Typical problems of an Allison transmission include abnormal shifts, abnormal stall check speeds, slipping of the clutch and abnormal responses. Other problems include the transmission throwing fluid out of the fill tube, excessive engine flares, intermittent noises and the retarder not functioning properly.
Abnormal shifts in an Allison transmission happen during shifting at high and low speeds. At high shifting, this problem may be caused by a malfunctioning governor valve, leaking vacuum hose or broken mechanical actuator cable. A loose valve spring adjustment or stuck modulator valve causes problems during low and rough shifting. Leaking oil in the converter housing originates from a defective real oil seal, spoiled input charging pump or cracked converter assembly weld.
A defective clutch, leaking clutching pistons and transmission fluid leaking past the control valve are the probable causes of the clutch slipping and chattering in only one range. Low fluid levels, broken seal rings on the front support hub and open lubrication valves cause slipping of the clutch in all forward ranges. If the shift selector linkage is unhooked, it may cause abnormal responses in the transmission. Failure to engage the shift selector at the selector valve also causes unresponsive movement of the selector lever.
High transmission fluid levels and a failed vacuum modulator may throw fluid out in an Allison transmission. A loose dipstick, clogged breather, improper dipstick marking and water in the transmission are also probable causes of the problem. Weak trimmer valves and leaking forward clutch seals may cause excessive engine flares. Lack of air supply, leaking air lines, malfunctioning check valves and a sticking pressure regulator valve are probable causes of a faulty retarder.