The training vehicle was on the hard shoulder of the northbound carriageway, between Junction 10 for Walsall and 10a for the M54.
No one was injured and there were no passengers on board. Twitter users posted images of the smoke, which was visible from several miles away, and the aftermath of the fierce blaze.
The fire, which happened at around 2pm on Saturday, lead to delays on the busy stretch as smoke started billowing out and could be seen across Essington, Bloxwich and New Invention.
Despite the on-going strikes the fire service was sent to the scene.