Testing an Ignition Coil

The Ignition Coil is the unit that generates the sparks to be sent to the plugs. They are quite robust units and rarely cause trouble, but if consistent rough running presents itself, and all other diagnostic tests have revealed nothing, the coil should be suspect. The following test will determine whether or not it is working correctly.




Before carrying out any work on the ignition coil, it is essential that the car battery be disconnected and the coil unit removed from the car to be tested separately.

1
Using a multimeter, first test the primary circuit by placing the meter across the two terminals of the primary coil. A reading of between 0.75 and 0.81 ohms (Ω) should be given.

2
Next test the secondary circuit by placing the meter between the positive terminal of the primary circuit and the central terminal of the secondary circuit. A reading of between 10,000 and 11,000 ohms (10 - 11k) should be given.

Should either of the readings fail to fall within these tolerances, the coil is defective and should be replaced with a new unit. Consult the vehicle owners handbook to ensure that the correct type of coil is ordered. A component removed from a car is not necessarily a reflection of the component that should have been fitted originally.

Diagnostics
Beyond ascertaining the working order of the coil, there is very little in the way of diagnostic testing that can reveal the cause of its failure.
If it is obvious that the coil is many years old (or even an original component), the main culprit in its failure is clearly age. Provided the low-tension circuit is thoroughly tested (or ideally replaced), the classic owner can be reasonably confident that the root cause of the problem has been cured in the replacement of the coil.

If age is not the issue, the entire ignition system should be checked thoroughly. Begin by checking the details of the coil against the factory recommendations: ignition coils are commonly available in both 6v and 12v forms, with or without the need for a ballast resistor. If the details do not match those of the handbook, replace with the correct type.
If the correct coil is fitted, check that the connections on the primary coil are the right way around and tightly secured. Check the security of the spade connector on the distributor's low tension block, and proceed to carry out a full continuity test of the ignition system.
The most common cause of coil failure is overheating caused by either an absent or defective ballast resistor (where fitted), or intermittent low-tension circuit. In the case of the latter, this is most commonly caused by a defective condenser or sections of high resistance caused by broken or dirty electrical contacts at any point between the distributor, coil and earth.

10 comments:

  1. Hello,
    Your post is very nice. This multi meter use proper way good result. Thank you for your nice shearing.

    ReplyDelete
  2. Trying to troubleshoot a 67 corvette 327 V8 L-79 engine miss and stall problem on the road before the tow 60 miles back home. Everything else checked OK including fuel pump pressure (5 psi), carburetor floats/metering, spark plugs, wires and battery. This left only three (4) other items, the rotor, the coil, the points and/or the condenser. My 3-year old coil tested 0.27-0.30 ohms (primary) and 9,700 ohms (secondary). Since your website identifies a secondary range of 11,000-12,000 ohms resistance, my first thought was the coil needed to be replaced. However, new coils tested at 9,100 ohms. My 49-year old coil tested at 11,500 ohms. So, I re-installed it. Next, the pitted (not burnt) points and condenser were replaced with new. So, that finally resolved the problem. My question is..how does one know whether the problem was the coil, the points or the condenser? Appreciate any incite on this. A flat bed tow without extra insurance cost me $130.

    ReplyDelete
    Replies
    1. I had same problem with my 67 -327. Engineer miss then stall then flat bed truck home. My 1st thought was a carb clog. 2nd check was a cropped hose from gas tank to fuel line impacting gas to carb. 3rd issue was old wiring to plugs. Replaced wiring.
      4th issue was pitted points. 5th issue was old coil. Replaced with original better than new. eemed like all had had something to do with it. 6th issue was reset timing. Welcome to Corvette trouble shooting. They don't make them like this anymore. Still historic. Wouldn't trade for anything.

      Delete
  3. Good it covers all important data in short. Nive post, thanks!

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  4. Nice information for this post, thanks!

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  5. At one time that i attained onto your web-site regardless that becoming interest quite simply rather tiny bit submits. Delightful strategy for new, My organization is bookmarking all at once get hold of versions last part increases higher. 2003 ford escape coil pack

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  6. Why do we check the secondary with connecting the + terminal with the secondary winding.dosent that measure the primary together with the secondary?

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  7. Do these value apply to all coils ?

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  8. how does one know whether the problem was the coil, fan motor or the condenser?

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