I searched for Graduation Test... there are a lot of threads but not finding one that specifically the procedure for this test. Also looked in the FAQs but no luck.
Can someone point it out please?
Good practical description by
@TexLexican -- I have posted similar information previously but buried in multiple other posts, searching "Hi/Lo Test" would reveal.
The FSM guidance is a bit cryptic -- see attached -- same for LX470 and LC100 (where AHC is fitted).
It is hard to find because it is filed under "On Vehicle Inspection" for the Damping Force Control Actuators, with the heading INSPECT GAS CHAMBERS which is FSM-speak for 'inspect globes'.
The test simply measures the volume of fluid displaced by the height movement in the 'shock absorbers'
plus the push-back from the 'globes' at LO versus HI (AHC pressure is lowest at LO, because more weight is carried by compressed springs and torsion bars, less weight carried by AHC system).
Assuming that nothing is out of order, the displacement from the 'shock absorbers' is constant each time it is measured at "HI" and "LO", so long as the vehicle height at "N" is the same whenever/wherever the test is done. The test should be done on a level surface (think garage or similar, not in the street). Cross-levelling the Front should be done first. Front and Rear AHC pressures also should be in the correct ranges at "N" height for the test results to be comparable with the FSM recommendations. AHC Fluid should be in good condition -- if polluted with gas and/or moisture and/or debris, then readings can be unreliable, particularly if the AHC Fluid contains air, or nitrogen from failing 'globes'.
The idea is to repeat the test over time (say annually over 10 years) under the same conditions each time. Then it can be assumed that the displacement from the 'shock absorbers' is the same each time, and, the declining
differences in the number of "graduations" observed at the AHC Tank from time to time in each test is due to the declining push-back from the 'globes'.
Over time (years) progressively fewer "graduations" will be observed, as the 'globes' slowly lose their nitrogen pressure from behind the membranes. This also will cause damping and 'ride comfort' to deteriorate as the pressure in the 'globes' slowly declines.
Declining nitrogen pressure in the 'globes' means that damping (which is done by the Damping Force Control Actuators controlling the rate of fluid inflow and outflow) becomes less and less effective.
Note: The 'globes' by themselves are not 'shock absorbers' but actually are gas springs. The
'globes' combined with Damping Force Control Actuators are the actual shock absorbers in an AHC-equipped vehicle. The items called 'shock absorbers' in an AHC-equipped vehicle are simple hydraulic struts used to adjust the vehicle height in real time all the time -- the auto levelling function alias Active Height Control (AHC) -- as controlled by the ECU responding continuously to the Heght Control Sensors and the driver instructions via the height selection switch at the centre console, and deploying the AHC Pump and Levelling Valves as required.
When the 'globes' are new, this test under standard conditions should reveal a difference of around 14 graduations between HI and LO.
When the test result declines to a difference of 7 graduations between HI and LO under standard conditions, the FSM recommends replacement of all 'globes' to restore damping performance and 'ride comfort'.
The limitation of this simple test is that it gives an outcome indicating the condition of the four 'globes' as a whole. It does not distinguish between the individual 'globes'.
