Four wheel drive not working

I needed to use 4wd today in my series 3, but found it's not engaging - yellow lever isn't staying down, and I've tried driving in low range, but 4wd still doesn't engage.
After looking around online, I've stripped the floor out to get to the front of the transfer box. I've removed three bolts that secure the front cover, but something is still holding it.
Do I need to remove the 4wd locking pin to be able to take the cover off?
Maybe there is something else that I've missed, as I've been working outside with a lamp, and it's not ideal
If anyone can advise I'd appreciate it very much.

is the red leaver all the way forward ..make shure it isnt fowling the floor ...

yes ,pin needs to be removed buy undoing pivot bolt and removing yellow knob linkage ,you will see 3 shafts they have a pivot inside which can come out of sync which would require taking complete housing off to reset ,but might be one shaft stuck in housing oil and hammer might cure , the selector for 4wd is connected with springs so not as easy to unjam as hi/low

Do you have free wheeling hubs ?????? ................

Thanks for the advice, I hope it's fixed.
Red lever wasn't fouling.
I removed dust cover enough to get to selector shafts, smothered them with oil and managed to get them moving, let them soak for 1/2 hour, after which I pushed them in and let them slide out under spring pressure several times until they seemed to run free.
When I put it all back together, on pushing yellow lever down, it returns slightly, but when the red lever is pulled back to low range, yellow lever releases by about 1 inch, which I think is how it should be?

Yes, I do have free wheeling hubs fitted.
Am I right in thinking that if I jack up one front wheel, if it won't turn it's in 4wd, and if it turns it's not?

scorpion king said:

Yes, I do have free wheeling hubs fitted.
Am I right in thinking that if I jack up one front wheel, if it won't turn it's in 4wd, and if it turns it's not?

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Correct , but it wont matter whether its in 4 wheel drive on the levers - you have to get out and turn the centres of your free wheeling hubs to the ' LOCK ' position . If you dont then your still running in 2 wheel drive ( even tho your in x4 on the levers ) .