Hi Guys thanks for the replies. The Xtool has been very handy with both the LX and my BMW's and as yet haven't come across anything it hasn't been able to scan and clear in a general sense and the live data feed seemed to be very comprehensive. Correctly the underbody is very clean on my vehicle as it was a highway queen for most it's life so far but in 19 years of service records I haven't seen any springs changed or torsion bar work. So if I am understanding correctly as the suspension sags the system applies more pressure to compensate? My recent experience with my 740li on the same crap roads (gravel and poor quality bitumen) was exceptionally good and the LX has seemed to get worse in the last few months on the same roads and especially the corrugated gravel. Minor weeping on one front shock absorber is the only obvious fault. A special shout out to Indrocruise from the best state in Australia to the second best state
First, kudos to
@LJE who has summarised the situation accurately and succinctly.
Meanwhile, I had been tapping out the following long-winded notes in between watching TV movies -- and now that they are done, I may as well offer these notes as well ….
Wow!! 19 years and 520,000 kilometres (say 323,000 miles) is a great testament to the longevity of the AHC/TEMS system, LX470/LC100 vehicles and the “100 series” generally.
On comments and questions by
@westaussielx470 …..
“minor weep on front shocks”
This is unsurprising, actually inevitable, given the very high AHC pressures now measured (Front: 9.4Mpa and Rear 7.5Mpa) and which probably have been that way for a long time. Have bars or sliders or a winch or rooftop tent or something else been added to the vehicle?? Anyway, there is a very good chance that correcting the AHC pressures will cure the leak, unless the internal seals inside the ‘shock absorbers’ really are permanently damaged. On the AHC/TEMS-equipped vehicles the so-called ‘shock absorbers’ are no more than hydraulic struts which carry a share of the vehicle mass and are used to manage the ride height of the vehicle, in conjunction with the ECU and the Height Control Sensors (two in the Front, one at the Rear). AHC/TEMS ‘shock absorbers’ have no damping function -- damping is regulated in real time by the ECU and the Damping Force Control Actuators to which the ‘globes’ are attached.
See diagrams and more information in the “AHC/TEMS General Description” document attached to my previous post #12 in this thread.
(In my own case, I learnt this by experience the hard way!! Years ago I added a steel ARB Deluxe Front Bar and ARB Intensity Driving Lights, probably not much change out of an additional 80 kilograms at the Front. THEN I unnecessarily replaced the overloaded, suddenly weeping, over-pressure Front ‘shock absorbers’ BEFORE I discovered the knowledge and wisdom available in in IH8MUD, initially from
@uHu and
@PADDO , then many others after that. To ‘give back’, I have tried to be a contributor since then).
In addition to internal seals suffering from over-pressure, there is another caveat concerning what should be the ‘life-of-vehicle’ lifetime of AHC/TEMS ‘shock absorbers’ alias hydraulic struts (provided not rusted out). Unlike conventional shock absorbers, these struts carry a share of the mass of the vehicle 100% of the time, even including when the vehicle is at rest! Unlike conventional shock absorbers, the AHC/TEMS units are part of the ‘vehicle support system’ and are not dampers. The consequence is that the top and bottom bushes work harder and may break up more quickly over the years than bushes on conventional shock absorbers which respond to movement forces but are not static weight-bearing. Disconnecting the units and replacing the bushes on the AHC/TEMS Front ‘shock absorbers’ is not too difficult on a DYI basis, but removing the AHC/TEMS Rear ‘shock absorbers’ to replace the upper bush can be a PITA job!
“when on corrugated gravels or very poor sealed roads the ride becomes pretty harsh”
“the LX has seemed to get worse in the last few months on the same roads and especially the corrugated gravel”.
This is not how things should be. I find that ‘my’ 2006 LC100 Sahara with AHC/TEMS (now in the hands of my son-in-law in Western Australia) is MUCH better on corrugations and on rough winding trails, and in turns, and in reduction of roll, squat and dive anywhere, than my 2020 LC200 Sahara with KDSS and ARB BP-51 adjustable shock absorbers. This was tested most recently on the Plenty Highway, the Great Central Road, the Strzelecki Track, and various trails in the North Flinders Ranges and elsewhere across Australia in a Brisbane>Perth>Brisbane round trip of ~10,000 kilometres (~6,200 miles) in 2024. True -- the 19 years old LC100 would benefit from new bushes all round!
Better than stopping and fiddling with the C-spanner on the BP-51’s on an LC200, you should be able to feel by seat-of-the-pants the difference in a AHC/TEMS-equipped LC100/LX470 when using the four-position “comfort” switch on the central console. If the differences are not discernible and the ride quality is harsh, then that is telling you that either
- stiff multi-ply Light Truck tyres at high pressures (eg BFG KO2 “E” rated or similar, at 40psi) are in use, or,
- the AHC pressures are too high (as is now known in your case) – this impairs damping performance and system response, or,
- some or all ‘globes’ are past their ‘use-by’ date and have lost nitrogen pressure (highly likely, almost inevitable, if the ‘globes’ are 19 years old) – this also impairs damping performance and system response, or,
- various bushes in the linkages throughout the mechanical suspension system, Front and Rear, are past their ‘use-by’ date, or,
- ball-joints and or wheel bearings are in need adjustment or replacement, or,
- there is an AHC/TEMS fault which has caused the ECU to place these systems in the designed and protective “fail safe function” which prohibits variable damping by TEMS (and usually prohibits AHC operation as well). Has Xtool D7 recealed any DTC's?? There are multiple possible causes but ancient Height Control Sensors are a cause for suspicion -- and no, absence of a Height Control Sensor Diagnostic Trouble Code (DTC) on any scanner DOES NOT provide assurance that these Sensors are healthy, or,
- some or all of the above -- and the recent change in ride quality as reported may be due to an ancient ‘globe’ finally giving up -- or it may mean that a Height Control Sensor (or its circuit) has developed a fault -- or it may mean that some other fault has developed -- such as breaks in wiring, water ingress in connectors – hopefully not!! Anyway, best to start at the top of this list with simple, basic things first, then proceed systematically.
“So if I am understanding correctly as the suspension sags the system applies more pressure to compensate?”
Yes. As the mechanical parts of the suspension age and tend to sag, or, if additional load is added to the vehicle, and also if vehicle movements cause the ride height to reduce while the vehicle is in motion in real time, then the ECU directs the AHC Pump to increase AHC pressure, causing the vehicle to rise until the voltage signal received by the ECU from the Height Control Sensors matches the correct voltage (2.25 volts) for “N” height, and the vehicle is returned to the set ride height for “N”, +/- 5mm per FSM – see new attachment. (Or 4.05 volts for “HI” height or 0.45 volts for “LO” height if either has been selected at the centre console switch. The height
differences for “HI” and “LO” are permanently set in the ECU).
Note: Most scanners do not give Height Control Sensor readings in volts but instead display in inches or millimetres. At “N” height, the display in inches or millimetres should be as close to zero as possible -- FSM tolerance +/-5mm or +/-0.2inches.
If movements have caused the vehicle to become too high for the “N” height setting, then the ECU will cause the Levelling Valves in the Control Valve Assembly to open and allow AHC Fluid to flow back to the AHC Tank, and the vehicle lowers until the ECU receives the correct voltage signal for “N” height from the Height Control Sensors (ditto for “HI” or “LO” if selected).
This feedback loop and matching of voltage signals and system responses is how the
Active Height Control (AHC) system works in real time to
‘actively’ maintain “N” height (if selected), all of the time the vehicle is operating (unless there is a fault condition). This is the fundamental design principle used in this auto height control system, first used in the Toyota Soarer circa 1983, before Lexus was invented. The ability to choose “LO” height or “HI” height was/is an added feature included when the system was adapted for use in the LC100/LX470 models circa 1998 (not available in all markets for LC100). The
Toyota Electronic Modulated Suspension (TEMS) system -- nowadays called AVS in later models -- also emerged in 1983 as the precursor to the AHC system. Both systems have evolved alongside each other successively in LX470, LX570 and LX600.
The last few pages of the “AHC/TEMS General Description” document attached to my previous post #12 in this thread show how the hydraulic circuits actually work in these systems on LC100/LX470.
All good in theory unless the AHC system is overloaded -- which means accounting for the weight of the factory-delivered vehicle
plus the weight of everything in or on the vehicle -- accessories, fittings, fuel, other fluids, persons, baggage -- everything. The FSM offers the following guidance:
The characteristics of the AHC/TEMS systems themselves are set by Toyota/Lexus and are not adjustable or reprogrammable -- or at least not by mere mortals like me! Instead, if significant permanent additional weight is to be carried, the idea at the Front is to compensate for this with additional pre-load at the Torsion Bars by winding the adjusters clockwise (as seen from below), possibly re-indexing if there is insufficient thread at the adjuster bolts, or in the extreme, by upgrading to higher spring rate torsion bars. At the Rear Coil Springs, packers sometimes are advocated but often are insufficient. A better approach is to replace the ancient OEM Rear Coil Springs with higher spring rate Australian-made King KTRS-79 Coil Springs:
In all of this it is important in Australia not to exceed the Gross Vehicle Mass (GVM) limit of 3,260kg (7,188 pounds) for LC100/LX470 (LC105 is different) so as to avoid regulatory, legal and insurance repercussions of having an overweight vehicle deemed unroadworthy and consequently illegal to use on public roads. A GVM Upgrade would need a lot of re-engineering and statutory re-certification on a AHC/TEMS-equiped vehicle and probably is not feasible.
Also attached is the Electrical Wiring Diagram for the AHC/TEMS systems -- hopefully not yet required -- but may be worth adding to the file with the documents previously supplied post #12 in this thread.
“A special shout out to Indrocruise from the best state in Australia to the second best state”.
Thank you for the acknowledgement -- just to have a ‘bet each way’, I can disclose that I was born in Derby in the Kimberley region in the North-West of Western Australia, too many years ago to reveal, now resident in Brisbane in Queensland on the East Coast!!
Some context for others:
