This SAE Recommended Practice establishes uniform test procedures for the defrosting systems of enclosed cab trucks, buses, and multipurpose vehicles. It is limited to tests that can be conducted on uniform test equipment in commercially available laboratory facilities. Current engineering practice prescribes that for laboratory evaluation of defroster systems an ice coating of known thickness be applied to the windshield to provide more uniform and repeatable test results, even though under actual conditions such a coating would necessarily be scraped off before driving. The test condition, therefore, represents a more severe condition than the actual condition, where the defroster system must merely be capable of maintaining a cleared viewing area. Because of the special nature of the operation of most of these vehicles (where vehicles are generally warmed up before or garaged in preparation for road operations) and since defrosting under steady state, over-the road operation is the main concern, test conditions have been adopted which eliminate the engine warmup phase of vehicle operation. This recommended practice will be reviewed and revised as technological progress in vehicle defroster test procedure requires.