Jackhammer is one of the few brave enough or stupid enough to willingly remain on Cybertron during the threat of the grand Black Hole caused by the destruction of Unicron. He appears to be the brains between him and Crush Groove, but this seems like a fairly low bar.
Jackhammer's alternate mode in unknown; he has hints of perhaps a walking vehicle similar to a Scrapmetal, though circular objects on the backs of his legs might be wheels.
Fun Publications Cybertron Fan Club Comic
With Cybertron largely evacuated, Jackahmmer and his friend Crush Groove had moved into Maccadam's Old Oil House, stockpiling a mass of Energon cubes and high-grade oil that had been left behind. The two were sitting pretty until they were approached by Ramjet and Nemesis Prime, who "tested" the two to determine if they would become warrior-slaves to Unicron.
Five seconds later, both Decepticons were dead, and Ramjet and Nemesis Prime moved on.