Just after the start of Production on the 1990 W car, the inlet and outlet fittings to the evaporator were changed from a standard "O" ring design to a captured "O" ring design to reduce leaks. When making necessary repairs at the evaporator, if new "O" rings are needed make sure the correct captured design "O" rings are used.
The captured "O" ring part numbers are as follows:
1/2 Pipe "O" Ring - 52454645 3/4 Pipe "O" Ring - 52454646
The opposite end of these fittirigs are the standard "O" ring design.
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.