Try helping it along as it attempts to extend. If that works, perhaps it just needs a good cleaning and lube.

May be worth taking it apart to get the gears and 'mast cord' lubed. Could pay off down the road.

The drive belt and mast are actually available and replaceable parts.

Mine had similar symptoms, and I found that a plastic part that engaged the spring had snapped. I epoxied it back together.

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1986 300E 5-Speed 240k mi.