This device is installed near the control unit in some models operated at hight altitudes. See the Electrical Wiring Diagram for more information.
The resistance should read 1-10 kOhms between terminals 6 and 9 of ECU plug and 0.2-2 kOhms between terminals 6 and 12. If the readings are out of specification, check cables to and from altitude compensator. If cables check ok, compensator is defective.