Author Topic: Spark and coil lead resistance readings  (Read 2302 times)

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Offline Matt G

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Spark and coil lead resistance readings
« on: July 15, 2017, 04:57:17 PM »
Hi guys, Does anyone have a 'typical' and 'bad limit' ohm reading for my spark leads and for my main coil lead (if different). I've had some issue with missing and spluttering when warmed up recently (it's a fight to keep it running at lights and misses badly at loaded and unloaded higher revs). My distributor cap was filthy, and had some moisture residue. It's all cleaned up now, but thought that I'd check the leads for issues while I'm at it.
Any info/experience would be great.

Offline Vettech

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Re: Spark and coil lead resistance readings
« Reply #1 on: July 15, 2017, 06:01:44 PM »
Obviously an older kettering system style car (C1,2,3).

Lots of issues. Are leads solid wire or resistive type. Solid = 0ohms, resistive = very high (meg ohms).

Try running up to temp at night, lift the hood in the dark and check for crossfire, arcing, breakdown.
Will appear as blueish crackles and flashes. Look at plugs if one is lightup or flashing, it maybe damaged internally.

I usually change Plugs, leads, points, cap ........AND the capacitor/condensor in the dissy.

If still faulty, only one thing left, the Coil, now you may also have a dropping resistor, which is by-passed when starting (gives full 12v hotter spark).
Comes into cct to drop volts to coil when running, as coil is designed to run extended periods at lower volts therefore cooler etc etc.

It is not unusual in electrical car ccts for heating to come into play after a time, any component can get hot enough to expand a crack or joint
and give an intermittent HR. This type of fault is the bane of the auto-electrician.

BOB
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Offline Vettech

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Re: Spark and coil lead resistance readings
« Reply #2 on: July 15, 2017, 06:04:25 PM »
Sorry - another thought, is the timing ok?...
Vettech.

Offline Matt G

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Re: Spark and coil lead resistance readings
« Reply #3 on: July 16, 2017, 01:17:58 AM »
Thanks Bob, lots to look into.  Thanks.  Yes C3 (70), but not standard ignition (previous owner's choice).  No points, MSD Ignition (6AL I think - need to check), Accel 8.8 Silicone leads with spiral core.  Accel Coil as well. I had planned to pull leads, to check for changes in idle, etc, which I think would give the same result?  I'm guessing that the blue arc is around the cap join to the spark plug/distributor cap, or do you mean along the lead length as well?  I remember running solid copper cables on Dad's old Datsun 120Y... Made a HUGE difference in power (which still wasn't that much), but it also killed the radio signal :).
I've dropped a cylinder previously via a burnt through spark lead, and it was more of a consistent power down experience.  This feels like multiple cylinders at once (2 to 3), so i went straight for the cap, and figured that I'd work towards the coil and the ignition box.  Timing is a little too advanced, but I don't think that's my problem.  I need to check it again anyway.  I haven't gone for a spin since I cleaned the cap, so maybe I should do that also... ;P
I found an NGK site that indicated between 1 and 6 kilo Ohms.  I checked a couple earlier this morning and got 1.35 and 1.7, so maybe all good...
How do you test a coil?



Offline Bfit

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Re: Spark and coil lead resistance readings
« Reply #4 on: July 16, 2017, 10:15:22 AM »
Go back to basics ,  be prepaired with tools etc take that car out for a drive  when it starts to play up turn engine off and coast to a safe place and pull all the plugs. They will tell you a story .
Bfit

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Offline Vettech

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Re: Spark and coil lead resistance readings
« Reply #5 on: July 16, 2017, 04:29:19 PM »
Well Well Well, yes! not a simple system. Each item can give a lot of problems.

1. With high energy systems crossfire can be a real issue, on cap, plugs, but also where leads cross or touch earths, look for holes/damage.
2. Solid wire are not an issue if,
    a- large earth braid is used to earth the radio case. To a SOLID EARTH thats a clean earth - no paint.
    b- capacitor/condensor is connected near radio - from (radio)  power +12v to earth. (2uf 50volt working or greater)

If miss is at idle, pulling leads is great on 4 and 6's, but harder on 8's, but sometimes works.

Coils -  you may need to defer to the maker, normally low primary and much higher secondary (spark side).

But your electronics could be breaking down, also points - be they Optical or "Hall Effect" (magnetic) could also breakdown.
Had a car once where the reluctor pick up was loose in the dissy.
You can also use a Laser Temp gauge pointed at each Exh port looking for one with a lower temp.(not firing as much).
This only works if the miss is in one cylinder only.


BOB = "Best Of Brittish"
Vettech.

Offline Bfit

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Re: Spark and coil lead resistance readings
« Reply #6 on: July 16, 2017, 06:41:36 PM »
Did I read this car has a 6al that will put out 530 volts and 135mJ of spark energy
I'd be careful  when pulling leads
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Offline Vettech

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Re: Spark and coil lead resistance readings
« Reply #7 on: July 16, 2017, 07:08:47 PM »
Yes Bfit is correct, and of course this is only the Primary side.
However you should not be able to come into contact with this.

BUT!...

The Secondary side is around 45,000Volts and this will be at the plug.

Contact with this could be lethal especially if you have health issues.
The main issue with this type of cct design is that it will continue to pump out the volts and constant current even into your now limp body.

Be wary,
Vettech.

Offline Bfit

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Re: Spark and coil lead resistance readings
« Reply #8 on: July 16, 2017, 08:35:52 PM »
Health problem
Like holding on to the spark plug lead.
And that sucker will jump a long way
 And deliver full voltage for 20 deg crank
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Offline Matt G

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  • Car: 1970 Coupe 350 manual.
Re: Spark and coil lead resistance readings
« Reply #9 on: July 16, 2017, 10:57:16 PM »
Ha, ha. So using my tongue (like on a 9v battery) isn't a good idea.....
Pulled my plugs today, and measured the resistance on all leads. #6 plug was bad, all others were good, which was strange because I'm sure that #2 was less than finger tight (still not sure how, but I'm glad that I found it).   Leads were all good for resistance and in sync with lead length (ranging from 1.15 to 1.97 kOhms) however I did notice that my lead on #6 had 3 different arc points and burning within about 3" of the und of the spark plug boot.  I cut the lead down past the last burn/crack, and re-crimped an end on.
I should now have plugs, leads, and distributor cap all good. Will go for a test run next weekend to see if I need to keep working on options for errors.
Thank again for the feedback!,