testing resistance on pencil coil with multimeter?

Ignore me! I think I have got the secondary as well now by switching to 20k setting.

The first coil has a value of 1.5 on the primary and about 10 on the secondary.

The second coil has a valye of 1.0 on the primary and between 9 and 14 on the secondary.

How do I interpret the values for the primary and how do I know which one is likely to be the failed coil? I have an idea which one it is but from the data how is it possible to know?

from the figures you give (assuming that your secondary figure is in thousands of ohms) they are both giving sensible readings, however coils break down such that their internal insulation becomes poor, so until they are actually running and giving sparks you cannot tell if they are ok without an expensive test rig. The best (easiest, quickest) way of testing a coil is by substitution.

Unless either the primary or secondary has gone open circuit, which is a very rare failure mode, you can't test a coil with a multimeter.

Some cars (VW et al) give a feedback signal to the ECU to "say" they have fired; in the event of a misfire it will be logged. If your car does not do that, and/or you don't have a fault code reader, you need to follow Mr C's advice and do it by trial and error.

Chris

That would only partially test the coil. Sometimes they leak from turn to turn when in use. You really need a dynamic tester.

Another useful test if you have access to a 'scope is to 'ring' the coils. You stuff a square wave in one end of the coil and see if the resultant waveform is damped. If so it indicates shorting turns. A bit technical but if you know an electronics buff they should be able to do it for you.

Indeed so. Several times I've had coils which tested ok on a DVM but were below par or non-working in situ. About the only thing a DVM can tell you is continuity or the basic ohmage is close to what it should be (which can be useful in itself).

John expressed precisely :

Your meter is not adequate to the task, to measure sub one Ohm resistances you would need a m-Ohm meter, which are quite expensive.

There is also not way for you to check if the coil's insulation is breaking down under voltage.

Really the only sensible way to test a coil, is by substitution with a known good one.

From your experience, do these coils sometimes fails after a run i.e. @ 50 miles ? I ask, because my Rover has twice, but some months apart, started misfiring but it has soon cleared itself.

I popped in two second-hand coils at the weekend and it began doing somewhat similar misfiring every time I tried to pull away. I'm wondering if mine are somewhat marginal !

I'm reluctant to buy two new ones and it not be the problem. Would any garage be able to test them independently?

Cheers

Andy Cap laid this down on his screen :

They can fail when they get hot, yes.

They could be marginal, but it is unlikely that all would be marginal. I would look elsesware if all coils exhibit the same fault - Check plugs, HT leads and distributor cap.

The normal test method is substitution with known good components. If a cylinder is missing, try unplugging the HT from one cylinder at a time, until the misfire doesn't get worse by unplugging - that then is the cylinder with a misfire.

I would suspect the TDC sensor before the coils (assuming leads and plugs are good)