I’m not familiar with ISTA, but is it capable of monitoring and recording live values for solenoid currents, speeds, etc.? If so, what are the chances that you could capture your low engine speed 5th gear event?
On the L322 Range Rovers with the 6HP26X transmission, a piece of diagnostic kit called the GAP IIDTool can be used to record exactly what’s going on and generate .csv files with the captured data so you can produce graphs similar to the ones below showing, for example, the solenoid activity. This can be very helpful for analysing exactly what’s going on. It’s worth bearing in mind though that the transmission doesn’t have any pressure transducers so you’re only measuring solenoid current and having to assume that the pressure is following the same characteristic. Clearly this may not be the case if a solenoid is faulty.
Just a few points from me, in the unlikely event they’re helpful:
As you have already mentioned, in 5th gear all the EDS solenoids are electrically ‘off’ (well, more accurately, they’re at their minimum current of 0.048A). EDS 1 & EDS 3 (yellow cap solenoids) will therefore be producing almost no pressure and EDS 2 & EDS 4 (blue cap solenoids) will be producing maximum pressure (regulated to 5 bar). The B-clutch (controlled by EDS 2) and E-clutch (controlled by EDS 4) are therefore engaged.
The EDS solenoids only modulate the clutch pressures during the actual gearshifts. Once the oncoming gear is engaged the holding valve for that particular clutch latches on and the clutch then operates at mains system pressure, which is dictated by solenoid EDS 5 (blue cap). The EDS solenoid (1, 2, 3 or 4) for that particular gear is then either electrically ‘on’ (0.848A) or ‘off’ (0.048A) and has no influence on the pressure applied to the clutch pack.
EDS 6 (yellow cap solenoid) controls the application of the torque converter lock-up clutch, and it’s also worth monitoring what it is doing as well, because a fault with the LUC can also feel similar to one of the transmission clutches slipping.
Phil