AHC LO While Towing (1 Viewer)

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Sep 12, 2022
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AL
I bought a well maintained 100 series over the summer. I’ve replaced the accumulators/globes with new OEM as well as the rear springs with new OEM. The rear springs also have Slee 30mm spacers. (This truck does have a sensor lift w/ diff drop).

I’ve noticed that with a full tank of fuel, towing a 16 foot airstream, 3000 lb, 450lb tongue weight, the AHC will indicate LO. It will also do this when I have 7 adults on board (not towing).

I know LO indicates the AHC is letting the springs carry more of the weight vs typical. Do others experience the same? I was hoping the new springs and spacers would keep the AHC happy.
 
Have you measured pressures? You need to.

Yes, it's common for AHC to go into low (or at least indicate LO on the dash) when you put a lot of tongue weight on the car. The AHC system when properly setup can handle nearly 1000 lbs over the rear axle and still maintain a totally normal ride height in N. If you have not checked and adjusted pressures then it's likely your system is constantly strained running too much pressure to maintain normal heights. When you add a trailer or 7 friends, you overcome the AHC limits and it sinks a bit - just like any normal car would under those loads.

Check and adjust pressures.

If after that you still have occasional forced "LO" situations, you should add air bags within the rear springs. The air bags act as helper springs and can increase the dynamic weight you can carry.
 
I bought a well maintained 100 series over the summer. I’ve replaced the accumulators/globes with new OEM as well as the rear springs with new OEM. The rear springs also have Slee 30mm spacers. (This truck does have a sensor lift w/ diff drop).

I’ve noticed that with a full tank of fuel, towing a 16 foot airstream, 3000 lb, 450lb tongue weight, the AHC will indicate LO. It will also do this when I have 7 adults on board (not towing).

I know LO indicates the AHC is letting the springs carry more of the weight vs typical. Do others experience the same? I was hoping the new springs and spacers would keep the AHC happy.

@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:

LC100-LX470 AHC Load Limits.jpg


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:


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.

AHC with Airbag Assist and KING KTRS-79 coils.jpg


LC100 Rear Arrangement.jpg
 

Attachments

  • AHC-TEMS for LX470 and LC100 - General Description and Diagrams.pdf
    319.4 KB · Views: 31
Last edited:
I tow a 4,000 lb RV. Never had the AHC light go into Low.
But, I do use a Weight Distribution Hitch (WDH).
Do you tow your Airstream with a WDH?
 
@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:


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
First, I definitely did not deserve such a lengthy and detailed reply. Amazing people here. Thank you.

I took a couple photos of the Techstream AHC readings with a full tank of fuel and no driver. I cycled LO to HI on level ground several places around the neighborhood. Pressures are under 7 both front and rear. They seem pretty decent.

Everything seems pretty healthy other than “LO” being lit when the trailer is attached. Anyone spot something I don’t? (Note: I bought this truck with a sensor lift, so the numbers may not be textbook.)

44D32654-21A7-452F-9C10-1891ABA85CCF.jpeg


4E2604DA-8B45-48BB-B9C2-E2B09BD17AF6.jpeg
 
I tow a 4,000 lb RV. Never had the AHC light go into Low.
But, I do use a Weight Distribution Hitch (WDH).
Do you tow your Airstream with a WDH?
I do not. I’m thinking this hitch may be acting like a lever.

E2510246-A6DB-4A57-8054-BFF78D576AC5.jpeg
 
First, I definitely did not deserve such a lengthy and detailed reply. Amazing people here. Thank you.

I took a couple photos of the Techstream AHC readings with a full tank of fuel and no driver. I cycled LO to HI on level ground several places around the neighborhood. Pressures are under 7 both front and rear. They seem pretty decent.

Everything seems pretty healthy other than “LO” being lit when the trailer is attached. Anyone spot something I don’t? (Note: I bought this truck with a sensor lift, so the numbers may not be textbook.)

View attachment 3167656

View attachment 3167657

Requesting a little more physical data if you are willing to measure and post??

. . hub to fender tape-measurements at each wheel with vehicle on level ground without trailer, and,

.. ditto with trailer added (but no other loads).

The purpose of these two data sets is to check whether physical tape-measured heights and the Height Control Sensor readings are consistent with one another.

Suspicious of the difference between FL and FR Sensor readings. Absence of a DTC does not provide assurance of healthy Height Control Sensors -- for reasons posted elsewhere previously.

Also relevant to see data showing what the front and rear are doing physically w,hen the trailer is added.

Stating the obvious, but assume the sinking effect is happening when the trailer is added with engine and AHC operating, all doors and rear hatch closed and foot not on brake??

Thirdly, when the trailer is added, does AHC Fluiid level rise in the tank?? If so, this would suggest a leaky Levelling Valve in the Control Valve Assembly (unusual but possible).

Finally, it is worth knowing what the rise times are from "LO" to "N" and from "N" to "HI" without trailer. Very slow times compared with those stated in the attachment to my previous post this thread and elsewhere in FSM might suggest a developing blockage in the internal strainers inside the AHC Pump -- and a pump struggling to do the job when the trailer is added.
 
Last edited:
Here is my Hitch without the Weight Distribution chains. Note that the hitch from my Trailer is pressing downward on my LX470's receiver:

Hitch Setup-4-no weight distribution.jpg



After I attach the WDH chains, note that the hitch from my trailer is pulling upward on my LX470's receiver:

Hitch Setup-5-lifting up at receiver.jpg


Effectively what the WDH is doing is distributing the hitches weight across all 4 tires.
Not only weight distribution but also sway control.
Have you sensed your trailer swaying?

All hooked up and ready to go camping:
Hitch Setup-6-WDH with Lexus and Winnebago.jpg
 
Amazing post again as usual, @IndroCruise. To answer your question about GVM; no there's nothing like re-engineering requirements that I know of in the USA, at least for private vehicles. GVWR as it's called here is only enforced for commercial vehicles. If you overload your vehicle and get into a crash maybe there would be some extra liability but I've not heard of that happening before.

To the OP: Lo, N, and Hi are only differentiated by the pressure in the system. If it's dropping to Lo then the pressures are too high at N. If you can reduce pressures and then it can operate at N all day long even with a heavy load.

I use also use airbags they work awesome and compliment the AHC perfectly. I don't use a WD hitch.

Also there is no point to a sensor lift IMO unless you want to pose hard on the street. My stock ride height AHC in Hi is taller than a lot of truck with lifts. If you activate ATRAC it will go into Extra Hi mode and lift even higher. I've never felt the need for anything more.
 
Also there is no point to a sensor lift IMO unless you want to pose hard on the street. My stock ride height AHC in Hi is taller than a lot of truck with lifts. If you activate ATRAC it will go into Extra Hi mode and lift even higher. I've never felt the need for anything more.
AHC can only be optimized for one height setting. If optimized for stock height and fully loaded, one might not even be able to get into HI mode (when you need it) due to the reduced load capacity.

AHC high is limited by speed (~20mph or so). Lifted height is desirable for washboard desert trails, whoops and sand dunes, all way over 20mph.
 
Amazing post again as usual, @IndroCruise. To answer your question about GVM; no there's nothing like re-engineering requirements that I know of in the USA, at least for private vehicles. GVWR as it's called here is only enforced for commercial vehicles. If you overload your vehicle and get into a crash maybe there would be some extra liability but I've not heard of that happening before.

To the OP: Lo, N, and Hi are only differentiated by the pressure in the system. If it's dropping to Lo then the pressures are too high at N. If you can reduce pressures and then it can operate at N all day long even with a heavy load.

I use also use airbags they work awesome and compliment the AHC perfectly. I don't use a WD hitch.

Also there is no point to a sensor lift IMO unless you want to pose hard on the street. My stock ride height AHC in Hi is taller than a lot of truck with lifts. If you activate ATRAC it will go into Extra Hi mode and lift even higher. I've never felt the need for anything more.

I also questioned the rationale of the sensor lift, but at this point, it has a Slee Diff-Drop and new BFG KO2 305/65/R18 tires installed. The oversized tires may be the reason for the lift (e.g. prevent rubbing while articulated or at lock), but that's only an assumption on my part. I bought the truck a few months ago and the previous owner had performed these mods.

It's had the timing belt, water pump, and heater-T service performed within the last 50K miles as well as an alternator.

What I've done since taking ownership:
Optima Yellow-Top
New Toyota CV/Axles
New Toyota AHC Accumulators
New fluids and filters throughout
LX470 cabin air filter mod
PowerBass 2ohm replacement speakers front/rear

We've already taken it cross-country and it's super comfortable, plush, and quiet. Glass smooth and has no issues whatsoever.

I'm going to take some physical measurements as described in a recent post and report back.
 
We had the same issue when we loaded the car up for camping, towing our Kimberley Kamper.
With a full fuel tank, the system would go down to Low.
The solution I did was to change he rear springs to used Toyota non-AHC springs.
That worked really well, downside was that the rear was way higher then the front but with the addition of the 182 liter long range tank and dual spare wheel carrier later this has reduced a bit.
Car has 299,100 kms on the clock (probably more due to bigger tyres) and still factory AHC components as far as I know.
I have replaced the fluid earlier this year.
 
I appreciate everyone jumping in and offering their advice. I decided to lower the rear sensor just a touch to see how that would impact rear pressure. As expected, the rear pressure is now within spec. Rear height is 21.5” center-hub to fender. FL is 20.75”. FR is 21.0”.

Ground to fender:
Front: 36.5”
Rear: 37.25”
 

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