The 280DP was introduced in 1983.
Gear oil would have been GL-5.
If not a cable/linkage adjustment...... here are several things that could prevent FWD gear engagement:
a.... Recent switch to synthetic gear oil.
b.... Metallic debris in the sliding sleeve oiling grooves.
c.... Glazed gear cup and/or sliding sleeve surfaces.
d.... Broken shift shoe.
If a, b or c, the transmission will need to be disassembled so that the gear cup and sliding sleeve can be cleaned and lapped.
If d, drain the gear oil and pull the shift mechanism housing off and inspect the shift shoe. This will cost you 3 new O-rings only.
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