Originally Posted by Firemanshort
Like all things Rover - this switch is stone-tool simple and primitive.
If you pull the three tubes off - and have the motor running - one of the tubes should be pulling a slight vacuum at all times. The other two are quite. Of the other two - they each go to different sides of a vacuum bellow attached to the center differential. This is located under the floor pan of the right side and can be viewed from below the trick or by taking the floor pan out.
So - step one is to check all of your vacuum tubes. They are all old and super brittle now if they are anything like mine. Make sure there are no breaks or cracks or leaks.
Step 2 is to test the switch. The switch works by joining the one vacuum inlet with one of the remaining two vacuum outlets. This is accomplished by a simple rectangular rubber block that slides back and forth inside the switch. I would put some short scrap hose on the switch and blow into on side and see it air comes out only one of the other holes. If both holes are plugged - then the air should stop and not leak.
If your switch is leaking then you need a new switch or the courage to carefully open your existing one. It is hard to explain but it can be opened by prying open some small metal tabs on the switch housing. Once it is open, you will see the small rubber block that slides back and forth. It is supposed to have a vacuum tight seal on the bottom of the housing. My rubber was old and slightly dry rotted. The sliding actuator was also worn allowing the block to not slide true and wiggle a little. I cleaned things up really well and coated the rubber block in petroleum jelly - think chapped lips and Chapstick. This sealed up the air leaks and sorted me out well.
New factory LT95 switched are very expensive if you can find one. Used ones are a crap shoot on if they will actually work or not.
McCar Master or other online stores sell a variety of air switches that could easily be adapted for this use. You would just need to find the right shape / size.
All of this assumes that the vacuum bellows are good and the the mechanical shifter works, too. If you are just going off the dash indicator light.- the switch goes bad for that, too. So it may be working but your light not turning on.
Thank you for this amazing and very extensive write up.
I know that the bellows under the truck works as does the indicator on the dash because when I swap the two tubes the diff will engage as it should.
I took the switch off prior to my initial post and attached an air tube to the vacuum inlet port. When I blew into it, air only came out one pipe. When I pulled the switch out and blew into it air came out from everywhere except the pipe. Sounds as if I may have chapped lips.
I was thinking about taking off the cover but wasn't sure about it. I saw the 4 little metal tabs that you were speaking of. If the switch is bad, I guess there isn't much to be lost by taking it apart and having a go.