Ignition coil resistance

[NutmegCT]

Earlier I had posted about having "hot coil, ignition loss" problems.

Someone suggested I replace the coil, which may have been failing.

I removed the old Lucas standard coil, and measured resistance across (-) and (+).

Got 8 ohms. I believe it's supposed to be 3.1 to 3.5.

I then measured the brand new Lucas standard coil.

Got 8 ohms.

Figuring my VOM was dying , I measured a resistor in a pack of new resistors.

Resistor rated as 10K ohms.

Measured 10K ohms.

Second one rated as 12K ohms.

Measured 12K ohms.

Am I completely losing my mind? Any ideas why both my old and my new coil measure 8 ohms from minus to plus? Coils are removed from the car.

With either the new or the old coil, the engine starts, idles and runs fine, but under load there's a frequent, random miss. Tried replacing distributor cap, rotor and plug wires. No change. Plugs are gapped at .025.

Thanks.
Tom

 

[NutmegCT]

Sure Tom - here's the solution!

Check the VOM first by shorting out the leads. If the meter doesn't show zero, then you have to subtract what it shows when shorted, from the results when you test the coil.

When shorted, my VOM shows 5 ohms.

So when I test the coil and VOM shows 8 ohms, subtract 5 from 8.

Result: 3 ohms.

Brilliant!

<insert icon of babbling idiot here>

T.
(of course, that doesn't solve the problem of engine missing when under load ...)

 

[Brosky]

Self inflicted wounds are always the worst.....glad you found the issue....sort of

 

[TR3driver]

First thing I thought of was a bad wire somewhere. Especially the little wire that connects the moving point plate to ground inside the distributor. It flexes every time you step on the gas and over time can break internally. Then even if the wire doesn't work, the plate may get grounded through the moving joint anyway, so it's really hard to test. What I do is to undo the mounting screw and check resistance from the wire end to ground, while gently tugging on the wire.

Repeat for the wire that runs from the points to the coil.

Does the problem get worse for more or less load? High/low rpm?

 

[NutmegCT]

Well, looks like I AM losing my mind.

Just for grins, I put the old Lucas coil back in. Kept the new dist cap, rotor, and plug wires in place.

Engine started, idled, and then ran great for a ten mile drive. No missing or bucking whatsoever.

I have no idea why the *new* coil seemed to cause the "missing on load" problem, but putting the old one back in seems to have solved it. I don't have a reliable DVOM, so was using my analog. Maybe using my neighbor's DVOM would show a difference in the coils' resistance values.

Time for a bit of Glen Fiddich.

Onward through the fog.
Tom

 

[Mickey Richaud]

Time for a bit of Glen Fiddich.

Tom

Funny how that seems to make everything right... :wink:

 

[TR3driver]

so was using my analog.

Every analog multimeter I've ever seen has an adjustment for zero ohms. Generally you need to re-zero for each scale (x1, x100, etc).

For example, on this one it's the thumbwheel at the lower left:

Hopefully I'm wrong, but I have a suspicion that your problem will come back eventually. Possibly all swapping the coil did was to cause the broken wire to make contact.

 

[mrv8q]

Time for a bit of Glen Fiddich.
Onward through the fog.
Tom

I find the Glenfiddich tends to start the fog .... But, that's just me ....

 

[dklawson]

When making resistance measurements it is also important to have a good battery in the meter. With digital meters you get a warning that the battery is low typically via a battery icon or a display that simply "acts up". With an analog meter you can easily fool yourself as the meter will typically give good voltage measurements but erroneous resistance measurements. Regardless, the meter needs a good battery for resistance readings.