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coil pack resistance how far off of 9.2kohms is bad 

 

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 Post subject: coil pack resistance how far off of 9.2kohms is bad
Posted: Sun Mar 01, 2009 12:53 pm 
Tyre Shredder
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basically ive allways had a miss at idle under no load it still slightly misses up to 1500rpm under load as soon as you accelerate it goes away

its had
new spark plugs
new leads
new fuel filter
new K&N air filter
new cat
compression test all at 11-12 bar, took 9ish strokes to get there and held perfectly
today i tested the secondary resistance values on the coil pack
results
1st bank 9.16kohm
2nd bank 8.7kohm
3rd bank 9.16kohm

the book says it should be 9.2kohm
i havnt managed to find anything about how far out is within an aceptable range

is 8.7 too low?
is it fd?
isnt it wierd that only one bank is down?
also if it isnt the coil pack any other ideas is that compression test bad?

Last edited by djcustomcomputing on Mon Mar 02, 2009 6:12 pm, edited 1 time in total.

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 Post subject: Re: coil pack resistance good or bad
Posted: Sun Mar 01, 2009 12:59 pm 
Tyre Shredder
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also are better coil packs available for these cars eg MSD
aprox price/place to get from

and yes i have seen that thread in the 6cyl workshop
i was just wondering wether there was a direct swapover/replacement
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Mar 02, 2009 7:59 pm 
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Tue Mar 03, 2009 4:08 pm 
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Tolerances vary amongst electronic components so its hard to give you an exact figure. For most electronics (resistors)it is 5% but some are even lower than that (1%). If you had a higher resistance I would not be concerned. The fact that you have a lower resistance is a concern. This is usually due to shorted windings in the coil (insulated copper wire breaks down under load from heat and age)reducing the overall resistance of that pack. The other 2 coils look spot on so you know your multimeter is functioning correctly. The lower resistance in the coils secondary winding will mean less induced voltage from the primary and so less spark at the 2 cylinders concerned. This will mean a poor spark on those 2 cylinders and hence ignition of the air/fuel mixture is not as complete. The end result is less power from 2 of your 6 cylinders.

As with all high voltage transformers, once the copper wires varnish coating is ruptured arching internally usually causes the coil to deteriorate over time. Arching causes more breakdown of the insulating coating and more windings to short out. Its a snowball effect.

I can't guarantee this is your problem but it doesn't look good for that coil. Remember coils are wound with a precise number of turns (give or take a turn or 2) of wire so resistance should be spot on.

 

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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Tue Mar 03, 2009 5:07 pm 
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thanks
i guessed as much
im going to look into a newy shortly even if the one i have is ok it couldnt hurt
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 8:05 am 
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Bump

Does anyone else have some insight to this?

My coils read 10.5, 8.5 and 10.5 on the secondary when they should be 9.2 and the primaries are 0.6, 1.0 and 0.9 when the book says they should be 0.6.

Are my coils stuffed or is this much variation ok?

The car started stuttering on acceleration yesterday and I'm trying to find out why.
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 8:35 am 
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Hey there,

The deviation from 9.2 wouldn't be all that bad... Is the deviation from the other two that is the worry...

You haven't provided units of measurement but I'm guessing the secondary is measured in kohms and the primary in ohms?

If so thats 2000 ohms that have just "walked" out of your second coil...

Primaries don't look too bad... .4 of and ohm isn't much difference... Your multimeter leads may have .4ohm in them...

The secondaries are the concern because when the field in the primaries collapses (12V applied across the primary)... This induces current into the secondary, there is a circuit present so potential difference is seen and to the turns ratio from primary to secondary the potential is some ridiculous figure... I've seen some HEI coils rated at 30,000Volts...

The higher the voltage the better (thicker) the insulation needs to be to contain it within the wire... Hence, the secondary coil usually goes... It tracks and bridges out on turn, then two turns and snowballs until it is easier (less resitance) to spark or discharge internally rather than accross the plug gap, or the induced voltage drops and compression in the cyl starts blowing the spark out... Remember you've got one coil dropped from 10.5 to 8.5 turns to volts is directly proportional, and resistance per meter does not change...

so:

(8.5/10.5)*100 = 80.95%

So if coil 1 and 3 had 100% (say 30KV for example, unsure what the coil pack actually is)
Then coil 2 has 80.95% = 24230kv roughly...

Suddenly that spark has lost lots of intensity...

I think that all makes sense?

Now what people don't understand is the reason this happens...

People with coil packs predominantly it happens to... When you get a high resistance joint to the plug, that is, spark plug lead is not clipped on properly, plug gap is too large, lead not clipped to coil properly, lead damaged (broken inside), or a combination of these thing which will lift the resistance... You get to a point where suddenly there is less insulation resistance inside the coil... So half the energy produced may go across the plug and half across the secondary winding... Once its happened once it will continue to happen...

I bet if you check your spark plug leads you will find one thats unusually high resistance, or one thats not clipped on right or has a dirty connection...

Point is if you replace the coil, you will kill the next one too until you rectify the high resistance circuit... Depending on lead cross sectional area, the manufacturer should be able to give and ohms/meter figure to test them against... work out lead length as a fraction of a meter and then that fraction of the given ohms is what they should be...

Just food for thought...

 

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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 9:11 am 
Getting Side Ways
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Wow, thanks for that reply Timmy. Very helpful.

The leads seemed tight. I measured them and all were the same resistance from end to end. I think they were 1.5kohms. They were much less than what the manual suggested they should be.

You were right about the secondary being kohms and the primary being ohms.

Im not sure when the coil pack and leads were last replaced, it was before I bought the car. The plugs are about 400kms old and all looked fine.

I might change them both and see if it helps.
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 12:38 pm 
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djcustomcomputing wrote:
basically ive allways had a miss at idle under no load it still slightly misses up to 1500rpm under load as soon as you accelerate it goes away

its had
new spark plugs
new leads
new fuel filter
new K&N air filter
new cat
compression test all at 11-12 bar, took 9ish strokes to get there and held perfectly
today i tested the secondary resistance values on the coil pack
results
1st bank 9.16kohm
2nd bank 8.7kohm
3rd bank 9.16kohm

the book says it should be 9.2kohm
i havnt managed to find anything about how far out is within an aceptable range

is 8.7 too low?
is it fd?
isnt it wierd that only one bank is down?
also if it isnt the coil pack any other ideas is that compression test bad?


I would be checking your inlet manifold gasket for leaks???

 

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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 6:40 pm 
Getting Side Ways
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Dansedgli wrote:
My coils read 10.5, 8.5 and 10.5 on the secondary when they should be 9.2 and the primaries are 0.6, 1.0 and 0.9 when the book says they should be 0.6.


Just tested the new one.

Got the same values on the primaries but 10.5's all round for the secondaries. Hopefully this new one sorts the issue.
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 10:21 pm 
Getting Side Ways
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Fixed!

Thanks Timmy.
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 Post subject: Re: coil pack resistance how far off of 9.2kohms is bad
Posted: Mon Feb 14, 2011 10:50 pm 
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Good to hear... :D

Like I said that 2000 ohms would start to make a fair whack of difference to the output voltage... Once under compression it blows the spark out... In the same fashion as a turbo does to a motor without the gap closed up a bit...

Dad works for Ford and he said soon as they get the faintest miss you need to swap out the leads or repair the problem because if it's left it will inevitably kill the coil...

Cheers,
Tim

 

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