Coil pack primary resistance checks: replace?

[gathermewool]

I just checked my primary coil resistance against the acceptable range in the Haynes Manual.

I read 1.0 ohm on 1-2 and 3-4 coils, while Haynes states a range of .68-.72 IIRC (Manual is not with me at the moment.) Should a defective coil show drastically out of range number or does this small of an increase indicate replacement?

thanks!

 

[jmac]

This shows a degrading coil or your ohm-meter is not that good at low resistance measurements.

If over 50,000 miles then the likelihood is on the coil, but I have seen both.

 

[gathermewool]

Using Equus 3310 multimeter

Car has 197kmi on'er now - bought with 173kmi. I have no idea if the previous owner replaced the coil, and I'd rather not spend money on something for old bessy unless it's absolutely necessary. I'm getting the misfire VERY infrequently, but have initial on-throttle stumbling occasionally.

I've had the p0400 -EGR Malfunction- for 15kmi now. The stumbling is more recent.

 

[PandaBear]

When my coil failed, the resistance go from 1 ohm to about 20ohm. a 0.68-0.72 to 1.0 is most likely the precision of your meter, because that's still darn low.

p.s. what is the temperature you measure at? At only 1 ohm, temperature could cause a change in the resistance. I think it is still good and you probably shouldn't worry about it.

Actually, come to think of it, the correct way to measure the life of the coil should be its inductance rather than resistance, but most people don't have the equipment to measure it.

 

[gathermewool]

I checked the coil after the car had been sitting for 4-5 hrs, so no temp effects. I'm guessing that the EGR issues is related some how, but explaining the more recent misfires is tough. : /

The car runs great most of the time so, for now, this is more of a challenge to see if I can trouble shoot and fix this with out spending any money (or as little as possible.) I didn't know how old the plugs were, and one of the wires came apart while I tried to remove it so I didn't remind replacing those, but I'd rather not replace the coil if it seems fine. I'll at least hold off until after I figure out the EGR issue.

 

[oilyriser]

Touch the probes of the meter to the same coil pack terminal to read the measurement error, and correct for that.

 

[Axe Man]

Originally Posted By: oilyriser
Touch the probes of the meter to the same coil pack terminal to read the measurement error, and correct for that.


Correct. When dealing with resistance measurements of approx 5 ohms of less you must take into account the resistance of the meter leads themselves which can vary between .1 to .4 ohms.

Short your meter leads together and measure their resistance. You'll probably see a reading of .1 to .3 ohms. Subtract that amount from the reading you get when measuring the coil primary and you'll most likely be within the spec.

Measuring the DC resistance of a coil is useful only as a relative indicator of it's condition. For example if the coil is open it will measure infinite resistance and you'll definitely have a misfire. As stated inductance and impedance measurements are more accurate at testing a coil's condition but are usually not needed.

I'd say to look else where for your problem.

 

[Axe Man]

Some other thoughts. Go after the P0400 EGR code first and fix that. That could be the source of the stumble.

Did the broken plug wire correspond to the misfire code? For example: was the broken plug wire on cylinder 1 and did you get a P0301 misfire code? If it did you most likely fixed the intermittent misfire.

 

[mechtech2]

Also, temperature can affect coil Ohm readings.
Stone cold vs. warm/hot will show a chahnge, for sure.

 

[gathermewool]

P0302 code. Wire was on cylinder 4. All of the plugs looked ok, but the gap on #2 seemed larger than the other 3. Unfortunately I didn't mark them as I pulled them or else I would take pics and show you guys. No misfire since replacing the plugs and wires.

As far as the P0400 code goes the system seems to work just fine. I've been chasing this thing down for over a year now and have done every single check, and it all works just fine. The only thing I didn't do was to clean out the metal tubing and replace the rubber tubing from the Xducer to the exhaust-side of the EGR valve. I did this yesterday. Of all the things I checked I'm not sure why I didn't do this originally : /

Readiness status still 'not ready.' I'll reply back if the code clears and the hesitation goes away. I'll be really relieved - and a little mad at myself - if something so simple as this little bit of tubing has been causing so much trouble haha.

I'm going to go drive to the beach now JUST to get this readiness checks over with. ; )

 

[gathermewool]

.2 ohm for sense lead resistance. Still out of spec, but not by much.

 

[javacontour]

Go measure a new one for comparison. If it's similar to the one you are testing, then it's unlikely that's the issue, I.E. you may be experiencing the limits of your multimeter.