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Ignition Coil Diagnosis

My 1976 Lincoln Mark IV died while running recently. I replaced the fuel pump a little while back and it ran great after replacing that, but now it won’t turn over again. My neighbor who is a retired mechanic suggested I check the ignition system, since the fuel system is probably in decent shape.

While checking the resistance on the ignition coil, the secondary resistance seems to fall in spec reliably, but the primary resistance seems to vary wildly. It will register in spec for 5-10 seconds at a time and then the resistance will shoot up to 40 ohms+ and then jump around and it usually returns to spec. This seems to happen more often if the coil is titled while measuring, but that could be a fluke.

Does this suggest the coil should be replaced?

measuring resistance, you have to disconnect at least one of the connections to the coil.

If you did that, and you are sure the ohmmeter is making a good connection, then try tapping or moving the coil. If it changes, than thee is an internal short or open.

I have it completely disconnected. When I move it (while keeping the probes of the ohmmeter firmly attached), the resistance jumps.

Sounds like you’re doing a proper job testing it

I would say it failed the primary resistance check

Normally, I would say if a coil is just marginally out of spec . . . for example, primary resistance is supposed to 40 ohms, but you reliably measure 41 ohms, each and every time . . . the coil MAY be just fine

But since you can’t even get a steady reading, that suggests a bad coil

When the resistance “jumps around” does your meter read “0.L” . . . ?

If so, that essentially means open circuit

Do you have a spark tester?

Get one at a parts store if you don’t . . . they’re very inexpensive, and this won’t be the last time you use it

I’m not familiar with your car . . . is the coil a canister type, or is it an e-coil?

If the coil has any kind of cracks, or if the coil is leaking any oil, it’s definitely bad

I’ve seen quite a few coils that ohm out just fine, but they do have those other flaws as I described. That’s the kiss of death, as far as I’m concerned

@db4690 when I say jumps around, it seems to go all over from 0 to OL. My ohmmeter is one that is part of a relatively cheap multimeter, so it isn’t super reliable, but I get steady readings on other things that I have tested (including the secondary resistance). It is auto-ranging as well, so something that can make the numbers look bouncier than they actually are.

I was considering getting a spark tester, but I have not done that yet. It is a canister type (I think. I’m still pretty new to this stuff. This is what RockAuto says replaces it:

I don’t see any obvious oil or damage, but the whole engine area is pretty dirty, so it’s got some gunk on it.

It can be difficult to get accurate readings measuring low ohm resistance. As I recall a coil on an engine like that would measure a little less than 10,000 ohms or so on the secondary, and 2 or 3 ohms on the primary, right? So first make sure when you touch the two DVM probes together you get zero ohms or maybe 0.1 - 0.2 ohms. If it doesn’t always show in that low range, but instead jumps around, sometimes to 40 ohms, then you know you got a meter problem to solve before you can test the primary on the coil. This is a common thing with very inexpensive DVM’s by the way. The connections inside the unit can begin to develop high resistance and that adversely affects low ohm measurement attempts.

If touching the two probes together yield consistent low ohm readings in the 0.1 to 0.2 range, but you get inconsistent readings on the coil primary, make sure the coil contacts you are probing are completely clean of any oil film, and have no oxidation on them. Suggest to use a little 220 grit sandpaper on them to remove any surface oxidation. If after doing all that you still get inconsistent readings, from 0.1 ohm jumping to 40 ohms, then you probably need a new coil. You could try cooling the coil in the freezer, then heating it up slowly with a hair dryer and see how that affects the readings. If it seems to work only at certain temperatures, that’s a common failure mode w/coils.

@GeorgeSanJose It does read 0 when I touch the probes together. It supposedly “auto-calibrates” when touching the probes together, but it seems to be fairly accurate based on measuring known resistors.

According to the Haynes manual and what I could find online, the primary is supposed to be 1 to 2 ohms, and the secondary is supposed to be 7k to 13k.

I cleaned off the contacts on the coil with a metal wire brush after taking the initial measurements to make sure it wasn’t anything on the contacts. They appear to be fairly clean after using the brush. I still got similar results.

I think you’ve probably got a bad coil then. Try knocking on the coil with the plastic handle of a screwdriver maybe, while making the primary measurement. Does that affect the dvm reading?

btw, the way I do a spark test – I have no fancy spark tester – is I pull a wire from an installed plug, connect a spare spark plug to the wire, then hold the spark plug against an engine ground. Ask a helper to crank the engine while you watch for spark at the tip of the spark plug. You should see a nice healthy spark every other crank revolution. Be aware that the engine might start while doing this. If you don’t see a spark, that doesn’t necessarily imply the coil is the problem, but with your experiments to do, that would be my first guess.

Based on everything you’ve said so far, it sounds like your coil definitely tests bad. It’s resistance is way out of spec on the primary side

I still urge you to test it one more time with that spark tester I mentioned, before condemning it and replacing it

And it sounds like your meter is not the problem, because you get steady readings when measuring those known resistors

Sounds like a bad coil. One more point about ohmmeter measurements, just to be complete.

The pointed probes that come with the meter are not very good for low ohms measurements, as it takes a steady pressure on the points to get a good contact… There is no way you can maintain that while moving or tapping the coil.

I always switch to “clip lead” type contacts. That is, an alligator type clip.

An ohm meter is never going to tell you if your coil is bad or not. It only applies a few volts to measure resistance. To test a coil, you need a megohm meter which can apply 5 to 10k volts to measure insulation breakdown.

I’d just buy a new coil and if it doesn’t solve the problem, then keep the old one as a spare. But first I’d make sure that the problem is with the spark. Remove a spark plug and plug it into the spark plug wire. Lay the plug on top of the engine on a metal part and then have someone crank the engine to see if there is a spark at the tip.

If there is not a spark at the tip, than inspect the distributor cap and the rotor. With the coil in place, check for voltage at the + and - small terminals on the coil while someone is cranking the engine.

I wouldn’t go quite that far. It’s true that a coil can’t be proved definitely good with an ohm meter, but it can sometimes be proved definitely bad with one, for example if the ohm meter shows an open circuit on either the primary or secondary. Concur however that best next step for the OP is to test w/ a new coil.