Starting circuit problems can be the easy to diagnose if a few simple rules are followed. First eliminate mechanical binding such as a seized engine or stalled transmission or hydraulic pump. Make sure the batteries are good. Then move on to the starting circuit.
Voltage drop testing is the best way to quickly break down the circuit to identify problems.
Start with a schematic of the system and work from one end to the other. You must not jump around.
Each connection and cable must be checked under a load.
First use your voltmeter to check for voltage drop (difference) at the positive battery post to cable connection. Load the circuit by attempting to start. There should be no voltage (.1 volt max) at this connection. If your volt meter reads more, there is a voltage drop. Remove the cable and clean or repair the connection.
Next check the cable by measuring the voltage from one end of the cable to the other.
The reading on the voltmeter should be no more than .1 volts with a load on the circuit. If the reading is higher, the cable is no good. Replace the cable. Test each length of cable and connection in the circuit this way until you have reached the ground connection at the battery. Even if you find the drop in the first part of the circuit, it is a good idea to check the whole circuit. If one section of the circuit has a problem, it is likely that there are more circuit sections that are in the same condition.
Don't forget that your volt meter measures voltage difference. In other words, the difference in voltage between the red lead and the black lead. There should be no difference in voltage from one end of a cable to the other. If there is, you have a voltage drop.
ęSteve Blankinship 2008