Technical Sideways mounted coil? Measuring Ohms?

"Mustard Top" coils. I saw a picture in an article in Engine Builder or Hot Rod, they had built up this monster engine with all the high dollar stuff sparing no expense, but they also had that good ole stock Ford ignition coil driving the spark. It looked sort of out of place. There's a bit of "snake oil" in the aftermarket ignition parts world. Bubba used to talk about this here, the spark energy required to jump the gap of a plug will only be what is required, and no higher.

The primary ohms thing is getting off into the weeds a bit, that is the low voltage main winding inside, of thick heavy enameled wire.

The secondary winding, made of a gazillion turns of magnet wire is what typically gets damaged, I expect, and here again you'd have to know what the exact ohms spec was to make a meaningful comparison test. Ignition coils (and condensers) also must be tested at normal operating temperature. All it would take is a few burnt turns to mess up the spark when it gets warmed up.

For most people about the best they can do with ordinary test gear like an ohmeter is rule out major defects and perform a Go/No go test or, basically determine that a part "should" work, or "might" be OK.

An ignition scope is a really useful tool, they aren't expensive. Nobody uses them anymore. Allen and Sun of course made the high dollar versions, but plenty of less expensive models were made. Most people probably want the solid state versions, but I eBayed a Heathkit model Ignition scope, a vacuum tube model, for about $40. A scope would show a defective secondary spark output.

You can tell quite a lot looking at the quality of the spark at the coil wire to ground and at a spark plug in the open air. With points ignition it should be a fat blue spark that jumps at least 1/2". Only perform this test long enough to verify spark quality, this is hard on a coil and can cause internal arcing.

But a spark in the open air, or idling in neutral, doesn't tell us what might really be going on inside the cylinder under load, or high speed under compression.

 

Reading the Stromberg instruction manual they are indeed very specific that the coil has to have at least 1.5 ohms resistance. eFire_Owners_Manual_2019_WEB_final.pdf (stromberg-97.com)
The troubleshooting guide also says that the module doesn't handle excessively high voltage if the charging voltage in the rig is high.

 

Yeah, they indicate that less than 1.5 ohms in the primary ignition circuit itself, will roast the module. Just like the Pertronix. And contact points, for that matter. The latter need about 3 ohms.

 

If you're measuring the coil primary resistance with a analog meter, be aware that most inexpensive units aren't really accurate enough or have a large enough 'scale' on the meter face to give a good reading. This is one place where a decent digital meter will do a better job as they usually will resolve down to about .1 ohm.

 

just be sure to touch the two leads together first, to see how much resistance there is at "zero". Sometimes it can be up to half an ohm

 

Exactly, just because they go down to 0.0 ohm in theory unfortunately does not mean they do in reality. I've probably got more digital multimeters than fingers, and only one of them goes down to exactly 0.0 when shorted, most stay at around 0.5, I think one ends up at 1.1 or something like that.

The ohm range on regular multimeters is NOT the tool for measuring such low resistances accurately. I described a 4 wire method on the previous page that gives way higher precision for resistance below a couple of ohms.

 

Yep, that's my choice for a coil. I've never had a bad one either.

Most automotive coils are about 1.5 ohm across the primary winding, varying between 1.3 and 1.7 ohm. There are some so-called '12V' coils that are 3 ohm that don't require a ballast resistor, the VW coils are an example.

The dielectric inside a coil, while there as an insulator, is also the means of cooling the windings as it more evenly distributes the heat preventing hotspots. If you can feel any 'sloshing' it has leaked at least partially out and the coil needs to be replaced.

 

I know it wasn’t mentioned, but the title of the thread seems to be asking about resistance how mounted.
That won’t change in any position.
I’ve seen a comment or two about “burning up the points “ with the wrong coil/ballast resistor, One or two in this thread and many other threads.

Can someone tell me what’s different between a set of points for a 6v ignition vs a 12v ignition?

My feeling is the coil is what is in danger, not the points.

 

Points, now almost exclusively made from tungston, differ from 6V to 12V in part numbers.
No ohm spec here...

 

Yeah, you're probably right.

Points will get nuked for sure though, if they happen to be closed and the ignition switch is left in RUN, with engine stopped. It will of course also ruin the ignition coil. With the engine stopped the current flow in the primary winding approaches 5 amps, not quite double the normal flow.

On the other hand, 1.5 ohm ignition coils aren't bothered a whit in the 12 volt applications when using the electronic modules. Pertronix Ignitor I is rated for about 8 amps continuous, a whole lot more than contact point systems.

That's why the need for (about) 3 ohms total primary circuit resistance in a V8 running points & condenser. If it isn't a factor for the coil, which is it? I will say that a fixed dwell makes for a very hot ignition coil if left to constantly idle or slow speed operation.

 

There is no meaningful difference in the actual points. The difference is the current that's being switched. As most coils had the same primary ohms, a ballast resistor is used on a 12V system to reduce the voltage at the coil from 12 to 6 volts. Without the voltage reduction, the higher current produces a bigger switching arc which erodes the point contact area faster; i.e. 'burns' the points.

There's several other factors that affect this also; how quickly the coil 'saturates', dwell time, operating RPM, and how well the coil 'cools' but these are not something the average guy can measure or fully control. In any event, these are generally disregarded as their effect is a smaller part of the problem. Generally speaking, running a 1.5 ohm coil on straight 12 volts with points (or without dwell control and/or current-limiting circuitry if using electronic ignition) will result in the coil running hotter but you'll probably burn up the points before the coil fails.

 

Suspect it’s a fitment issue. I see 6 volt with 1.5 ballast resistor and 1.5 coil. 6/3 is 2 amps 12 v has 1.5/1.5 would be 4 amps.
Looking at a set of points I can’t see a difference why the pints are at risk with more amps, coil yes I can see, points no. Again, just my opinion.
And as @Truck64 says constant current will cause issues with the points. Funny though, doesn’t really affect a relay with 480 on a continuous run motor until the contact open up.

Sorry for getting side tracked

 

I see what you’re saying, I can understand that, but if a 12 v system is switching 4 amps, and. 6 v system is switching 2 amps…why would a 6v system “burn points” on 4 amps?

 

where do you see that?

 

6 volt systems don't use a ballast resistor; they didn't come into use until the switch to 12V. The ballast resistor 'converts' the 12V system to 6V in 'normal' operation. The only real difference is the OEMs figured out that if they switched the resistor out of the circuit during starting when the battery voltage drops due to the large starter draw, this would 'restore' full voltage to the coil for a hotter spark and easier starting.

Look at it like this.... A 6 volt system when running with a properly operating generator is at nearly 7 volts. But during starting, battery voltage can drop to as low as 4.5 volts which reduces the spark the coil can produce.

A 12V system will have about 14 volts available when running, cut that in half with the ballast resistor and you're back to 7 volts, the same as a 6V ignition. But again, when starting battery voltage drops during starting and voltage to the coil through the resistor can drop to 6 volts or less, reducing spark. Switch the ballast resistor out of the circuit while starting and you now have 11+ plus volts available to the coil while starting. No hard starting issues... and the short time duration does little harm.

 

I'll also note that the OEM engineers can 'tune' the ignition system for spark output/longevity by varying the coil/resistor ohm values. While 1.5 ohms is the 'usual' value used for the coil and resistor, you may find some with say a 1.2 ohm coil and a 1.8 ohm resistor or even the other way around. Or total resistance numbers from as little as 2.5 ohm to over 3 ohms. You'll have to dig into a ignition spec book to find this info but don't obsess over it as it isn't that big of a deal.

 

The Ford shop manual I have here doesn't even mention voltages in that section on coil specs, it talks about current draw.

2.5 amps engine idling, 4.5 amps engine stopped. The ohms resistance of a coil varies a bit due to production tolerances, and the spec for the ballast resistor isn't exactly precision - "1.30 to 1.40 ohms" (at 75°) which is another thing, resistors aren't a constant current device.

 

Coil is mounted sideways on the firewall in the Ranch Wagon for almost 11 years and I haven't had any problems. HRP

 

An oil filled coil will never be totally full. If it was it would likely explode when heated up. Get a coil for a typical 50 Ford. 1.5 ohms.A quality coil can be mounted in any position.

 

The other thing we get into is impedance. An ignition system is really an AC system, it oscillates. The coil has impedance, not just resistance.

It gets pretty complicated to analyze this stuff....

 

UPDATE

So I went to 4 different places and checked multiple coils with my tester, found one that dialed in perfectly at 1.5 ohms and bought it. Swapped it into the car and made no change, if anything, it ran worse.

A friend of mine offered to come and take a look, and I was more than happy since I'm out of stuff to check. He looked it over and listened to it, and asked about trying to re-time it by ear. The timing had been set when it was put together last year, dead center of the adjustment mark on the distributor and hadn't been moved so I didn't see how that would help but fuck it, I'm up for whatever.

Not really knowing which way we were adjusting it we tried (what turned out to be) retarding the spark a notch at a time, which according to Stromberg is 4 degrees of crank rotation. Bad, bad, then worse. Then we tried it in the other direction, advancing it one mark at a time which is apparently 4 degrees. We ended up advancing it 3 marks (12 degrees) and it ran extremely strong! I don't understand how it ran so strong before if it had been off by 12 degrees the whole time, it had never been fiddled with after the initial drive last year. But the extra advance made a hell of a lot of difference.

Would changing the resistance of the coil also alter the built-in advance in this distributor? I don't really understand how that works. It had at one time run really well at the old setting, so I don't understand what changed.

Then on a test drive, the cap popped off and the rotor got messed up. The metal tab on the rotor is just held in place with a screw, and it got clocked in one direction, messing up the little raised plastic slots that keep it pointed straight. I was able to spin it back straight, put it back together, and drive home like that, and it still works, but I'm worried about that rotor moving around again. I ordered 2 more but I don't know how quickly I'll get them. The little clips that hold the cap onto the distributor are very weak and pop off easily, I'm currently using a zip tie to keep them from coming off again. Not impressed with that design.

So basically, adding in a ton of advance made the engine run great although I don't understand what changed that required such a dramatic change in timing. Coil may have been a non-issue the whole time, I don't know. Distributor seems to work great at the moment other than the mystery change in timing and the weak tabs that hold the cap down.

I don't know what to think, I'm thrilled it's running so well currently, I just would like to know why, and I sure hope it stays that way.

 

This is what the rotor looks like. When the cap popped off, it spun on that screw and messed up those little raised ribs. It was easy enough to clock it back straight, and I don't feel like it would move side to side but I still want to replace it anyway.

 

As for why I wanted to use this distributor in the first place, I had gone through multiple sets of new points from multiple sources for the old distributor and everything I could get my hands on was junk, including some old NORS made in USA stuff. Here's some pics of what I was dealing with. If good quality points had been readily available I may never have made this changeover to begin with.



This car has 6V positive ground with fabric covered wiring and even period correct hose clamps, it was kind of a big compromise to swap to a new electric ignition system, but I was trying to get around the lack of quality parts for the old distributor.

 

You experimented...and found a cure. I like that.

 

Years ago when I was student teaching, a guy that was demonstrating the flammability of different substances asked if we could make a sparking device to use in his lectures. My instructor and I discussed what we could to to make an automobile coil perform the task. We had a setup with a signal flasher and a relay with a 12 volt source. It did work, but it was a bit cumbersome. Our electrical instructor said it was possible to connect 110 volts across the primary to get plenty of spark. Inductive reactance should limit current flow in the primary. It did work and the coil put out plenty of spark, but we left it connected too long. It got hot and blew the seal in the secondary tower. We found out what burned transformer oil smelled like.

 

Battery voltage on a 12v battery shouldn't drop below 9v when cranking. If battery voltage drops to 4.5v when cranking, you've got a bad battery.

 

The Stromberg manual does say they come pre-set with 4 degrees initial advance and 22 degrees max mechanical advance at about 2600 rpm. I wonder if the advance in my distributor is not working properly and my manually advancing it just found a happy medium where the engine seems to run decently. If the advance was working last year and has quit working for some reason in the meantime, it would explain why I had to advance the timing so much from where it had been set before. I will have to check this with a light, just to see if it advances at all anymore.

 

Actually you don’t, but what’s the resistance in a 6 volt coil at? I’d think the current would be the same, right? And yes( as you mention later) you’re looking at a rise and fall of voltage acting as AC, similar to a DC oscillator and why the voltage can be transformed (from the expanding and collapsing fields) to the other side of the coil.

 

So is there any “resistance “ is a 6 v coil? I might have one around off a FH, but would take days to find it to check for myself.

 

I'd be surprised if it's much different from a 12v coil. The coils both operate at about the same voltage, once you add the ballast resistor to the 12v system.