1. Home
  2. Subaru Cars Forum
  3. Measuring ignition coil resistance

Measuring ignition coil resistance

I posted a few messages recently about a problem I'm having with a '96 Legacy Outback losing power on cold or rainy mornings, and having the check engine light come on indicating misfires on two cylinders. After much troubleshooting, the concensus was that the ignition coil possibly needs replacement.

I bought a new coil pack, but I haven't installed it yet. I thought I'd measure the resistance of the old and new coil packs, according to instructions and specs in my Haynes manual. Both the old and new coil packs are Diamond F-569 12V, and the Haynes manual says the primary resistance should be 0.62 to 0.76 ohms, while the secondary resistance should be 17.9 to 24.5 k-ohms.

On both the old and new coil packs, the secondary resistance is right in the suggested range. Both cylinder pairs on the old pack came out at about 23.24 k-ohms, and on the new pack, the secondary resistance was about 19.34 k-ohms.

However, the primary resistance is outside the Haynes specs on both the old and new coil packs, and this is what's bothering me. On the old coil pack, the primary resistance measures at 0.8 ohms, just above the high end of the range. On the new coil pack, it's much further outside the range, measuring at 1.7 ohms on both sets of pins.

I'd like to find out a little more before I install the new coil pack, because I can still return it for a refund if it hasn't been installed. Could I be measuring the resistance incorrectly? Do these measurements sound normal? Are the Haynes specs wrong? Can someone check their primary resistance on a working coil pack and post the results? (It's a

2-minute check if you have a multimeter and a repair manual.) Any other thoughts are welcome. Thanks!
Reply to
Dave N
Loading thread data ...

This may just be a matter of your meter not being that accurate at very low resistance readings. Typically there will be some resistance in the probes themselves, and/or the meter may not read exactly zero with the probes shorted together. What does it read with nothing between the probes? Also sometimes the meter has to be turned on for a while to stabilize at the proper zero point. It's usually not a factor except for very low ohm readings like this. If you find the meter does not zero out, just note what it does read with no resistance between the probes, and subtract that from the readings you get. I doubt there is anything wrong with the new coil. Have you also examined the old coil for signs of cracking or arcing? It would also be a good idea to replace the wires if you have not already done so, and to at least pull and examine the plugs and replace them if they look worn or fouled.

Reply to
mulder

Thanks Mulder. It's a pretty basic and inexpensive multimeter, but it does seem to zero correctly. When the probes touch nothing, it reads overload. With the probes touching, it registers 00.0 ohms.

Still, the meter could reach the proper zero point and still have trouble reading low ohm resistance. I don't see any obvious signs of trouble with the old coil, but I wouldn't know what problems look like. And yes, a mechanic already replaced all the plugs and wires. My original thread is at

formatting link

I think I'll take the plunge and just install the new coil. If the problem doesn't go away, I'll have ruled out the coil, and it only cost about a hundred bucks. I've already sunk 5 times that into this problem at the mechanic. :(

snipped-for-privacy@x.files wrote in news: snipped-for-privacy@4ax.com:

Reply to
Dave N

Accurate resistance measurements in the 0-2 ohm range are difficult without expensive 4-wire ohmmeters. I'd say don't worry about it.

Reply to
GrumpyOldGeek

Dave, I'm not sure if this applies in your case, but often problems with coils and cam & crank sensors, etc. don't show up until the unit has heated up a bit. Thought I'd throw that in. -Danny

Reply to
Danny Russell

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.