As we all know the V6 7 speed transmission is calibrated so that it upshifts at approx 1500 rpm when in 1st in manual mode, which is frustrating and it also has the unsafe behaviour of up-shifting when descending down hills. I have found you can fix this by implementing a work around that allows 4 low to be selected and manual mode to work as it should.
Basically the Navara’s have an electric transfer actuator device , which is a rotary actuator that rotates the shift rod inside the transfer case. There is a DC 12V motor and plastic gearbox arrangement that allows the shift rod to rotate through ¾ of a turn total travel, from 2H to 4LOW.
When the transfer case is in 2H the slot in the shift rod is in a vertical position. When the transfer case shifts to 4H it does so by rotating the shift rod ¼ of a turn clockwise. When the transfer case shifts to 4LOW it rotates another ½ of a turn clockwise.
On the transfer case actuator housing there is a connector which has 7 wires. The orange wire (pin23) goes to one of the motor terminals and the grey wire (pin24) goes to the other terminal.
When 12V +ve is applied to the grey wire and 12V gnd to the orange wire the transfer case actuator moves from 2H to 4H to 4LOW. (which takes less than 3 seconds, and should only occur when the engine is idling, the transmission is in neutral and your foot is on the brake)
Similarly when when 12V +ve is applied to the orange wire and 12V gnd to the grey wire the transfer case actuator moves back from 4LOW to 4H to 2H. (which again takes less than 3 seconds)
So what I have done is to cut the orange and grey wires and run wires up into the cabin, and fitted momentary switches so that I can change the motor polarity and manually shift the transfer case from 2H to 4LOW or back from 4LOW to 2H. (This works because the shift rod has fixed mechanical stops at each end of travel so that the motor stalls when it reaches either end of the ¾ range of motion. )
Effectively this is exactly the same as manually pulling on a transfer case lever for a 4WD that has one like a Patrol or D22.
Basically you never touch the 4WD selector on the dash from the 2H position as you are just applying electrical power to the transfer case actuator to initiate a manual change without the computer knowing.
So the procedure is:
drive to a 4WD track, and when you want to shift to 4LOW.
Follow these steps to select 4LOW
1. Let engine idle
2. Shift the transmission to neutral position
3. Put your foot on the brake.
4. Press momentary switches for 3 seconds only, (with the polarity 12V+ve on grey wire This will select 4LOW)
Drive around in manual mode or auto and everything behaves just how it should.
When you want to go back to 2H
Follow these steps to select 2H
1. Let engine idle
2. Shift the transmission to neutral position
3. Put your foot on the brake.
4. Press momentary switches for 3 seconds only with the polarity 12V+ve on orange wire (This will select 2H)
I have been using this for several months and I will soon be adding a couple more switches so that I can set everything back to std with a flip of a switch inside the cab, (which is good as this is the only way you can use 4H mode.) or press momentary switches and get a true low range manual mode where I can sit on 4000rpm and the transmission will stay in first gear if desired.
See pics below.