LT95 vacuum diff lock

[dave88sw]

Hi,

I'm trying to get the vacuum diff lock working on an LT95.  The background is, the vehicle has been fitted with a 6.2 GMC V8 (wouldn't be my first choice but it runs).  When i acquired the car there were no pipes attached to the diaphragm unit on the box and no switch.  I have no idea of the condition of the diaphragm on the box.  There are 2 pipe stubs on it, one in front and one behind, i connected each one to a vacuum source and nothing happened, i then tried sucking on them myself and there is no resistance felt on either so i concluded the diaphragm is probably split.  So i bought a brand new diaphragm assembly from Land Rover (over £100 ).  Now with the diaphragm off the car, if i suck on the front pipe, the selector shaft (integral part of the unit) pulls in, if i suck on the rear port, nothing happens, there's no resistance.  If i blow on the front port the shaft pops back out.

This makes no sense to me because i was led to believe the vacuum should be switched from one port to the other to make it go in/out of diff lock and that the original switch does just that, it should have vacuum in on one port and switches the vacuum between the 2 other ports.  Have i got a duff diaphragm?  Am i testing it wrong? I don't want to pull the selector off the box unless i know i have a good diaphragm to swap in.

Thanks

Dave

[disco2hse]

When I had to replace the vacuum switch for the CDL, I replaced the worn out switch on my 109 with a five-way vacuum switch because the new OEM switch was too expensive. One of the things I learned was that vacuum switches work very differently to pressure switches. For the switch to work properly, you need a vacuum (negative pressure) on both sides. The switch itself also needs an open "vent". The switch effectively pulls the valve one way, then the other. So, it won't work properly until the switch itself has been plumbed in correctly.

The picture shows the directional vacuum pulls required.

This is how it looks plumbed in. Note the air filter for the external air "vent". This stops crud from getting sucked into the switch. The switch engages the lock when the trigger is up and disengages when it is down.

 

Edited November 22, 2016 by disco2hse

[dave88sw]

Thanks very much for your reply, that's a great idea to replace the expensive original switch.

I'm still a little confused, am i right in saying that in the second picture, the black pipe on the left is the vacuum line from the engine and the right hand 2 pipes each connect to one of the ports on the actuator?

The switch then alters which of the 2 right hand pipes sees the vacuum from the engine and vents the other to atmosphere?  Is this not what i'm doing when i suck on the ports one at a time?  I can't quite get my head around why it moves in one direction when i suck on one port but doesn't move back when i suck on the other.

 

Thanks for your help, sorry if i'm being a bit thick.

[disco2hse]

Yes, IIRC that is right, the black is the engine vacuum line and the clear and red suck left and right. To be honest I am not quite certain either. 

[Eightpot]

The vac system is pretty simple and just sucks the diaphragm either way. 

The switch just has a sliding plate with a couple of holes in it - there is always vacuum and moving the button either way just slides the plate moving the holes till they hide/expose the relevant vac tube to the actuator.