In the meantime, Hannover had suffered relegation once again, but in season 1985–86 Hartmann was able to help his club back into the top league.
His jubilation, though, was short lived; he suffered a ruptured cruciate ligament and had to abruptly end his career.
In the years afterwards he turned out for SG Wattenscheid, after which his former coach, Werner Biskup, convinced him to make a comeback with VfL Osnabrück.
Hartmann, however, remained loyal to football following the end of his professional career: After a further stop at FC 1975 Damla Genc, he became player manager at his home club TSV Havelse between 1998 and 2000.
In the first and second Bundesliga he played a total of 206 games wearing the famous red shirt – scoring 53 goals.