Ignition coil
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
0.2 ohms is way too small. With a resistance that low, most of the voltage is going to drop across the resistance wire, giving you a smaller primary voltage, which leads to a smaller secondary voltage and hence a very weak spark. Additionally, with a resistance that small, the coil will draw more current than it should and will run hot. Your coil is close to failure and should be replaced. Your primary resistance should be around 1.0 ohms, not much more, not much less.
Thread Starter
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Tuned
Joined: Jul 2001
Posts: 432
Likes: 38
ignition coil
This is a brand new coil. I tested two in the store before I left with my purchase and both tested at .2 ohms, also. My old coil tested at .2 ohms before it was replaced. I replaced it because it was 33 years old, and I was replacing the connector cap so decided to go with a new coil.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
With only 0.2 ohms on the primary winding, the voltage divider created between the coil primary and the resistance wire upstream in the harness will only leave the coil primary with around 20% of the voltage. That's too small. Furthermore, every spec chart I have ever read specifies around 1.0 ohm for the coil primary resistance. A 33 year old coil, or coils picked up from the parts house (that are most likely foreign made) do not set the bar, nor do they trump solid circuit theory.
Please see the following:
http://econtent.autozone.com:24991/z...5280053a9a.gif
Please see the following:
http://econtent.autozone.com:24991/z...5280053a9a.gif
Elder User
Joined: Sep 2008
Posts: 563
Likes: 0
From: DeKalb, Ill.
I don't want to hijack this thread, but what contacts are used for this test? I'm going to replace my cap and rotor soon, but I never would have thought to test for ohms at the coil.
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Secondary winding: Probe between large center terminal and BATT terminal on the coil (separate test, see this link: http://econtent.autozone.com:24991/z...5280053a97.gif)
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Postmaster
Joined: Aug 2005
Posts: 2,760
Likes: 8
From: Gadsden Purchase
At this point it sounds as if his meter ain't reading right. :/
If it's an analog you blew on the face of it, right? ;)
Huffing and blowing on my meters' faces is automatic for me.
Glass, plastic either can build static and bogus readings can send you
looking in the wrong direction when hunting trouble.
Murph, now that you are thinking;) read some voltages and resistance of
stuff... like the stator coil and while you have the wires off the cap read
the ignition wires, even new ones can have an open.
Didn't know off hand the spec was 1 ohm for the coil, my post came from
my notes written in my shop manual. :)
IMO that's a cool thing about a book vs a CD manual.
Spec or no spec if my coil's resistance changes, it's out of there. ;)
Alvin in AZ
If it's an analog you blew on the face of it, right? ;)
Huffing and blowing on my meters' faces is automatic for me.
Glass, plastic either can build static and bogus readings can send you
looking in the wrong direction when hunting trouble.
Murph, now that you are thinking;) read some voltages and resistance of
stuff... like the stator coil and while you have the wires off the cap read
the ignition wires, even new ones can have an open.
Didn't know off hand the spec was 1 ohm for the coil, my post came from
my notes written in my shop manual. :)
IMO that's a cool thing about a book vs a CD manual.
Spec or no spec if my coil's resistance changes, it's out of there. ;)
Alvin in AZ
Elder User
Joined: Sep 2008
Posts: 563
Likes: 0
From: DeKalb, Ill.
Oh yea, just want to clear up one more thing (for all of us dummies). Do these numbers apply to all engines? I reviewed this thread and noticed it didn't seem to be specific to any one engine. Should I use these guidelines for testing my 300?
Thread Starter
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Tuned
Joined: Jul 2001
Posts: 432
Likes: 38
I'm using a digital meter that I have no reason to believe is inaccurate. I've also checked the resistance with an old Sears engine analyzer that I used back when one of my cars had a points-type ignition. It read .2 ohms also. I'll try a different parts place tomorrow and have them test one of theirs. This is becoming a good thread; I appreciate everyone's input.