Few studies provide conceptual frameworks that help teachers make decisions about the scaffolding they will offer their students in project-based learning (PBL) processes. To address this deficiency, an adaptation of the motivational development self-system model was used and a PBL experience involving high school students who had to create an electric vehicle that used solar energy was analysed. Applying a multi-group analysis with structural equations, in general, it was observed that the continued support of the teacher in the project activities strengthened the association of students' self-determination with their affective and cognitive engagement. The data suggest that scaffolding can compensate for deficits in self-efficacy, relationship and autonomy of the group of students. This study provides a model that can be validated in experimental investigations in which the level of self-determination of students is controlled.