@suprarx7nut has it covered.
@joebelth8mud -- your described experience is normal and AHC pressure measurements are essential. Then adjustments can be made and your vehicle caused to perform in the desired way, without continuing to exceed the capability of the AHC system. Here is some more detail and colour ....
See the attachment (only 20 pages) for a general description and diagrams describing how the AHC and TEMS systems are designed to work on a stock vehicle -- the second page deals with weights etc, as extracted below:
View attachment 3167448
Catch 22! Most of us no longer have stock vehicles -- inevitably things are added -- maybe including some or all of:
front and/or rear bars, towbars, sliders, long range fuel or water tanks, drawer systems, additional rear wheels, fuel or water jerry-cans, allowance for ball weight of trailer, and/or whatever else.
All of these permanent additions reduce the effective 'payload' compared to stock configuration -- unless something is done to cope with additional loads compared to the stock limitations described in the attachment.
The performance characteristics of the actual AHC and TEMS systems themselves (as described in the attachment) cannot be varied -- or at least not without massive re-engineering and re-certification.
However, the 100 series vehicles with AHC/TEMS (meaning LX470, and LC100 where fitted) respond very well to compensations made to the mechanical suspension (meaning, adjusted or re-indexed torsion bars, stronger coil springs, supplements such as spacers, airbags etc). In this way, greater loads can be carried while the required duty of the AHC/TEMS systems remain unchanged -- provided that AHC Front and Rear pressures are kept within FSM-specified ranges -- and provided that there are no faults lurking in the system.
'Stock vehicle' also means a vehicle with ride heights set per Factory Service Manual (FSM) -- or to the widely used IH8MUD approximations, hub-to-fender, Front: 19.75 inches; Rear: 20.50 inches.
Your story mentions a 'sensor lift'. This means ride heights are higher than stock at the chosen ride height at "N" height setting. In turn, this means your AHC pressures are higher at your chosen "N" height setting. The higher ride height means less deflection at torsion bars and springs and means that the mechanical suspension is carrying less share of the vehicle weight than at stock ride height at "N". To achieve this, the AHC system adds pressure from the AHC Pump and locks this in behind the Levelling Valves in the Control Valve Assembly, after the vehicle has found the chosen "N" height which has been defined by the settings of the Height Control Sensors. This 'sensor lift' arrangement requires the AHC system to carry a correspondingly increased share of the vehicle weight -- and at a higher AHC pressure than would occur at a lower setting of ride height at "N".
It is worth reviewing the necessity of a permanent 'sensor lift' -- there is already height flexibility provided by the "HI" height setting, albeit with the speed restrictions mentioned in the attachment.
Anyway, the AHC pressures are all quite manageable. At the front, AHC pressure reduction involves simple adjustment at the torsion bar adjusters -- and/or re-indexing of the torsion bars if the vehicle is unusually heavy. At the rear, you already have new OEM springs and spacers. You could consider change-out to higher spring-rate rear springs such as KING KTRS-79 -- but as mentioned by
@suprarx7nut, airbags inserted inside the new rear coils are the ideal solution to control rear AHC pressures for occasional heavy rear loads and towing. Airbags can be pressurised to carry part of the rear load and allow the AHC system to function normally as designed. When not required the airbags are deflated to a nominal pressure (usually around ~5psi) to hold their shape without carrying any load. In my case, I added both KING KTRS-79 rear coils and airbags. For me, this may have been overkill -- airbags would have been sufficient for now -- but that might change if an ARB Rear Bar, drawers, long range tanks and/or trailer ball weight actually are added some day. However, these items certainly would challenge the 'weight budget'!
As an aside, there is another Catch22! in Australia -- for 100 series (IFS models) the vehicle must remain within the registered Gross Vehicle Mass (GVM) limit of 3,260kg (7,172 pounds) inclusive of EVERYTHING in or on the vehicle including people, baggage, fuel, food, drinks, accessories, build, trailer ball weight, and whatever else -- UNLESS an engineer-certified, Government-registered "GVM Upgrade" has been obtained. Such a Certified and Registered Upgrade is impossible with AHC suspension -- and so for heavy builds, many people here opt for a conversion to a conventional suspension. In our case, with only two people travelling, we can operate just inside the vehicle GVM limit and retain the AHC/TEMS convenience. We prefer the much better comfort and road-holding provided by AHC/TEMS on long distance cruising. There may be more flexibility in other countries. Does Gross Vehicle Weight Rating (GVWR) cause similar restrictions in USA??
A point to note and as emphasised by
@suprarx7nut -- on AHC/TEMS-equipped vehicles, best if AHC pressures are
measured and managed within FSM-specified ranges
(neither too high nor too low) for two reasons:
1.
Active Height Control (AHC) system works properly, and,
2. Real time active damping performance in 16 steps via
Toyota Electronic Modulated Suspension (TEMS) is optimal and ride comfort is best. Damping can be poor (causing ride comfort to be poor) if AHC pressures are out of range because then the Damping Force Control Actuators (to which the 'globes' are attached) cannot work properly as designed. Excessive AHC pressures eventually will challenge AHC fluid seals especially in the 'shock absorbers', and possibly at valves elsewhere in the system.
If not already seen, in addition to the attachment, the following link is suggested reading. This thread has now grown since commenced by
@LndXrsr. It contains a lot of good information -- suggest skim the first couple of pages to get the gist, pause at post #5 and view the video and Cheat Sheet provided by
@suprarx7nut, then read on:
I initially put this in the Definitive list of AHC maintenance items thread, but at almost 800 posts long and growing, it's going to get buried and the thread itself is very unwieldy. In an attempt to make as focused a summary as possible, I present "The ABCs of AHC." It's intended as a...
forum.ih8mud.com
The pics below illustrate some of the above points. The airbags shown come from
Airbag Man - https://airbagman.com.au/. These basically are a US-sourced Firestone product. Similar products and arrangements would exist in other countries.
The airbags could be connected to an on-board or portable compressor -- but the air volumes are not large and maximum pressure is ~30psi. Realistically and practically, experience shows that a bicycle tyre pump and a hand-held low-pressure gauge are all that is required.
View attachment 3167477
View attachment 3167478
