The Stovebolt Site Forums The Shop Area The Electrical Bay bad coil?

i'm trying to start the engine for the first time. the coil(12v with internal resistor) has been in for around 3 years but was new when i bought it. multimeter testing the positive to the negative reads about 3 ohms (which i assume is normal). testing either post to center/distributor post gets no reading at all (a 1 all the way to the left). i'm assuming i should get a new coil - or i am doing it wrong? thanks, joe

Measuring resistance from either of the two primary terminals to the coil center tower should give a reading of 2,500 ohms or more. Try setting the ohmmeter to a higher scale such as "K-ohms". You're probably just out of range of the current setting on the meter.
Jerry

1. just tried it again at the 20k setting and got 7.4
2. what's the best way to check and see if the plugs are firing? can i just pull a plug and have someone hit the starter? this is probably a really stupid question but this electricity stuff makes me nervous!

Yes you can...but you have to ground the plug and hold it at the same time...might introduce you to that spark voltage rather directly. Use a good glove and you should be OK. Easy way is to pull the cap on the dizzy and with the key on, open and close the points by hand. should see a small spark at the points. If your ears are good you can hear it to.

Lot's of iffy ways to check spark at the plugs, take one out
ground the shell to the black, hold a screwdriver between the
terminal and the block with a slight gap, get your kid brother
or wife to grab holt of the wire and the fender, unless they've
got a pace maker. Try the previous methods while
pulling on the lawn mower starter rope, good luck.
Or you can free up your hands and do it the easy way.
For five bucks these are a great item to test for
spark to the plugs, I've got a couple in the test
equipment drawer:
http://www.harborfreight.com/inline-ignition-spark-checker-69014.html

DG

thanks for the advice. i'll try the spark plug tester and avoid frying myself!

DG's idea is the best way. You say "trying to start the engine", but we don't know what that means? Why are you testing everything? Does it rotate? Does it fire some? Does it backfire some, does it smell like gas?............ Gotta know the problem if you need advice. oops, I mean symptoms then tests, then treatment, then cure, Doc. The diagnosis on old trucks is after they start and run good we say "yeah, that's what was wrong".
I believe your reading is 7500 Ohms.
Two sources from the web: They didn't say internal/external or engine/era.

0.75 to 0.81 Ohms for the primary winding and 10,000 to 11,000 ohms for the secondary winding is correct.


The Ohmic resistance of the coil is about 0.2-3.0 Ohm primary and 5-20 kOhm secondary.

Your number 7500 ohms does not match any number I could find or HotRod's

The quick, simple, and relatively safe way to check the coil output is to remove the coil wire from the center of the distributor, hold it about 1/4" from the cylinder head, and have somebody crank the engine with the ignition on. There should be a good spark jumping to the head every time the points open. Then replace the wire in the cap, remove a wire from one of the plugs, and use a spare plug held against a ground with a pair of insulated-handle pliers and check to see if the distributor cap and rotor are functional.

All the above assumes the distributor is turning, so pull the cap off and bump the starter to make sure you don't have a stripped timing gear or a distributor that's not installed deep enough to engage the drive gear on the camshaft. Be sure the points are clean and properly gapped while you've got the cap off!
Jerry

HotRod: Quick and easy method. Good to remember. Maybe I'm missing something but I still would like to know the answers to my above questions re: What the heck is the motor doing!!!!! I realize his only question is how to test a coil so I guess I don't need to know what's going on with trying to start.

And still another way to check a coil on the bench. Use a jumper
wires to connect the coil in a circuit with a new or known good
condenser, a new or known good spark plug and a known good
spark plug wire. Use a fully charged 12 volt battery. Connect
a common ground to the negative post on the battery.
1. Connect the positive (+) terminal on the coil to the positive (+) post on the battery.
2. Connect the wire on the condenser to the negative (-) terminal on the coil.
3. Connect the condenser body to common ground.
4. Connect the spark plug body to common ground.
5. Momentarily touch and remove the wire coming from the
negative (-) terminal on the coil and condenser to common
ground to simulate the opening and closing of the ignition
points. The coil should produce a strong spark at the
plug each time the wire is removed from common ground.

dg

If I'm not sure if a coil works or not, I go buy new one.

You could get a [url=https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSqq4rVmv8v3P_Cra_A60datkEULcnRI9z7I90VVQ8mCgl0u73Jfw]Spark Plug tester[/url]

This one is the [url=https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSbvQkTq3WkkTbr051dNMZcEbmEvbaWpUmxxFxYLtQAbrgdkeudAA]OLD School unit[/url] that I have and have used for 30 years.

Here's looking up your old thread...
I heard of a simple test for a bad coil today. I believe I have an intermittant spark problem - the engine heats up, I park it ten minutes, after restarting it seems the spark occasionally disappears.
I mentioned it to a local parts dealer, he said either coil or condenser. I asked about how to test a coil. He said take the coil out, check the resistance cold, then put it in a warm oven (say 150 degrees) about half an hour. Then check resistance again. If the resistance is noticeably lower then the coil is faulty.
Anyone heard of or tried this?
Karl

They're oil filled, I've boiled them right out of the box before, old trucks and new vw's as a tech. Sometimes ya get a bad batch of parts. Like stated above, if it's iffy, spend some coin on a new one. Worst case ya got a spare.