How To Test an Ignition Coil with a DVOM

Batteries, magnetos, distributors, wiring harnesses and more.

Moderators: Barnyard, Team Cub

How To Test an Ignition Coil with a DVOM

Postby cubguy's dad » Sun Jan 25, 2009 9:00 am

Testing a Automotive Style Coil with a DVOM is easy to do and is another diagnostic tool that you can put in your toolbox and another use for your DVOM.

Again the goal is to show the test procedure along with some insights into the operation of the coil.

The coil is in its simplest form of a step up transformer. The coil converts system voltage from 6 or 12 volts at high amperage, (this is called the primary circuit), to 20,000 to 30,000 volts at a low amperage (this is called the secondary circuit). The high secondary voltage has enough PUSH to jump the spark plug gap under compression pressure.

The coil primary circuit has a + (positive) terminal and a - (negative) terminal. The coil has to be wired into the circuit with the correct polarity. The correct polarity is needed so that the spark will JUMP from the spark plug center electrode to the ground tab. If the polarity is reveresed the spark JUMPS from the ground tab to the center electrode (most spark plugs are not designed to operate this way).

The correct coil polarity is determined by the battery ground. The points are the ground side of the primary circuit. 6 volt cubs are + positive ground, so the + positive terminal on the coil will be wired to the points. On the other hand if your vehicle is -negative ground (12 volt cubs) the - negative terminal will be wired to the points.

TESTING

To test the coil primary circuit for continuity, set the DVOM to ohms, and use the 20k scale

Image

A good primary circuit will have NO or very little resistance, This coil shows .01 on the 20k scale (almost immeasurable resistance) this coil tested okay.

To test the coil secondary circuit, again use the 20k scale. One lead is on a primary terminal while the other lead is in the coil tower.

Image

In this test we are testing for internal shorts or high resistance that will prevent the coil from putting out it's 20,000 volt secondary spark. A good coil will range from 7,000 to 10,000 Ohms of resistance. This resistance value is one of those general rules of thumb that works for most coils.

Testing the coil to see if it is shorted to ground.

Image

A good coil will show infinity or open circuit. We do not want the primary circuit shorted to the case.

These are all the tests for this style coil.

Mark
Last edited by cubguy's dad on Sun Jan 25, 2009 3:32 pm, edited 1 time in total.
Experiance is knowing what NOT to do the next time.......
1937 John Deere A (Big John)
1953 Farmall Cub (LiL Red)
Lots of projects.
cubguy's dad
5+ Years
5+ Years
 
Posts: 556
Joined: Sun Jan 04, 2009 5:41 pm
Location: MI Onsted
Zip Code: 49265
Tractors Owned: 1938 John Deere A
(owned 38 years)
1953 Cub
(owned 21 Years)
1963 Olds Cutlass
1970 Ford F-350
(Cub Hauler)

Return to Ignition/Electrical

Who is online

Users browsing this forum: No registered users and 1 guest