LINK 8J2285 - Caterpillar

A low reading is an indication that the battery needs to be charged. But for a very low reading, a more thorough test is needed. Test the voltage again and the specific gravity of the electrolyte in the separate cells. Use the 5P957 or 5P3514 Coolant and Battery Tester to check the specific gravity. Follow the recommendations below when voltage and specific gravity are used to check the condition of the battery or batteries:
Use the circuit voltage test to check for normal operation of an electrical system. The lamps of the tester will be activated when a complete circuit is made. The lamps are for a 24 Volt system and are not visible (activated but not seen) in the range of 10 Volts or less.
In this voltage range, a meter can be used for the Circuit Voltage Test to check for complete circuits. The meter can also be used to check the voltage decrease in a circuit. Normally, there is a 1.0 to 1.5 Volt decrease when a solenoid is activated. This is caused by a current flow through the solenoid back to the battery.
Use the Circuit Continuity Test-Ground Seeking to find a damaged (frayed) or broken harness. This is a common problem which causes a short circuit. If a continuity test lamp becomes visible as the serviceman turns the selector switch, this is an indication of a ground on the circuit respective to the selector switch position. This same test can also be used to check for open circuits. The opposite end of the harness is grounded. If there is an open circuit, the continuity test lamp will not become visible at the selector switch position.
Use the Circuit Continuity Test-Short Seeking to find internal continuities. Turn the selector switch of the tester. If any of the lamps other than the one respective to the selector position becomes visible, there is continuity between the selector position and the one or more circuits respective to the other lamps which have become visible.
To check a solenoid for its minimum rate of voltage, use the Solenoid Test on the Special Instruction. When the tester is given a full 24 Volts, the voltage decrease over the resistance that is built into the tester will give an indication of the voltage rate of the solenoid.
All of these circuit tests are in schematic form on Special Instruction SEHS7153. There is also a schematic of the 5P6590 Wiring Harness Tester.
Special Instruction Form SEHS7153 is to be used with the 5P6590 Wiring Harness Tester Group.