Abstract: A truck trailer interior washing system with a washing head mounted on an extendable and retractable boom horizontally mounted for automated washing cycle extension into and back out of the trailer interior. A truck body or trailer is backed into exact position, centered and aligned with the boom, with the use of guide rails and a centering plate structure automatically moving trailer wheels laterally in a centering action as required. When the system is activated, a pump is started to deliver hot detergent solution to the nozzled cleaning head from wash fluid back-up tanks and heaters as the extension boom is moved into the trailer at a predetermined rate (50' per minute, for example). A feeler bar and microswitch system senses when the inside nose of the trailer is reached and acts to stop the boom in the extended position for a short interval of time during which the pump and feed mechanism clears itself of detergent solution with microswitch activated clear hot water flush to the nozzles.