What are the symptoms of a bad or weak ignition coil?

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What are the symptoms of a bad or weak ignition coil?

Rembrant
Gentlemen,

I'm going to do some engine testing this week but was just curious what are the symptoms of a bad or a weak ignition coil? Mine is the original 1984 coil sitting on top of the intake in the stock location.



I have never had to replace one before so I don't know the symptoms of a bad one. Do they just outright fail and go from 100% to 0%, or do they weaken over time?

If I was to buy a replacement, do I just get a typical aftermarket replacement? Or is there a better one to install as an "upgrade" that will work with a stock DSII ignition system?

1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

Gary Lewis
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Coils can go from 100% to 0% in a heartbeat.  Mine did on my '72 F250.  Turned it off and it then wouldn't start.  Had a broken wire inside.  (Long story.)

But, that was on the primary where there are a few turns of large wire.  If a wire breaks you stop.  On the secondary where there are lots of turns of small wire it is possible for there to be bad insulation that effectively shorts out several of the turns and the voltage output drops.  That gives a weak spark, but a spark nonetheless.

And the symptoms of that are a miss under heavy load.  As the load goes up so does the cylinder pressure, and that increases the difficulty for the spark to jump across the plug's gap.  If the voltage in the secondary of the ignition system isn't high enough, at some load it won't spark.

As for replacement, the DS-II system doesn't like a hotter coil than stock as that usually means more current for the box to sink, which means more heat.  To me, spark is spark.  If you have enough spark to light the mix off a bit more doesn't make much difference.  I'd go back with a stock style coil.  It worked and the box was happy.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: What are the symptoms of a bad or weak ignition coil?

ArdWrknTrk
Administrator
One symptom of a failing coil is extreme heat.
If the internal insulation starts to break down, shorting coils out of the loop, it will saturate much quicker and become a filament.

Missing and exhaust explosions (as opposed to backfire in the air cleaner) are another symptom.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
Thanks Gents,

On the same topic, but different question...with otherwise all stock ignition components, what do you guys recommend for plug wires?

I currently have Ford 8mm wires...nothing special.
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

ArdWrknTrk
Administrator
I'm rocking NAPA Belden premium wires.

You don't have anything insane like a top fuel magneto.
You don't need anything too special.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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Re: What are the symptoms of a bad or weak ignition coil?

Gary Lewis
Administrator
ArdWrknTrk wrote
You don't have anything insane like a top fuel magneto.
You don't need anything too special.
Agree.  The ones you have should be fine, assuming the were knew with the engine.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: What are the symptoms of a bad or weak ignition coil?

grumpin
You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html
Dane
1986 F250HD SC XLT Lariat 4x4 460 C6
1992 Bronco XLT 4x4 351W E4OD
1998 Suburban LT K2500 454 4L80E
2000 Nissan Xterra XE 4WD 3.3 RE4R01A
Oregon
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
grumpin wrote
You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html
Cool. Thank you sir, I'll check it this evening.
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
In reply to this post by grumpin
grumpin wrote
You can check your coil with a multimeter.

https://itstillruns.com/test-ford-ignition-coil-5941137.html
 Looks like my coil is still within spec...

According to the link you posted, a good coil should read between 0.4-2 ohms on the primary side, and between 6,000-15,000 on the output side.

My existing 35 year old Ford coil reads 1.3 ohms and 8,460 ohms.

The new aftermarket one I picked up today reads 3.0 ohms and 11,870 ohms.

Maybe I'll install the new one and keep the old one as a spare. I've been meaning to put together a little behind the seat kit with the old truck essentials...lol.
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

grumpin
Sounds good!

 IIRC the one on my Bronco was like 13K, a little high I thought. Put the new one on and no more cold start problem!
Dane
1986 F250HD SC XLT Lariat 4x4 460 C6
1992 Bronco XLT 4x4 351W E4OD
1998 Suburban LT K2500 454 4L80E
2000 Nissan Xterra XE 4WD 3.3 RE4R01A
Oregon
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
My new coil is clearly oil filled.

Of course there were no instructions with it, but while looking at other coils on Summit, I did see a note saying that oil filled coils can only be mounted in the vertical position. Is this true?

The stock coil location on my '84 302 is horizontal on top of the intake in front of/beside the carb.

Any comments from the experts?
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

ArdWrknTrk
Administrator
Oil filled coils should be mounted vertically for two reasons.

1) it keeps the windings submerged, instead of the whole top side without a cooling transfer medium.

2) if the insulator cracks, or its seal fails, the liquid will remain in the cup instead of leaking everywhere.
***remember this used to be really nasty PCB's***

Epoxy filled coils don't have these shortcomings, and if you want it horizontal perhaps you should find one that is.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
I guess I'll have to look around for an epoxy filled coil....hmmm

Do you know of any brands that are off hand?
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

ArdWrknTrk
Administrator
My Standard Automotive Products coil is direct replacement for the stock DSII coil

It doesn't slosh.
I can try to read the faded number, but I'm pretty sure it will come right up.
It is the lighter blue, of the two they sell.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
OK, some interesting findings, I guess...

I went on Rock Auto and looked at the various coils available for my 1984 F150 w/302 and in the OEM replacement group there were basically two to choose from. Standard Motor Products (The blue one) and United Motor Products (Black, like the original).

I called both manufacturers, and they both told me that their coils were oil filled. The interesting part is, the guy at Standard told me that the oil filled coils cannot be mounted horizontally. They must be mounted in the vertical position. The guy at United said sure they can be mounted horizontally, Ford did it that way for years. NBD.

So, I thought why not, I'll swap in this new oil filled coil, even if only temporary to see if it makes any difference. Low and behold, my original Ford coil is also oil filled (it sloshes a little bit) and IT has been laying on it's side for 35 years...lol.



Interesting I guess, if nothing else.

At the end of the day, the new aftermarket coil is on there, and if it made any difference in performance, it was too small to notice. I may stick the original back on and keep this one for a spare.

I checked my original while still installed (and when hot) and the primary side reading was 3.0 ohms, which is out of spec. The output side was showing approx 9600 ohms, which seems OK. The new coil, cold, was reading 3.0 ohms on the primary, and 11,880 ohms on the output, so not much difference between the two I'm afraid.

I think I'm just skinning my knuckles for the sake of skinning knuckles...
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

Gary Lewis
Administrator
Interesting!  (No, I'm NOT going to quote Arte Johnson.)

Yes, I've seen lots of oil-filled coils on their sides over the years.  Never have seen them leak.

But I think your test proves one thing - you don't have a coil problem.

So, what are you treating the knuckles with?  
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: What are the symptoms of a bad or weak ignition coil?

LARIAT 85
In reply to this post by Rembrant
Ford mounted the [DSII] ignition coil vertically on the back of the air conditioner compressor bracket if it had the 5.8L H.O. engine.  Even though I have a 5.0L H.O.(!) engine, it has the same bracket and the hole for the bolt is already there, so I chose to mount it here instead of horizontally on the intake manifold.  I think it looks better in this location, and is much easier to get to if the need arises:


Lucille:  1985 Ford F150 XLT Lariat

*Colors:  Dark Canyon Red exterior, Canyon Red interior
*Engine: 5.0, CompCams 31-230-3, "Thumper" E7 heads, Edelbrock Performer intake, Autolite 4100 carburetor, DuraSpark II ignition, Thorley Tri-Y headers, Flowmaster dual exhaust, H-pipe.
*Drivetrain:  AOD transmission, 3.55 gears, 2wd.



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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
Rick,

Out of curiosity, what spark plugs are you running in your 302?

I would think our engines are fairly similar. I know you're running Thumper E7 heads and I'm running GT40's, but overall they'd be similar.

I'm running Autolite 764 plugs, and I chose those somewhat randomly by reading online what the guys were putting in the SN95 Cobra cars.
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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Re: What are the symptoms of a bad or weak ignition coil?

LARIAT 85
I am running Motorcraft copper plugs SP415 (ASF32C), with a 0.050" gap.  

This is a colder plug than what the stock 1985 5.0L (2V) used, but is the stock plug for the 1984 - 1987 5.8L 4V H.O. engine.

And these plugs have the same heat range as your Autolite 764.
Lucille:  1985 Ford F150 XLT Lariat

*Colors:  Dark Canyon Red exterior, Canyon Red interior
*Engine: 5.0, CompCams 31-230-3, "Thumper" E7 heads, Edelbrock Performer intake, Autolite 4100 carburetor, DuraSpark II ignition, Thorley Tri-Y headers, Flowmaster dual exhaust, H-pipe.
*Drivetrain:  AOD transmission, 3.55 gears, 2wd.



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Re: What are the symptoms of a bad or weak ignition coil?

Rembrant
In reply to this post by LARIAT 85
LARIAT 85 wrote
Ford mounted the [DSII] ignition coil vertically on the back of the air conditioner compressor bracket if it had the 5.8L H.O. engine.  Even though I have a 5.0L H.O.(!) engine, it has the same bracket and the hole for the bolt is already there, so I chose to mount it here instead of horizontally on the intake manifold.  I think it looks better in this location, and is much easier to get to if the need arises:
I've seen some SBF's with the coils mounted vertically in front of the head(s) if I recall correctly?

Was there a Ford bracket for this install on a Non-AC 302, or was it something homegrown by the owner?
1984 F150 4X2 Flareside. Mild 302 w/GT40 heads, BBK headers, Holley 4bbl, XE256H Comp Cam, M5R2 5spd swap, Magnaflow full duals. Fresh Medium Blue Metallic in August 2019.
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