HELP! L4 not engaging BJ73 Cruiser

[Bruiser70]

Help needed recent outing low range wont engage it will engage H4 off the button but no luck with L4.

Any ideas/advice would be much appreciated ive tried few things but no solution found yet.

Truck is a BJ73 MWB cruiser

[Bruiser70]

Ok update after pulling all vacuum lines off solenoids and tracing vacuum feed back to transfer case diaphragm i decided it was a leak in diaphragm and pulled the unit off cleaned checked and re installed etc etc! After no luck started on wiring and quickly discovered it was most probably the low range sender behind leaver input. Pulled sender unit out pulled apart and dirty contact points the culprit!!! So end result is easy fix shame i didnt think about it before pulling the vacuum system apart!

[bang-thud-thump]

It would have been the vacuum if you started at the other end anyway