7. Whatever, they worked seemingly fine, but just know, they are made in China if you buy NGK coils from anywhere. Bastards got me. That's what I get for becoming an ngk fanboy after getting this truck.

 

If you think you have stock coils don't change them. How you would even know if you have stock coils unless you're the first owner, I don't know. It would seem to me impossible to know. What I've come to learn is that the resistance readings for any given coil compared to the service manual specs is only good for a ball park idea of what a coil should read. Those specifications are for stock coils. What you and I will likely ever know is how many winding turns they used for the primary and secondary coils, the diameter of wire, how much "dwell" the ICM accounts for--if it does--at a given rpm, or exactly how much voltage a stock coil was intended to output. Dwell was a knew term for me. If you're not familiar it's the amount of time it takes the coil to charge fully, usually measured in degrees of crankshaft turn. At lower rpms your coil might be sitting charged too long and at higher rpms it may not be charging fully before discharge. This all depends on your coil and ICM from what I've gathered. Correct me if I wrong.

 

I figure since they recommend NGK spark plugs then NGK coils would likely be within spec to some degree, maybe not primary resistance wise, but at least output voltage wise. It's hard to say. A lot can change in 40 years. If you're buying new coils it's a literal shot in the dark. You'll never really be able to know how well they will pair with these ignition systems.

 

I took apart the more "out of spec" of the coils I replaced. I had the idea of possibly shortening the primary coil winding to lower it's resistance to be within range. It was wound ~36 times per layer of 6 layers for a total of 216 turns. Starting at 1.6 ohms thats ~.0074 ohms per turn. I was able to get it down to an 1 ohm after removing a little over 2 layers. Let's say the winding ratio of primary to secondary was 1 to 100 to begin with so 216 to 21600 for a voltage uping of say a 250v spike from the collapsing magnetic field we'd have a 25,000v output. Lets say I removed 80 turns to leave 136. 136 to 21600 is about 1 to 159. 250v multiplied by 159 is just under 40,000v. I don't think that's gonna work but I really don't know what volatages we should be working with in a z24. It will also likely charge to fast, but it was fun tinkering a bit. Does anybody know what voltage our spark plugs like best and what our ICMs can handle before they fry? Of course each ICM manufacturer probably varies. I thought this would make for an interesting discussion.

 

Internals of canister coil for the curious. All that needed to be done for dissassembly was prying the top lip with a flat head and the rest slides out. Maybe I'll make a mini rail gun or something and I now have some pretty red wire.

[bottomwatcher]

I love experiments! Nice job! If I have time I always love disecting malfunctioning components. A couple of weeks ago it was the AC compressor in my daily driver. Although the carnage inside didn't instill confidence my filter caught all the metal bits. Anyways nice write upon coils.

[Thomas Perkins]

I did the fuel pump relay not long ago.The copper wires were messed up.I bought a new one.I have pictures of it on my page.Stay away from Oriellys coils.the metal piece where the cable. goes in will rust.I will.one day take them.back and get my money back

 

 

[datzenmike]

Removing coils will lower the resistance but you are also removing that which makes the magnetic field. Slightly larger diameter wire maybe. Same ratio of primary to secondary coils but more power through them.

 

I only have the information for the earlier 'matchbox' module but there is no reason this was significantly changed in the later Z series distributors. The module simply supplies a path to ground for the coil and breaks that connection at the proper signal to make a spark.

 

Power Switching circuit: switches power on and off with input from the Duty Control circuit below.

 

Duty Control : sets the make and break rate and is equivalent to the 'dwell time' on a points distributor. It's shorter at low RPMs when there is more than enough time to charge the coil and lengthens the time at high RPMs.

 

Spark Timing Signal Monitoring circuit: Detects the magnetic signal from rotor/coil sends signal to Duty Control.

 

Preventative circuit: turns off the module if the ignition is left on but the engine not running.

 

Current limiting circuit: controls the power flowing through the coil so it does not exceed a per-determined level.

 

The stock EI does a good job and has built in advantages. Something to think about before replacing with after market systems.

[Stoffregen Motorsports]

I remember when I had time to mess around in the shop...😒

[Foxhound]

I really want to understand this system fully. In another thread I made recently I spoke about breaking the magnet beneath the stator. The crack isn't visible here as it broke very clean right in the middle as I was tightening the two screws that hold the stator and magnet in place. I pushed it back together, screwed it down and it still functions, but I don't know if it functions at 100% and I don't know a way to test it or even what it functions as really.

 

My understanding/guess is that as the reluctor spins and lines up with the stator some type of magnetic signal is sent to the ICM (Ignition Control Module for anyone unfamiliar), with the magnet acting as a sort of magnetic field signal booster. The only physical connection the stator and ICM share is through ground and I doubt the ground is used as communication, but I really don't know. I'm pretty sure the ICM is grounded by the 2 screws holding it in place. When I first pulled it out I originally thought the bottom metal surface of the ICM was for ground and cleaned off what I thought was grease mucking up it's ground connection. I only drove it to the store once like that before learning it's actual use. It's not grease for anyone who does the same, it's thermal paste and the metal bottom is for heat dissipation as far as I know.

 

I can't wrap my head around how the ICM knows when to ground the coils. As the relcutor lines up with the stator is there a magnetic field created that the ICM senses? Is that why the ICM is wrapped around the shaft? Could I mimmick this signal for testing purposes? Why is the reluctor/stator gap important?

 

I'd love to pick one apart, but goddam are they expensive. I've read somewhere that they are just transistors and wondered if they even needed to be inside the distributor housing and if I could make one myself if need be having seen other vehicles use external ICMs..I think. I thought maybe the EBCI letters on the ICM stood for Emmiter, Base, Collector, as is common terminalology for transistors, but now I'm pretty sure it's Exhaust, Battery, Control (to vacuum switch), Intake after looking at the wiring diagram.

 

I filled in the missing wiring colors after the terminal block that go to the ICM. You might also notice that the (R)ed and (W)hite-B(L)ue wires on the terminal block are labeled wrong. These can't be switched on the terminal block because the eye hole connectors are a specific size for each of the 3 bolts.

 

I have my vaccum control wire to the ICM unplugged so both coils are firing regardless for the time being until I figure some other vacuum things out for the weber. I didn't think my ICM was shutting off the exhaust coils under load. Turned out a previous owner had cut the clutch switch wires and spliced them together so the vacuum switch thought the clutch was always depressed and would never send a voltage through the vacuum switch control wire connected to the ICM. I thought it was only one wire capped off for the longest time and that the clutch switch grounded directly to the clutch bracket. Fuckin asshole POs lol. We should have a "POs do the darndest things thread".  I thought the ICM was toast. After fixing that the exhaust side does shutoff now. Good ol multimeter testing at the vacuum switch.

 

I don't see how this ignition system could account for varying charge times of different coils at different rpms, but my understandings of these things is basic. With my cracked magnet I wonder how that would effect the signaling, if at all. I might try to meause my voltage from the coil outputs and spark plug wires at different rpms if my multimeter allows voltages at that range. I've been dying to get an oscilloscope, but they're a little out of my budget right now. Onward with research.

 

 

ICM wiring for anyone who needs to know:

 

(E)xhaust - Blue wire to terminal block then red wire to (-) on exhaust coil

 

(B)attery - Black-white wire to terminal block from brown wire from ignition for direct 12v to ICM as well as 12v to intake coil (+), no fuse

 

(C)ontrol - White wire (looks yellow from age). Comes from vacuum switch (the one with a little black box in the wiring before it)(the other vacuum switch is for fuel shutoff solenoid). Shuts off exhaust coil under load when clutch is not depressed and vehicle not in neutral. If your exhaust coil doesn't shutoff check your neutral, clutch, and vacuum switch before you suspect the ICM. If you pull off the vacuum hose to the vacuum switch it should shut off the exhaust coil if you also have your neutral switch harness disconnected. Its a round connector right behind the battery. That harness also goes to things such as your oil pressure switch and temperature sensor. Disconnected your neutral switch can't ground. If its grounded the vacuum switch signal won't go to the ICM.

 

(I)ntake - Red wire to terminal block and then White-blue wire to the (-) terminal of intake coil. White-blue also goes to the resistor for the tachometer.

 

[datzenmike]

On the dwell time I said it wrong I think. Obviously there is way more time to charge the coil at idle between firings than when revved up. The module does not allow current flow in the coil through most of that extended dwell time. It knows to switch it on just in time to fully charge the coil before discharging it. At 600 RPMs the coil fires 10 times a second or every 1/10 second. At 6000 RPMS that's 100 times or 1/100th of a second. No need for it to be on for 9/10th of a second more than it needs to be on. 

 

Here's another freaky thing. When there are two plugs firing the timing is reduced as the fuel/air is burnt sooner than with a single ignition source. When in single plug mode the module knows to increase the ignition timing from 5 degrees to 10-12* degrees or power would drop off.

 

*The amount isn't published but an L20B timing is 12 degrees so somewhere around this amount. Probably slightly less as this is a hemi head and the plug firing is very close to the center and the flame moves out to the sides more equally where the L20B has to cross from one side to the other.

 

The matchbox module used in the 280zx varies the timing depending on load and RPMs and other sensor inputs to the EFI's ECU.  I took one apart...

 

 

Normal everyday E12-80 matchbox module...

 

 

 

280zx E12-93 matchbox module. Some serious shit in there....

 

Note the lower right side has an I and W input from the ECU

[Foxhound]

1 hour ago, datzenmike said:

Some serious shit in there....

No shit...and thats for a single coil ignition right? I wonder how different these after market ICMs are for the 720s compared to stock. What I would give for some schematics on these things. I wish the service manual actually delved into them. The only mention of them is to replace them 😑. What 40 year old technological secrets are you hiding Nissan?!

 

So what your saying is: If I hook up a timing light, when I turn off the exhaust coil I should see an advance of ~5 degress if the ICM is functioning the way its suppose to?

[datzenmike]

The timing will advance. How much? you find out and tell me, I'd like to know what it is at 700 +- idle speed. Less that 12 could be as low as 8 I really don't know.

 

I have a cheap 1980 Datsun 200sx manual and these are Z20E single plug engines and the timing says 8 degrees, so probably the larger Z24 is similar when in single plug operation.

[Foxhound]

I'll have to repair my tach first before I can tell you what rpms or find a replacement. The wire I need is on the way. 12$ for the wire, if it fixes it, ain't bad. Here's the little winding that was toasted. Theres no continuity at all between the wire ends. That dark patch is all toasted. I blame the POs. Hope I can remember how to put this back together. It's been disassembled for a couple weeks. If your tach isn't working check this little winding. The wire gauge is 40 awg.

 

I got one of those little 10$ digital tachs off amazon that you wrap the wire around either a coil or plug wire, but it seems to be a piece of garbage. You live and learn.

 

Still, I should see /some/ timing change. The ICM in the distibutor might actually be stock. I haven't seen any on rockauto that look like it.

[john t heilman]

last picture of ICM ... yes it is stock. I have the same 4 spade ICM in my 1985 Nissan 720 ST. (Manufactured 2/85) I can not find a wiring diagram for this ICM 4 spade setup. The 3 spade set up is common.  You are correct about the E B C I. My 1985 is carbureted, and came with an ECCU (20 wires) under the driver seat. My truck "was" California ... not any more! ... I pulled everything and anything that was smog off of it including 18 wires out of the compete wiring loom- vacuum lines included. I tapped the Japanese threads to NPT and put in NPT plugs. I put an AFR Gauge on it. I put a 1974 carburetor on it. With out doing anything to the timing or the 1974 carburetor, it fired right up and was in the high 15's - low 16's on the AFR Gauge. Wow! It's got potential! I will need to adjust the jets. The condenser(s) for my truck are 2 x .47 microfarads. (2 coils).

[datzenmike]

The 4th wire is for shutting off the exhaust side coil under load.

 

You mean an '84 carburetor. The Z24 bolt pattern doesn't match earlier patterns. Idle in the 15s won't hurt anything but under load it should drop. Full throttle with secondary open should drop to high 12s low 13s. To go richer increase the secondary jet. I'd leave the primary for good mileage.

[Foxhound]

I think a possible way to tell if you have stock coils is to check the size of the terminals. They are different and the eye hole connectors will only fit on the correct terminals unless someone messed with the wiring. I put the 1.3 ohm coil back in my intake side which I'm pretty sure was stock unless some aftermarket companies do the same with the terminals which I can't know for sure.

[datzenmike]

 1.05- 1.27 ohms for the 720. I would run as low as possible if after market.

[Thomas Perkins]

The stock coils have rubber top dust covers.When I replaced my coils,them rubber dust covers would not fit them.I still have the rubber covers.I have been using Auto Zones coils,never had a problem with them and they have a lifetime warranty.Never use Oriellys  the metal piece inside the top will corrude.Mine did.

 

Edited March 7 by Thomas Perkins