Re: Enough battery for starting motor but not for spark?

Posted by John T on April 01, 2019 at 07:09:29 from (174.255.199.161):

In Reply to: Enough battery for starting motor but not for spark? posted by JohnV2000 on April 01, 2019 at 06:28:33:

If you stick a voltmeter on the coils input (from ignition switch NOT to distributor) terminal and turn the ignition on there should be near battery voltage there..... HOWEVER If there's an inline series ballast resistor and a good set of points are closed it could be around 1/2 of battery voltage IE a set up with a 12 volt tractor and a 6 volt coil and series ballast

If the voltage on the coils input with ignition on is a lot less then battery voltage (or none at all) and if there's no inline series ballast resistor:

A) The ignition switch may be bad or resistive (burned or loose or corroded contacts)
B) The wires open or bad FROM battery source or ammeter TO ignition switches BAT Input
C) The wires bad or open FROM ignition switches IGN output TO coils input terminal.

Its possible to temporarily jump hot wire direct to the coils input to see if that fixes it

If you measure the voltage on the coils input while the engine is cranking over that can show if the starter is drawing excess current maybe dropping battery voltage too low. If it has a good battery ??? and the starter motor isn't bad ??? the coils input when cranking should be around 10 to 12 volts, if too low the spark may be weak possibly caused by a weak battery or a bad starter or else a bad loose resistive connection like a battery or ground cable or part of the ignition wiring.

A test lamp over on the coils other output (to distributor) terminal with engine slowly cranking over should flash ON when points are open but go OFF when they are closed.

Check voltage on coils input with ignition ON both when setting and when cranking then check wires and connections. Its free n easy to have a shop due a load test on the battery. Maybe its a bad battery ??? or a bad battery or ground cable ???

If all else fails work through my Test Procedure linked below

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

John T

• John T's Ignition

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