The 80-series after around August 1992 introduced a viscous coupler over the centre differential. In order that this coupler doesn’t get stressed when in low range, Toyota deleted the manual centre diff lock switch and instead linked its operation to the transfer lever, locking the diff when in low range and opening it in neutral or high.
While this protects the transmission from erroneous usage, it does rather limit options, for example using low range when maneuvering trailers on hard surfaces etc. This needs to be rectified.
In order to bring the CDL back under driver control, two things are needed: Firstly a centre diff lock switch, and secondly a way of preventing the centre diff lock control ECU from knowing when it’s in low range. Rather than explain the process in extreme detail, Slee Off Road have a brilliant how-to here. This procedure is known generally as the Pin-7 mod, as it’s pin 7 on the centre diff lock ECU connector that carries the “low range engaged” signal.
Toyota conveniently left the centre diff lock switch loom tied back under the dash, so all that is required is to fit the new switch, connect the loom, and disconnect pin 7 from the transmission control ECU.
Once done, the CDL can be engaged and disengaged manually in high, neutral or low. Simples!
One other thing to do is to modify the ECT (main gearbox) ECU to allow the shift pattern to be switched between the factory-intended low range pattern (which is absolutely nuts – the engine revs to 3500 RPM before shifts occur, at which point they near enough break your neck due to the ferocity). This mod can be accomplished in a similar manner to the CDL mod in that a pin to the ECT ECU needs interrupting, however I haven’t yet got aroudn to digging the ECU out from under the dash…