Abstract: A warning device for the detection of a bearing malfunction in wheel assemblies of large commercial vehicles such as tractors and semi-trailers is disclosed. The warning device is preferably mounted on the stationary axle inside the wheel housing adjacent the brake mounting assembly to remain stationary. The device includes a probe which protrude outwardly so as to have a sensing end extend until it is a predetermined instance from the rotating brake drum, say 25-50 thousandths of an inch. As long as the brake drum is running true, the distance between the sensing end of the probe and the inner braking surface of the brake drum remains constant. If either wheel bearing becomes incapacitated for some reason which permits the brake drum to run out of true, the inner braking surface will contact the end of the sensing probe which activates a signalling device in the cab of the vehicle to alert the driver of the presence of a dangerous condition.