Lesson 2/Learning Event 2
Normally, the charging system of a vehicle will keep the vehicle's batteries charged. Occasionally,
due to excessive cranking, repeated short drives, or a fault in the electrical system, the batteries will
become discharged. If the vehicle is in the field where no external method of charging is available,
you will probably have to start it by towing or slaving the electrical system of another vehicle.
To slave a vehicle with discharged batteries, you must use heavy-duty jumper cables and connect its
dead batteries to the good batteries of a second vehicle. Stay alert when connecting the jumper
cables. The hookup must always be positive to positive and negative to negative. If either of the
vehicles has an AC charging system, the rectifier will be burned out if you even momentarily touch
the cables to the wrong terminals.
Never try to slave one vehicle from another one that has a power source with a smaller ampere-
hour capacity rating. In other words, do not slave a 10-ton tractor with four 6TN 100 ampere-hour
batteries from a - ton truck with two 2HN 45 ampere-hour batteries. This will only lead to more
After connecting the jumper cables, run the live vehicle with good batteries at 1,800 revolutions per
minute (RPM) for a short while. This allows its charging system to put a slight charge in the
discharged batteries which helps a lot when you attempt to crank the dead vehicle. While cranking
the dead vehicle, keep the live one running at 1,800 RPM so you can use the power created by its
Remember that batteries will be damaged if left in a discharged state for long. After you get the
slaved vehicle started, make sure the vehicle is kept in operation long enough to fully recharge its
batteries. If this is not possible, arrange for the batteries to be charged from an external source. If
some defect caused the batteries to discharge, get that defect corrected immediately.
Some vehicles are equipped with slave receptacles so a slave (jumper) cable can be easily plugged in.
Both vehicles involved must be equipped with these receptacles. A special two-conductor slave
cable that fits the receptacles is provided. The proper positive-to-positive and negative-to-negative
connection for slaving is made by merely plugging the one-piece slave cable into the slave
receptacles of the two vehicles.