You can join B.Tech course (regular or distance) in Chennai by D2D process after completing your Diploma in Mechanical Engineering. B.Tech programme allows Diploma students to take entry directly into second year of engineering course.
For taking admission in B.Tech, you must have done at least 3 years diploma after 10th in any branch of engineering.
Some colleges take admission through lateral entry exams like Joint Entrance Examination of Lateral Entry (JELET), Diploma to Degree Entrance Competitive exam, etc. so you need to appear and crack these exams. Some colleges do conduct their own entrance exam to provide admission in their engineering courses.
Admission procedure varies from college to college. So, you will have to contact colleges individually.