WpgDSMer

So does anyone know what a good measurement is for our ignition coils? I thought I had some bad readings, so I ordered a new set of coils. I measured them, and they might be similar readings to my used ones (I'll check once I swap them out).

I did find a very strange thing thou. The readings are different depending on the resistance measurement mode I use on my multimeter. The 2000 ohm (diode) measurement setting gives very different results. If anyone understands electronics, and/or ignition coils better they can explain it.

For reference, I'm including a picture of the meter I used (a cheap one, but seems to read fairly well on other things) and the readings of all four new coils. I did both forward and reverse readings on the terminals, the first letter I list is the terminal which the positive lead was attached to. For the readings in Mohm, the value was steadily dropping, after a few seconds it would drop very slowly and that was the reading I took. All other values were stable. Unless it was the 2000ohm diode setting, all other modes would read the same value. The mode listed provided the most accurate reading.

A-B (20K res) : 1.54Kohm, 1.42Kohm, 1.38Kohm, 1.56Kohm
A-B (2000 diode) : 745ohm, 713ohm, 701ohm, 755 ohm
B-C (20M res) : 16.2Mohm, 18.6Mohm, 18.2Mohm, 16.5Mohm
B-C (2000 diode) : 487ohm, 495ohm, 495ohm, 494ohm
C-A (20M res) : inf, inf, inf, inf
C-A (2000 diode) : inf, inf, inf, inf

reverse readings
B-A (20K res) : 1.57Kohm, 1.43Kohm, 1.39Kohm, 1.57Kohm
B-A (2000 diode) : 577ohm, 575ohm, 575ohm, 573ohm
C-B (20M res) : inf, inf, inf, inf
C-B (2000 diode) : inf, inf, inf, inf
A-C (20M res) : 14.6Mohm, 17.6Mohm, 17.2Mohm, 15.6Mohm
A-C (2000 diode) : 572ohm, 570ohm, 565ohm, 570ohm

WpgDSMer

Well, it looks like my coils were probably fine. These are the measurements taken from my old (I believe original, 45k mile) coils.

A-B (20K res) : 1.71Kohm, 1.71Kohm, 1.73Kohm, 1.75Kohm
A-B (2000 diode) : 788ohm, 810ohm, 799ohm, 805 ohm
B-C (20M res) : 15.5Mohm, 15.1Mohm, 15.2Mohm, 14.9Mohm
B-C (2000 diode) : 486ohm, 486ohm, 488ohm, 485ohm
C-A (20M res) : inf, inf, inf, inf
C-A (2000 diode) : inf, inf, inf, inf

reverse readings
B-A (20K res) : 1.72Kohm, 1.72Kohm, 1.74Kohm, 1.76Kohm
B-A (2000 diode) : 579ohm, 577ohm, 574ohm, 573ohm
C-B (20M res) : inf, inf, inf, inf
C-B (2000 diode) : inf, inf, inf, inf
A-C (20M res) : 14.6Mohm, 14.5Mohm, 14.6Mohm, 14.1Mohm
A-C (2000 diode) : 582ohm, 583ohm, 578ohm, 581ohm

These look about the same as the new ones. So, when people say that their readings show the coils are going, what are they measuring and what values are they seeing?

dannobre

iTrader: (3)

They don't tend to show anything till they quit working

wcs

iTrader: (2)

@ dannobre ... looks like I'm following you around lol


Service Bulletin for Ignition Coil inspection
http://www.finishlineperformance.com...16-07-1793.pdf

My quick review of it, didn't mention anything about measuring the ohms ...

WpgDSMer

Section C talks about inspecting the coils. Step 2 is to measure the resistance between each terminal. Regular resistance isn't directional (doesn't matter which terminal the +ve lead is connected to), so I'm not sure if they are implying the diode test.

wcs

iTrader: (2)

Try pm'n Jedi54 or Jon316G
Those guys seem to have some skillz at the old coil inspection deal.

Or Charles R Hill ...... he's my go to guy

dannobre

iTrader: (3)

I've had coils that spec out perfect..and have misfires and don't work properly...

If you have problems that you think are coils and they are 30K miles or so...just replace them and be done with it....

WpgDSMer

So it looks like it probably wasn't the coils. I took it in to the dealer, and after a decarbonization and some other attemped fixes, nothing changed. A compression test revealed low numbers across the board.
It took a while for them to determine if the engine would be replaced under warranty, but they eventually decided that it would be, and it's currently being swapped. I had to supply proof of service, even for the period before I owned the car. Luckily the previous owner did their service at a Mazda dealer so I was able to obtain the records. Not sure what would have happened otherwise.