Caterpillar 621B Manuals. If you are not finding what you need for your Caterpillar 621B Contact us here.. Where available, the Manual Kit is a great choice for all the manuals written for your equipment. Service Manuals will help you fix what is broken. The Parts Manual will show you exploded views of all the parts and help you find what you need. The Operator Manual will give you operating instructions and critical everyday use and preventative maintentance procedures such as fluid changes, capacities, and lubrication points. In need of a Wiring Diagram? Be sure to check the manual preview on the product page or call our friendly staff. Sometimes these wiring diagrams are in the Operator's manual and sometimes they are in the service manual of your Caterpillar.