Systematic functional testing is a fundamental step of embedded control systems development cycle, as it allows to verify and validate their final implementation. Various approaches to black-box testing have been proposed, however they either involve test-case explosion or do not ensure the correctness of system behaviour in scenarios not covered by system specifications. To cope with such issues, a methodology which better suits both complexity and safety-criticality of the target system is needed. This paper describes the ASF functional testing methodology, based on a grey-box approach aimed at generating and reducing an extensive set of influence variables and test-cases. The methodology, embracing different aspects of system test process (code coverage verification, regression testing, etc.), was successfully applied to validate ASF implementation of SCMT (an Italian project for an Automatic Train Protection System). The results obtained in our testing experience proved the time effectiveness and extensive coverage of the proposed approach.