[max 93]

I was testing the resistance on my coil to make sure it was compatible with the Pro Flo 4 that I’m installing. The ohm reading was jumping all over from 5 to 120 ohms but a few times it would just read between .9 and 1.2. It is rated at .6 ohms. The car ran fine with it before. I just don’t want to damage the Pro Flo. It it normal to get readings like this, or should I replace it?

[alleyCat]

How are you testing it ? All wires off the terminals ?

[max 93]

The coil is not connected to anything. It’s sitting on the workbench. I used two different ohm meters and they do the same thing

[Shiny]

Definitely not normal to have a reading jumping all over but may help to first clarify your goal.

When you say "verify compatibility", is there a statement from Pro-Flo about your specific coil or about generic coil characteristics? It's not clear if Pro-Flo is telling you to verify your coil resistance is 0.6 ohms or if you know your coil should be in that range and are trying to make sure it is healthy.

The jumping readings may just be measurement technique. What type of coil are you using? How are the connections made to the coil when installed in the car? How are you connecting your meter leads to the coil contacts?

Before you reach any conclusions about the source of the described variation in resistance, gain confidence in your DC resistance measurement. Here's what I'd do:

- short the two test leads together and wiggle the wires. If the resistance is stable, you can eliminate the leads and meter connections as a source of instability. if you can do this with both of your meters and get the same stability, you gain even more confidence.

- if stable, note the resistance reading when your leads are shorted. This will include both the lead-wire resistance and the "contact resistance" at the 3 contact points (both leads-to-meter and leads-to-leads). This resistance will give you an idea of potential for inaccuracy when you measure the coil. 0.6 ohms is not easy to accurately measure with an ohm-meter and the contact resistance will always be included and will add resistance.. but I'm talking <0.5 ohms, not tens of ohms

- find a way to securely clamp your test leads to the coil terminals... screw clamps, alligator clips, connectors, etc. Soldering would be even better if you can swing that without damaging the connectors or leaving solder where it doesn't belong

If you are confident in the stability of your test leads, if you assure good mechanical clamping to the coil terminals and still see jumping readings, I'd be suspect of the coil... but that's a LOT of IFs

[Schurkey]

Resistance testing of ignition coils also includes testing the secondary winding. Some coils would be tested from either of the two primary terminals, to the secondary output (coil wire) terminal. Other coils would have an output terminal, and a "ground wire" that comprise the two ends of the winding.

Either way, the secondary resistance will be far higher than the primary.

Lastly, remember that an ohmmeter can tell you if an ignition coil is faulty. It can't confirm that a coil is GOOD. That's why you also need to verify spark output using an oscilloscope and pickup; or (way less expensive) a commercially-available spark tester calibrated for ballast-resistor or non-ballast-resistor style ignitions, whichever matches your system.
Photo 1. HEI and Conventional spark testers. http://hbassociates.us/HEIsparktesters.JPG


Photo 2. Extended (recessed electrode) gap of HEI tester vs. Conventional tester. http://hbassociates.us/HEIsparktestergaps.JPG


For the record, I was having significant trouble getting stable ohm readings when checking DC resistance of some speaker driver voice-coils. Discovered that my multimeter's test leads had broken conductors under the insulation. New test leads fixed what I had thought was a faulty meter.

[max 93]

Pro flo does require a coil with at least.6 ohms primary resistance. It is a Pertronix coil. For some reason, when I test it today, it is not jumping around and is testing fine

[Shiny]

Good deal. Sounds like a better day!

[max 93]

I still would like to know what caused the erratic measurement. Nothing changed except for it was laying on it side for a while before the first test and it sat upright until the next test. Being an oil filled coil, could that cause the doin the readings?

[Bermuda Blue]

If the meter is a digital auto-ranging meter, large capacitive or inductive loads can make the meter change ranges with a subsequent change in the display. If its a digital meter, lock it in a range that which will display the expected value before connecting it to the device to be measured.

[Schurkey]

Originally Posted by max 93

I still would like to know what caused the erratic measurement. Nothing changed except for it was laying on it side for a while before the first test and it sat upright until the next test. Being an oil filled coil, could that cause the doin the readings?

No. The oil is non-conductive, There'd be only the tiniest air bubble inside--if there even is an air bubble.

And those oil-filled coils work just fine right-side-up, leaning over, on their side, or even upside-down, despite folks saying the just have to be completely vertical.

[Ben M.]

Originally Posted by Bermuda Blue

If the meter is a digital auto-ranging meter, large capacitive or inductive loads can make the meter change ranges with a subsequent change in the display. If its a digital meter, lock it in a range that which will display the expected value before connecting it to the device to be measured.

With a digital meter this is what I would suspect as the cause, especially if the probes are dirty or not fully clamped on to what they are measuring.

And as Schurkey says, a meter will only show you a hard short or broken wire, it won't tell you if the coil is actually any good or not. But honestly, in all the years I've driven points cars I have yet to have a coil fail that wasn't because someone dropped\damaged it and cracked the housing, causing the oil to leak out. A stock AC-Delco coil or other similar non-"performance" coil should work just fine for your application. If it was running without issue before chances are it'll be fine now for the Pro-flo.

[Shiny]

Originally Posted by Bermuda Blue

If the meter is a digital auto-ranging meter, large capacitive or inductive loads can make the meter change ranges with a subsequent change in the display. If its a digital meter, lock it in a range that which will display the expected value before connecting it to the device to be measured.

Good thought. Wouldn't you expect this to be a "transient" reading that would settle out vs the original comment of "jumping around"?

No reason I can think of for orientation to matter unless there was a loose connection inside the coil that was moving around. This seems highly unlikely if the stable reading was near 1 ohm.

[Lee]

Originally Posted by Schurkey

No. The oil is non-conductive, There'd be only the tiniest air bubble inside--if there even is an air bubble.

And those oil-filled coils work just fine right-side-up, leaning over, on their side, or even upside-down, despite folks saying the just have to be completely vertical.

I always installed them vertically, because I've seen the oil leak out of coils that were not vertical. They can work in any position, unless the oil runs out.

[Schurkey]

The oil-filled coils should be sealed. If they leak...something's broken.

[Will]

the oil is what provides the insulation between the windings. If there's an air bubble in there you lose your insulation and get a short between the windings. So, the coils should be completely filled and sealed. Can't imagine them being made any other way...

[Ben M.]

Originally Posted by Will

So, the coils should be completely filled and sealed.

Filled to a point. The oil heats up and expands, it has to have some room to expand to as the windings and coil body heat up.

[Schurkey]

Originally Posted by Will

the oil is what provides the insulation between the windings. If there's an air bubble in there you lose your insulation and get a short between the windings.

No. The windings are varnished for insulation or they'd short-circuit. They're wound tight--oil wouldn't surround each wrap of wire.

The oil is non-conductive, but it's primarily there to move heat from the windings to the steel case.

[Greg Reid]

Another thing that can cause flaky ohm readings at higher resistances is putting your fingers on the leads to hold them in place. Easy to do when trying to hold something steady while checking, like an unstable device lying on a workbench.

[Will]

Originally Posted by Schurkey

No. The windings are varnished for insulation or they'd short-circuit. They're wound tight--oil wouldn't surround each wrap of wire.

The oil is non-conductive, but it's primarily there to move heat from the windings to the steel case.

Okay, makes sense. I was just doing some maintenance on a medium voltage to 480v transformer that's filled with oil where the oil does work as an insulator as well as cooling. On those it's very important to not let any air get sucked in at the sample port which is down at the bottom of the transformer. I just kinda figured that coil works the same way, but it would probably be three times the size if it did...lol.

[Shiny]

Originally Posted by Will

Okay, makes sense. I was just doing some maintenance on a medium voltage to 480v transformer that's filled with oil where the oil does work as an insulator as well as cooling. On those it's very important to not let any air get sucked in at the sample port which is down at the bottom of the transformer. I just kinda figured that coil works the same way, but it would probably be three times the size if it did...lol.

Just speculating but I suspect the oil (or resin today) in an ignition coil is also an important insulator. The varnish on the windings insulates wire-to-wire but on the high voltage side the oil may be needed to prevent arcing internally. The voltage difference between adjacent windings will be too low to break down the insulation but the voltage between any wire and ground (or to the low-voltage windings) will be high. Obviously high enough to ionize air and arc across a large gap. I think oil would insulate much better than air inside the coil.

The varnish on magnet wires can degrade from heat and time. When the varnish fails a short can happen between windings. This short can be very hard to detect with an ohm-meter. An open will be easy to detect, but detecting a short depends on where in the length of the winding it occurs. This is why Schurkey commented a good resistance reading doesn't guarantee a good coil.