80 Series Land Cruisers come with an electrically actuated centre diff lock, and depending on specification locking front and rear axle diffs too (my Japanese import HDJ81 had all three diff locks fitted).
On my truck the centre diff lock stopped working (which also stops the front/rear diff locks form engaging, as the system checks that the centre lock has engaged before actuating either of the axle locks). Here is some info on the system and how to go about troubleshooting it:-
The centre diff lock actuator is a motorised unit that sits on top of the transfer case and actuates a small gear via a worm and wheel gearbox driven by a permanent magnet motor. There is a clock spring arrangement to allow the motor to preload the gear until the lock can actually engage (sometimes you need to rotate the centre diff by driving the truck before the lock can physically engage) and limit switches so the gear only rotates a certain amount in either direction. The gear physically moves the diff lock mechanism within the transfer case. There is also a plunger switch fitted to the transfer case that engages when the diff is actually locked.
Here is an pic of the transfer case showing the actuator and the switch:-
There is just enough room to get to the actuator and switch by crawling under the truck and reaching up above the transfer case, but you have to work by feel as it is very difficult to see what you are doing!
The diff lock will normally operate automatically when selecting low range, or can be engaged from the ‘centre diff lock’ button on the dash if you have one. When the lock is engaged the yellow centre diff lock light in the instrument cluster will light up.
In this picture you can see the centre diff lock light and the button, together with the red lights for front and rear axle diff locks (picture is from a left hand drive truck):-
Failure Mode 1 – Faulty diff lock sensor/switch
If you can hear the actuator motor operate when you select low range, or press the button the the dash, but the yellow light on the dash does not come on, it is possible the sensor/switch has failed (make sure the bulb hasn’t gone in the dash first! – it should come on briefly during the ‘bulb check’ when you first turn the ignition on)
With the wiring disconnected from the switch you can unscrew the switch from the top of the transfer box. Using a multimeter or test lamp check for continuity between the two electrical contacts on the top of the switch, you should get continuity (a short circuit between the two contacts) when the switch plunger is pressed in. If there is no continuity you can sometimes get it working again by spraying contact cleaner, brake cleaner or similar into the plunger mechanism and actuating it a few times. If you can’t revive the switch it is at least easy to replace it for a new one!
Failure Mode 2 – Faulty actuator motor
If there is no action from the actuator at all one thing that can cause it is the magnets falling off the inside of the motor housing and jamming it up. Once you remove the actuator from the transfer box it is fairly easy to strip it down to check it out. There are four bolts holding the unit to the transfer box (you can see these in the picture at the top of this page) – it is a bit fiddly to get to these while the transfer box is still in the car but it just about possible to undo them, unplug the electrics and wiggle the whole unit out from above the transfer box. The unit is sealed onto the transfer box with instant gasket type material so it might need a bit of persuasion to come loose.
You can see the gasket (red material) in this picture:-
The motor is the lump sticking out of the right hand side of the unit in the picture above, unbolting the fasteners allows you to withdraw the housing. If you have a loose magnet in the housing it is easy to fix it back in place using two-part epoxy (it should be obvious where it has come from)
Failure mode 3 – Seized shaft
You may find that the motor is actuating but the gear is not rotating, this can happen if the unit isn’t used for a while. If the shaft has become seized the clock spring will allow the motor to move to both to both engaged and disengaged position without the actual gear moving. Stripping the unit down and cleaning/greasing the working can usually sort this out.
My unit had all of the faults above but following the steps above got everything working normally again for just the cost of a few consumables (grease etc.) Second-hand units are very expensive and may have the same problems as your own unit!
Once you have everything working again it is worth engaging all the diff locks every few months even if you don’t need them – this will help to stop them seizing up from lack of use. Make sure you only actuate them on a low friction surface (grass, mud etc,) to reduce the stress on the axles/locks.
Hopefully this info has been useful – any questions of comments let me know below.
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