4WD not engaging / transfer box playing up

Another one that turns up regularly: four wheel drive will not engage, engages intermittently, or the transfer box behaves oddly when selecting.

Owners in the club have traced this to wiring and switches around the transfer box more than once — corroded connectors, and in some cases a switch needing replacing — rather than the box itself being finished. Worth ruling out the cheap causes before anyone quotes you for the expensive one.

Useful detail when you post

• Model and year — K74, B40, Series 4, Series 5
• Is it not selecting at all, or selecting and not holding?
• Does the dash light flash, stay on, or do nothing?
• Manual lever or electronic dial?
• Anything changed recently — off-road trip, jet wash, winter salt?

The more of that you give, the fewer rounds of guessing everyone has to do.