Measuring ignition coil resistance

[Dave N]

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!

 

[mulder]

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!

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.

 

[Dave N]

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 http://groups-
beta.google.com/group/alt.autos.subaru/browse_thread/thread/6ab1b1c754741be
8.

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.

 

[GrumpyOldGeek]

Dave N wrote:

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.

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

 

[Danny Russell]

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!

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