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testing a 12 volt coil

A

Anonymous-0

Well-known Member
my farmall 350 was converted to electronic ignition befor i bought it . it now has no fire.i have current to the coil.it was running and back fired now no spark . how can i test the coil.i dont have anything to borrow a nother one from.cant retunrn a new one if that isnt the problem.
 
Put a test light on the wire to the dist. Turn it over & it should flash on & off. If it doesn't, the electronic device is bad.
 
A low energy low voltage simple ohm meter continuity test can show if a coil is definitely BAD but it cant tell you if its GOOD. IE If it fails its bad but even if it passes it may still be bad.

The resistance of the LV primary between the small + and - terminals should be around 2 to under 4 ohms (many around 3) but if its an open circuit the coil is BAD.

The resistance of the HV secondary should be in the vicinity of 3000 to under 10,000 ohms and again if its open its BAD.

HOWEVER it can pass BOTH those but still be bad due to HV breakdown especially once warmed up.

If the switch is good if you place a test lamp on the coils output (To distributor) terminal and crank the engine over slowly, it should flash ON and OFF which simulates when the points opened (coil fires and light is on) and when they close (coil conducts current lamp is OFF)

You can also bench test a coil and use a plug and plug wire and each time after the coils LV primary conducts current but then its interrupted (like points opened) it should fire.

John T
 
I had problems with coil on 300 monday. I conected to ground and had 12v on pos side of coil. Checked neg side with points open and had 12v. Coil bad. Put new coil on checcked neg side with points open and had nothing . Coil good. Cleaned points with emery cloth and tractor started after 4 months parked.
 
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