Re: spark coil questions

Posted by John T on November 24, 2021 at 06:12:57 from (174.197.11.95):

In Reply to: spark coil questions posted by Bob N.Y. on November 23, 2021 at 14:06:20:

Great questions, here's my response

1: With the key on but engine not running, should the voltage on the distributor side of the coil be the same as the battery side?

ANSWER NO the voltage on dist side to points SHOULD BE NEAR ZERO as the closed points (which is typical if engine not running) should create a short to frame ground. With key on the voltage on the coils input from ign switch should be battery voltage unless theres an inline series ballast and points closed.

YES it would be same as batt voltage IFFFFFFFFF points were open but see the above

Question 2: If the voltage on the dist. side is considerably lower than the batt. side, does that mean the coil is hooked up backwards?

ANSWER NO if the points are closed AS TYPICAL WHEN AN ENGINE IS NOT RUNNING there should be ZERO VOLTS on the coils output to
distributor terminal because the closed points creates a short circuit to frame ground

NO it DOES NOT MEAN coil is wired backwards

Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? Thank you.

ANSWER NO However the coil should be wired at the correct polarity since due to the effects thermionoc emission its easier to arc jump current from a hotter surface to a colder one. I will still work wired backwards but its more efficient if wired correct.

For NEG ground the coils + goes to ign switch and - to distributor

NOTE if ign is on and engine slowly cranked over a test lamp on coils output to dist should flash ON when points are open but OFF when closed

Read through my Troubleshooting Procedure which can help understand and better answer this

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=farmall&th=5745

Hope this helps, any questions???

John T

• John T's Ignition Troubleshooting

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