Windshield Defrosting Systems Test Procedure and Performance Requirements--Trucks, Buses, and Multipurpose Vehicles

  • 2009-01-27
  • SAE International

This SAE Recommended Practice establishes uniform test procedures and performance requirements for the defrosting system of enclosed cab trucks, buses, and multipurpose vehicles. It is limited to a test 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 and left- and right-hand side windows 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 kept in a garage or warmed up before driving) and since defrosting under steady-state over-the-road operations is the main concern, test conditions have been adopted which assume that the engine is warm before the vehicle is driven.

There are two options for producing hot coolant in this recommended practice. Testing using these two approaches on the same vehicle will not necessarily provide identical results. Many vehicle models are offered with optional engines, and each engine has varying coolant temperatures and flow rates. If the test is being conducted to compare the performance of one defroster design to another defroster design, then the external coolant source approach (Test A) will yield the most comparable results. If the test is being conducted to validate the defroster installation on a specific vehicle model with a specific engine, then using the engine to heat the coolant (Test B) will be more appropriate.

This document will be reviewed and revised as technological progress in vehicle defroster test procedure requires.


The current SAE J381 test procedure provides two separate means of heating the coolant during testing, but these two approaches can provide very different results. If the engine is used to heat the coolant, the engine rpm and engine load criteria are neither clear nor consistent.

There are two distinct reasons to test a defrost system:

a. To compare two or more alternative defroster systems during the design process.

b. To compare the defroster performance of a specific vehicle and engine configuration to an established standard.

This revision recognizes these differences in the reasons for testing, and provides specific test conditions depending on the test intent. Variations of the test between gasoline and diesel engines are eliminated with the rationale that how well the window clears should not be a function of the engine fuel source.

Diesel and gasoline engines in the past ran at very different rpms, but today most truck gasoline engines run high idle rpms at similar speeds when compared to their diesel counterparts.

The chamber wind condition was increased to up to 5 mph because this is easier for all chambers to meet and this should not significantly affect the results

Updated the test report template to match the procedure

References to the type of cameras that could be used were removed because such specific recommendations do not keep pace with advancements in technology.

Standard Published Revision Status
J381_200901 2009-01-27 Latest Revised
2000-09-29 Historical Revised
1984-06-01 Historical Revised
1974-05-01 Historical Revised
1968-02-01 Historical Issued
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