Posted by John T on December 03, 2010 at 13:38:48 from (66.244.97.31):
In Reply to: How to Check a Coil? posted by Doug Kieta on December 03, 2010 at 11:19:55:
Lets answer your direct question first:
How do you check a coil to see if it is bad?
A simple Ohmmeter will tell you if a coil is BAD like a complete open or way out of spec LV primary or HV Secondary winding resistance BUT IT CANT TELL YOU ITS NECESSARILY "GOOD" because typical problems may not happen until it warms up and/or it may be high voltage insulation failure/breakdown which the quick low voltage ohm meter test wont detect.
A "real" coil tester does it better
"Typical" not all and the answer depends on the coil voltage etc:
LV primary winding between little + and - terminals may be in the 1 to 4 ohms range (say 1.5 to 3 ohms on many, depends if for 6 or 12 volt use)
HV Secondary may be in the 4,000 to 10,000 ohms range
IF EITHER IS OPEN ITS A BAD COIL although many coils will test good for the above but once warmed up she will suffer a HV insulation breakdown and fail to produce a spark
ANOTHER CAUSE OF SPARK FAILURE AFTER WARM UP IS A BAD CONDENSOR Check that first as its much cheaper to cure !!!!!!!!!!!!!!
Its easy to bench test a coil, you simply rig a circuit to pass current through its LV primary but then interrupt/stop that current flow and it should produce a HV spark out its top HV terminal/tower. You can rig plug wires or even a spark plug to do this easy simple bench test. NOTE if a good condensor is in paralell with the simulated points (where you close but then open the coils low to ground connection, i.e. what a set of points does closed then opened) theres a better spark mind you, she still fires with no condensor (unless coil bad) albeit weaker then if the right condensor were in the circuit
You can do it on the tractor with a plug and/or plug and wire and have the ignition on and points closed and uee an insulated stick to open the points and each time they open the coil should produce a spark. One can put a plug wire in top the coil and have other end to a side grounded spark plug and see if she fires. Or just pull plugs and/or wires and turn her on n crank her over to see if the coil fires and/or all the plug wires/plugs
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