[nkaminar]

Most all the tests that Bob described can be done with the DC volt scale on your multimeter. Assuming you have a 6 volt system, the battery voltage should be about 6.3 or 6.4 volts without any load on it. The voltage at the switch should be about 5.5 volts with the starter running and ignition off. If it is low, such as 3 volts, then the battery is bad or there is a loose connection or the cables are too small. If it remains high, then you have a bad switch or starter. To test the switch, do as Bob suggested and connect the battery cable directly to the starter nub. To see if the battery is the fault, measure the battery voltage with the starter running. If that is good, 5.5 volts or so, try to find where you are loosing voltage. Measure the voltage between the ground terminal of the battery and a good ground on the engine while running the starter motor with ignition off. If you measure any voltage at all you have a bad ground connection or small cable. Start by cleaning the connection at the ground terminal at the battery. If there is still some voltage then clean the location where the cable mounts to ground and if it is still on the frame, add a second cable to connect to a bolt on the transmission. Continue to measure voltage at various places to see where you are loosing voltage when the starter is running.

By methodically going through the electrical system with the multimeter, you can find where problems are. The starter can be tested with the ohm setting. There should be very little resistance between the nub on top and ground. If the starter has high resistance to ground then you can put it on the bench and use to ohm setting to find out why. I once had the wires come off a brush in my starter and this was really easy to find with the ohm setting.

The multimeter is your friend. Learn how to use it to find problems.