Good morning, I'm having more AHC problems on my 07 LX 740. Front end is bouncier than usual but not unbearable. If parked for a few days the front end will slowly sink down like a leaky air suspension, upon startup it raises up quickly and maintains correct height. No errors on dash. Reading some posts on here I looked for external leaks at the shocks and checked my pressures and all was good there. My thoughts are if pressures and heights are correct a rough ride means new globes, right? Nowhere have I seen that globes can be the cause of suspension dropping overnight. Am I dealing with two separate issues here? Trying to figure this out on my own but don't want to waste money throwing the "parts cannon" at it. Thanks in advance.
If the Front of the vehicle is dropping in the manner described (or the Rear for that matter), it means that the AHC Fluid in the ‘shock absorbers’ (really hydraulic struts on an AHC vehicle) is going ‘somewhere’.
There are only three places for the fluid to go:
1. Fluid returns back to the AHC Tank (and the fluid level in the tank rises as the vehicle drops) – important to check this because if the vehicle is dropping when stationary and with engine OFF for a long period as described
AND if the fluid level in the AHC Tank is rising at the same time, it would mean that one or other of the two Front Levelling Valves in the Control Valve Assembly are leaking and bypassing fluid back to the tank.
This seems very unlikely in a relatively young 2007 vehicle -- but can be a possibility if dirty, never-changed or rarely changed AHC Fluid has caused a blockage in one of these valves -- yet another example of the importance of good ‘hydraulic hygiene’ and following the Owners Manual direction to change out AHC Fluid at the earliest of each 60,000 miles or each 6 years (or better still, twice as often),
OR
2. Fluid is escaping out of the vehicle (and the fluid level in the AHC Tank is observed to be noticeably lower) -- a thorough inspection is necessary of
ALL pipe joints throughout the system for leakage, not just the ‘shock absorbers’ –
this also seems unlikely in a relatively young 2007 vehicle -- but can be a possibility where rust has affected AHC lines or joints,
OR
3. Fluid is moving into the space formerly occupied by nitrogen which has escaped from one or more failing ‘globes’ (and the fluid level in the AHC Tank is observed to be noticeably lower) -- if the ‘globes’ are original in a 17 years old 2007 vehicle, then it is more likely than not that they are way past their best. It is inevitable (over say 10+ years, can be longer or shorter) that fluid will slowly pass through the membrane in each ‘globe’ and equally inevitable that the nitrogen space then will be occupied by AHC Fluid under pressure and also equally inevitable that damping performance will deteriorate. In old ‘globes’ in the situation described (vehicle drops slowly when left parked for an extended period), nitrogen is escaping from ‘globes’ quite quickly and the vehicle drops -- then on start-up the system responds to the now-incorrect position of the Height Control Sensors, meaning that the AHC Pump starts, adds pressure to the system and raises vehicle to “N” height (if “N” has been selected). Eventually, when the ‘globes’ have lost ALL nitrogen (and this may happen quickly when membranes ultimately tear or rupture with old age and repeated use) and the suspension will then be solid and springy with very little if any damping.
Alternative #1 and #2 above are obvious and easy points for basic inspection and should be done anyway.
May I suggest that, whether this is the first time this has been mentioned or not, Alternative #3 is by far the most likely cause of the observed symptoms (which are not unique) and would require replacement of all four ‘globes’.
Some indication of the overall condition of the four 'globes' can be obtained from the “HI/LO Test" -- by observing the difference in AHC Fluid levels in the AHC Tank at “LO” height (most fluid in the tank) and “HI” height (least fluid in the tank) -- a difference of around 14 graduations marked on the side of the tank indicates new ‘globes’; a difference of around 7 graduations is the FSM prescription for changing out all four ‘globes’. Be aware that, this test aims to give an indication of the AHC Fluid displaced by the 'globes' between "LO" height (when AHC pressure is lowest) and "HI" height when AHC pressure is highest) -- and as noted in the FSM, the FSM-quoted readings
only apply when the Front and Rear AHC pressures at "N" height are set to the FSM-specified ranges. In other circumstances, this test is misleading.
Also be aware that while correct Front and Rear AHC pressures are vitally important for all sorts of reasons,
AHC pressures give no indication whatsover about the internal condition of the 'globes'. AHC pressures relate only to the share of the vehicle weight carried by the springs/torsion bars and the share of the vehicle weight carried by the 'shock absorbers' (actually hydraulic struts) in the AHC system. For the same vehicle weight distribition and the same conditions/settings of the springs/torsion bars, the Front and Rear AHC pressures will remain same regardless of whether the 'globes' are new and fully charged with nitrogen, or, completely 'flat' (all nitrogen charge lost).
Here are a few pictures which may help ....
AHC/TEMS hydraulic circuit with vehicle at rest -- in this condition and assuming the 'globes' are healthy, the AHC Fluid is "locked" behind the closed Levelling Valves unless and until release is caused by the ECU or by opening a bleeder valve, including the fluid in the 'shock absorbers'. Fluid movement requires either failed and leaky Levelling Valve(s) OR leakage from pipes and joints downstream from the Levelling Valves OR fluid moving from the 'shock absorbers' into the space behind the membrane(s) previously occupied by nitrogen gas:
Leakage points:
Details of 'globes' a.k.a. 'spheres' a.k.a. 'accumulators' a.k.a. 'gas chambers:
