When you test the cranking voltage in a 6-, 12-, or 24-volt series system, connect the voltmeter leads of the (ester, as shown in figure 4-23. Observe the polarity as you make the connections. Then turn the voltmeter selector switch to 8 volts for a 6-volt system, 16 volts for a 12-volt system, or 40 volts for a 24-volt system. When a vehicle is equipped with a 24-volt series parallel system, the voltmeter leads arc attached to the two terminals on the starting motor. Before cranking the engine with the ignition switch ON, connect a jumper from the secondary terminal of the coil to ground to prevent the engine from starting while testing. While cranking, observe both the voltmeter reading and cranking speed. The starter should crank the engine evenly, and at a good rate of speed, with a voltmeter reading as follows (UNLESS OTHERWISE SPECIFIED):
4.8 volts or more for a 6-volt system
9.6 volts or more for a 12-volt system
18 volts or more for a 24-volt system
When the cranking voltage and cranking speed are good, it is reasonably safe for you to assume that the starting motor and starting circuits are in order. If the cranking voltage is lower than specified, test the battery capacity, starter circuits, and starter cranking current. However, if the cranking voltage is high but the starter action is sluggish, check for starting circuit resistance, as outlined in the circuit resistance tests given later in this chapter.
Provided the engine cranking load is normal, excessive starting motor current indicates trouble in the starting circuit. However, increased current is normal on new or newly overhauled engines or where the cranking load is above normal.
To check an excessive starting motor current, you can perform a starting motor current draw test of the 6-, 12-, or 24-volt series system.
To conduct this test, turn the battery starter tester control knob to the OFF position. Then turn the voltmeter selector switch to 8 volts for a 6-volt system or 16 volts for a 12-volt system. When a vehicle is equipped with a 24-volt series system the voltmeter selector switch is turned to 16 volts if 12-volt batteries are used or to 8 volts if 6-volt batteries are used. On a 24-volt series system, connect the voltmeter leads across one 6- or 12-volt battery ONLY. Connect the VOLTMETER leads of the tester, as shown in figure 4-24.
Before you crank the engine with the ignition switch ON, connect a jumper from the secondary terminal of the coil to ground to prevent the engine from starting during testing. While cranking, note the exact reading on the voltmeter. After cranking, turn the control knob of the battery tester clockwise until the voltmeter again reads exactly as it did during cranking. The test AMMETER should indicate the starting motor current within the normal range of the vehicle being tested, as determined from the manufacturer's specifications.
Figure 4-24.-Starting motor current draw test.Continue Reading