Re: Farmall A no fire through coil

Posted by John T on September 28, 2005 at 19:43:58 from (66.244.90.98):

In Reply to: Farmall A no fire through coil posted by GettingStarted on September 28, 2005 at 16:37:23:

I may be able to help you diagnose the problem AFTER I figure out if you have a Magneto,,,,,,,or a regular battery powered external coil ignition system,,,,,,,,,,,,or if you have a Mag but only using its internal points to operate (switch current on and off) an external coil?????????????

If youre using an external battery powered coil and regardless if the points inside a distributor or the points inside part of an old Magneto are used to switch coil current, the coils high input (NOT to points) terminal requires hot battery voltage to be present when an ignition switch is turned to ON,,,,,,,and then the coils other (to points) terminal must be wired to the points which serve as a mechanical switch to make (points closed) and break (points open) coil current and each time they have been closed to conduct coil current but then open, the coil should fire. The coils low to points side terminal CAN NOT BE PERMANENTLY GROUNDED OR SHE WONT FIRE. The points must serve as the switch to make n break coil current and she will fire each time they break open. ARE YOU WIRED THAT WAY???????????

To save some time I will cut n paste a portion of my generic Troubleshooting Procedure for non spark on a coil ignition. Then once we figure out what you have if this dont help post back more info and your findings n we will go from there....... This is for a battery powered coil remember, regardless where the points are (in an old mag or a distributor)

TROUBLESHOOTING A BATTERY POWERED EXTERNAL COIL TYPE IGNITION SYSTEM:

1) THE VERY FIRST THING YOU GOTTA HAVE is voltage to be present on the coils high supply (NOT to distributor) lil terminal when you turn the Ignition switch ON. If not she cant ever fire, but in the event the ign switch or circuit/wire down to the coil is bad, you can HOT WIRE it by jumping a hot ungrounded battery voltage source to the coils high input supply (NOT to distributor) side.

2a) When the ign switch is turned on, voltage should appear on the coils high input side. That would be 6 volts on a straight 6 volt system, 12 volts on a 12 volt non external ballasted system, or around 6 volts on a 12 volt system that used a 6 volt coil plus an external ballast resistor and if the points are working and closed.

2b) To insure the coils low voltage primary winding is not bad/open, use an ohmmeter and measure its DC resistance between its lil + and -terminals. If its an open circuit (no continuity) its bad/open and will NOT work. It should measure around 1.2 to 2 ohms or so if its a 6 volt coil and maybe 2.5 to 3.5 if its a 12 volt internally ballasted coil. NOTE CAUTION have all leads and any voltage source attached to the coil for this simple primary winding continuity test.

3) Next, place your voltmeter or test lamp on the coils other low to distributoir terminal side, turn her on and crank the engine over.

4) A test lamp there should flash ON (when points open) and OFF (when points are closed)

5) a) If the lamp never comes on there, the coils primary is bad/open,,,,,,,,,,or the points are never opening,,,,,,,,,or theres a shorted/bad condensor (remove its lead to points and see if lamp comes on, if so, bad shorted condensor or its wiring),,,,,,,,or the points wire is shorted,,,,,,,,or the distributirs side pass thru sud is grounded (use ohm meter to test that),,,,,,,,,or the points may have a shorted spring.

b) If the lamp never goes off as engines cranked, the points are not closing,,,,,,,,or the wire or circuit is missing from the distributor to the points,,,,,or the distributors not well grounded to the tractor.

She cant fire the coil unless its low side is getting a conductive ground return path via closed points and then the circuit is open when the points open.

Be sure the condensor or its wiring is NOT shorted out and see if the lite comes on (when points open) with the condensor disconnected.

To check for possible cap or rotor problems (if all the tests above pass)............

Remove the coil wire from the distributor leaving coil end inside coil and place the exposed end to within 1/8 inch away from tractor iron like block or head etc. Turn her on and crank her over and a spark should jump that gap nice n blue each time the points open. If so there but NOT out the plug wire terminals,,,,,the cap or rotor is bad. Check it for cracks or carbon traces etc. If the cap n rotor are good but the plugs dont fire, the wires or plugs could be bad. Try different plugs or wires and by swapping sides see if any plugs fire as she is cranked. If fire comes out the coil wire per the above but no plugs fire, its gotta be a cap or rotor or plug wire or plug problem. NOTE if its the right cap, when the points break open the rotor tip should be lined up next to one of the caps internal pickup points.

Be sure the points open on high cam,,,,,,,theres voltage present on distributors high side at all times when ignitions on,,,,,,,voltage on coils low side flashes on and off as distributor is cranked,,,,,,,,,condensors not bad/shorted,,,,,,,,no shorts in wires to points and no shorts in pass thru side out distributor stud,,,,,,,,coil has continuituy.

John T, retired electrical engineer

Follow Ups:

Post a Followup Name: E-Mail: Subject: Comments: Optional Link URL: Link Title: Optional Image URL: Email Notification: If you check this box, email will be sent to you whenever someone replies to this message. Your email address must be entered above to receive notification. This notification will be cancelled automatically after 2 weeks.

TRACTOR PARTS	TRACTOR MANUALS

We sell tractor parts!  We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today. [ About Us ]