I have been reading of late several posts on starting problems.
First thing is having the proper tools in everybodys toolbox you NEED a DVOM (digitail volt ohm meter) a good one will set you back about less than $100.00 Craftsman has one in that range, I have a Fluke Meter MOST DIYers do not need this expensive of a meter. With a DVOM(digitail volt ohm meter) you can diagnose anything electrical on the tractor.
First start with the battery.Check the voltage at the top of the posts (DVOM set to DC voltage setting,all these tests will use this setting), a fully charged 6 volt should be about 6.5 to 6.6 volts, (12 volt battery needs to be 12.6-12.7 volts )if the battery does not pass this test it will have to be charged BEFORE you can continue.(a 6volt battery at 6 volts has less than a 10% charge in it).
With a fully charged battery (6.5 volts)in your cub, again set the DVOM to DC volts, observe meter polarity and measure from the top of the battery posts, disable the ignition and crank the engine over for 15 seconds(we are testing battery capacity or amperage draw) at the end of 15 seconds observe the voltage should be above 5.5 volts, the higher the better(means your battery has enough capacity to effectively operate the starter) remember on a distributor ignition you need voltage reserve to make the coil function as well as operate the starter. You need a minimum of 4.5 volts to have enough battery capacity to start the tractor if not the battery needs replaced.Always buy the largest battery that will physical fit in your application (see Bob's post in this section for his recommendations on batteries).
Next test the Ground and positive circuits. Remember that electricity will always take the path of least resistance and we will provide an alternate path and will measure how much electicity will take the easy route. Starting with the ground circuit put one on the top of the battery post end (observe meter polarity) put other end on the starter case, crank the engine(ignition disabled) and observe meter it should show no more than .5 volts (1/2 volt) the less the better on a 6 volt I like .2 volts or less. You just tested the complete ground circuit for total resistance. Now you can clean, replace and retest to correct the resistance in the groung circuit and you can test for your results.
Do the same thing with the positive circuit one end on the top of the battery post and the other end on the starter post and see what you have. Again you need to see less than .5 volts. Clean replace and retest as needed. Again less than .2 volts or less is desirable.
Add up your total resistance. For example if you have .5 volts on both the ground and positive circuits you have 1 VOLT resistance, couple that with a only 50% charged battery (a (reading of 6.3 volts on the battery voltage test)and you have a problem. You only have 5.3 volts (6.3 minus 1volt combined resistance) available to the starter BEFORE you even run the starter to crank the engine.
This test works for any vehicle 6 or 12 volt. 12 volt systems start out with more voltage (electrical Pressure) and can handle more resistance up to 3 volts and still operate where a 6 volt system is done at 1 volt total resistance and that is not much.
Correcting the resistance in the battery circuit will help your charging system, as it has to operate with the same resistance as your starter cicuit does. Measure charging voltage at the battery to get the true reading of the charging system.The automotive style battery needs 1.5 volts over fully charged battery voltage to fully charge the battery. A 6 volt battery needs a charging voltage of 7.8 to 8.1volts at the battery, and a 12 volt battery needs 13.8 to 14.2 volts at the battery to keep it charged.
Measuring voltage drop is not hard it just takes a little practice. Everybody on this forum knows the importance of clean connections and the right size battery cables. Taking the time to test and repair the resistance in your electrical system will solve many starting and charging problems
Last edited by cubguy's dad on Sat Jan 17, 2009 2:59 pm, edited 2 times in total.