I was testing the coil because I just changed the plugs and wires and wanted to know if the coil was also in need of replacement .

The Chilton manual says primary resistance should be 0.3 to 1.0 ohms. Secondary resistance should be 6.5 to 11.5k ohms.

My old coil measured 2.4 ohms primary, and 11k ohms secondary. I thought this was indicative of a bad/weak coil, so I ordered a new one. I don't know the age of the old coil, it might be the original.

The new one arrived and I find it has the same resistance readings as my old coil.

I found a similar situation when I changed the spark-plug wires. Chilton spec is 5000 ohms per foot (417/in). The new MC wires are just barely within spec with an average of 380 ohms/in. (My old wires were 260 ohms/in.)

Are the Chilton specs just bogus, or do MC parts just have poor specs ?

 

Maybe your meter is wrong.

 

Resistance checks of coil is usually a waste of time unless the thing is dead open.

I'd blame it on the Chiltons for having fairly useless information in a fairly useless manual.

 

I thought of that too and tested with another meter which produced the same results.

 

I don't have any other manuals. Maybe someone can cite the info from the Haynes manual for these specs.

 

The only way to test coils is with an expensive OBD II scan tool that has live missfire mode 6 capability. They can bench test good and still missfire randomly.

 

Coil resistance is written into manuals by writers who don't know any better.
They are only writers.
A coil is an inductance device that works on magnetic principles and properties around a metal core.
DC resistances have no measurable value to the owner using a multi meter of any kind except an open coil winding.
If the coil is open, you will have the code therefore no reason to do a test for the obvious.
.
A coil with shorted turns still cannot be picked up with a DC resistance test using a multi meter.
The reason is the value is so low you cannot pick the fault out.
A shorted turn or turns affect the magnetic field build up and limits the voltage output from the coil.
Again you cannot measure that effect with a meter.
These checks should not even be put in a manual for the above reasons.
The resistances from production variations are all over the range, so what would you consider a standard?
Good luck.

 

Quick question... Did the truck run better when you plugged the new coil in?

 

I did not install the new coil because it has the same readings as the old coil.

I'm also not going to throw out my Chilton manual, (yet). This resistance test procedure has been around for decades. It's apparently still being taught in mechanical training programs. The impression I'm getting from reading other sources, besides this one, is that the procedure can be used for finding bad coils but passing the test does not mean the coil is good.

So it appears the test can be valid but is not definitive. I know this is contrary to above comments but I'm still researching, don't flame me.

As for the numerical values in Chilton's manual, I have not found any explanation for the discrepancy, probably because of the reasons given above.

 

I'm on the fence here. I have melted two coils in my F150 and it didn't throw a code. Maybe it would if it were open, maybe the misfire detection isn't as good in the V10. For those who may be wondering, I had an intermittent short of the trigger wire from the coil to the PCM causing a very difficult to troubleshoot misfire. It's my own fault, I pretty much made the harness, so I must have messed something up. I ran a new wire to the PCM to solve the issue instead of tearing into the harness.

 

Note that we are talking about a coil pack here - not a coil on plug. The 4.2 V6 uses a single coil pack for all 6 cylinders, it's set up as a waste spark system with 3 coils in the pack.