testing a coil

[Kdancy]

What is the proper procedure for bench testing a 12v coil?

53commander HDTP
53 Champion HDTP
61 Cursed Purple Hawk
64 Champ long bed V8
64 GT

[Alan]

Take a good Ohm meter and the primary side should read 1.5 to 10 ohms or so and the secondary side should be around 7,000 to 12,000 ohms. Neither side should show an open or short. If you have resistance on both sides it should work. A coil is an item that either works or dosn't. The primary winding normaly handles a current of around 4 amps, so the wire must be relativly large. The secondary winding only carries a low current, so the wire can be small. A typical coil will have a secondary winding made up of 20,000 turns of very fine copper wire wraped around an iron core that concentrates the magnetic field. Secondary wire must be almost hair thin to get this many turns into one small package. It is usualy this wire that gets broken. One end connects to the primary terminal the other to the center.

[Kdancy]

Thanks Alan, I'll give it a shot.

53commander HDTP
53 Champion HDTP
61 Cursed Purple Hawk
64 Champ long bed V8
64 GT

[Roscomacaw]

Alan, your diagnostics are valid - however, the "it works or it doesn't" idea is a bit to decisive from my experience. I've had more than one instance of a coil that would work fine when cold and then start to act up [}] (if not just plain quit[xx(]) once it started to get warmed up.
I would suspect that such a coil would yield acceptable ohmmeter readings when cold.

Miscreant at large.

1957 Transtar 1/2ton
1960 Larkvertible V8
1958 Provincial wagon
1953 Commander coupe
1957 President 2-dr
1955 President State
1951 Champion Biz cpe
1963 Daytona project FS

[Alan]

Mr Biggs, I have a lot of stories I could tell myself of coils that people thought were bad, when 90% of the troubles were traced to the ballast resistor or that dam pink wire as Stude people like to call it. Coils are so cheep it is just as easy to replace, but if you had a Stude coil that came with the car OEM, or if your grandfather bought it from Hot Rod Hennry's in 1960 go for it.

[Roscomacaw]

That pink wire is notorious for causing grief, and I don't know why. It's just a wire, albeit a resistance wire. But it's resistance SHOULD not be altered with age. Maybe it's the connectors that are the real gremlin![}]

Miscreant at large.

1957 Transtar 1/2ton
1960 Larkvertible V8
1958 Provincial wagon
1953 Commander coupe
1957 President 2-dr
1955 President State
1951 Champion Biz cpe
1963 Daytona project FS

[gordr]

Biggs, I think you are right about the problem with the resistance wires being in the terminals. It's probably similar to the problems with aluminum wiring as was used for few years in house construction...it goes bad at the terminals due to heating where dissimilar metal make contact.

To bench-test a coil: connect the (+) terminal of the coil to the (+) terminal of a 6-volt battery or charger (set to 6 volts). Connect the coil's bracket to the (-) terminal. Find a spark plug and wire, and put the other end of the plug wire in the coil tower. Clamp the shell of the spark plug to the coil bracket. Using clip leads, connect the shell of a spare ignition condensor to the coil bracket, and the condensor lead to the coil (-) terminal. Use another clip lead to momentarily ground/unground the coil (-) terminal, just a quick amke-and-break to simulate the action of the points. You should get a nice snappy blue spark at the plug, assuming it's a clean one. A good 12 volt coil, if it's the type requiring an external resistor, ought to be able to fire the plug working on 6 volts. Likewise a 6 volt coil, naturally. If the 12 volt coil is they type that needs NO external resistor, then you might need to use 12 volts for this test; at least try it before condemning the coil as bad.

I suggest using 6 volts for the test because it approximates a "worst-case" scenario for starting the car in cold weather with a weak battery. Applying full 12 volts to a coil meant for use with an external resistor might get you a good spark from a coil that is actually past its best-before date

Gord Richmond, within Weasel range of the Alberta Badlands

[Kdancy]

Thanks fellows, I did put my ohm meter on the coil and both terminals showed some resistence. I would up pulling the distributor and putting a rebuilt one in that I had. After the carb got gas it fired up. 70lbs oil pressure on guage cold, I let it warm a little and still had 55 or so. Can't put it under load as car needs all new brake job, but just wanted to see if I would hear any rattles. Sounded pretty good except for exhaust leak. It hadn't been started in over 3 1/2 years. Looks like I will be doing a frame off as the frame really needs blasting and repainting along with a-arm bushings and the lot. It is a 63 cruiser and the customer wants to drive it 40 miles to work every day.

53commander HDTP
53 Champion HDTP
61 Cursed Purple Hawk
64 Champ long bed V8
64 GT