School Bus Alternator Application Guide - TSB-1025

TSB-1025 school bus alternatorThis technical bulletin is a detailed document that focuses on the entire charging system in school bus applications. The bulletin covers the alternator, batteries, and interconnected cabling in this type of demanding environment. Here are some examples from the bulletin:

"The battery is a reservoir of chemical electrical power, whose primary purpose is to provide the electrical energy needed to crank the engine. Once the engine has been started the purpose of the battery is complete, and the bus amperage requirements should be managed by the alternator. This will ensure that batteries are charged properly and are healthy for the next time they are needed for engine cranking."

"It is important to note that there are alternators being used in school bus applications today with ratings of 200 amps that produce less than 120 amps at idle. These units may run at a low voltage during times of high amperage demand, which will cause the premature failure of other parts of the electical system."

"Imagine a fire hose that is pumping the maximum amount of water that it can accommodate. Now increase the size of the pumps that are supplying the water to that hose. The fire hose can not pump any additional water, as it is already at maximum. Instead of pumping more water what you will see will be an increase in the pressure of the entire system.


This same phenomenon applies to the electrical system on your school bus. Increasing the size of the alternator without also increasing the size of the connected cabling will increase the electrical pressure (known as resistance) within your system. Technically speaking, you should not have more than 1/4 volt of resistance in each output circuit in your system with the alternator supplying 75% of its rated capacity."

The TSB includes information on:

  • A worksheet that will help you to determine the electrical requirements of any school bus, then how to read an output curve so that you can make sure that you select the correct alternator
  • The purpose of the batteries, and how they fit into the overall 'electrical system'
  • How to make sure your bus has the appropriate wiring for the electrical demands that will be placed on it

You can download TSB-1025 here, or you can find it along with all of our technical bulletins here

The information contained in this bulletin is intended for professional technicians who have the proper tools, equipment, and training to perform the maintenance described. This information is NOT intended for 'do-it-yourselfers', and you should not assume that this information applies to your equipment. If you have any questions regarding this information please contact our customer service department at:
Leece-Neville Heavy Duty Systems
a division of Prestolite Electric Inc
7585 Empire Drive
Florence, KY USA 41042
(800) 354-0560
(859) 525-8801