ZF 4HP-18: Differential
WARNING: This page does not describe the selected car, but rather 16 other vehicles, including the 1990 Plymouth Sundance, 1990 Plymouth Laser, 1990 Plymouth Horizon, 1990 Plymouth Acclaim, and 1990 Eagle Talon. However, it is still accessible from the selected car via links, so may be relevant.
With vehicle on level surface, fluid level should be at the bottom of fill plug hole. See Fig 1.