AHC stuck in low...emergency help (1 Viewer)

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Joined
Dec 23, 2022
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Location
Nicaragua
Hi all! I am brand new here, first post.

2004 Toyota Land Cruiser Turbo Diesel

We are in Nicaragua and I found some good information of how to go through the system and debug the issues, but we are currently stuck in the middle of the jungle and the ride height is set to LOW and won't go back up.

Are there any hacks for fully disabling the entire system or locking it in HIGH? We can't make it out of here in LOW. We don't have a tester and minimal options for debugging and fixing.

Any ideas or hacks for just getting back to civilization?
 
Few more things to try:
These deal with AHC pressure set to high (out of adjustment).
If carrying to much weight on hydraulics, excessive heat builds in fluid. Which can put system into fail safe (OFF). Cranking clockwise on t-bar adjusters, will take some load off front AHC (reducing front pressure). Removing some weight (cargo) may also help. Adjusting rear height sensors, moving down in slide. Puts more weight on rear coils, taking pressure off rear AHC. Same can be done evenly on both front adjusters.

Additionally:
We've seen some issues with wiring, under rear LH quarter. Unplugging the wire harness housing block and re-plugging in may help.

Tip:
If you get back to N. Maybe best to turn AHC OFF, for now!
 
One of my front sensors went out causing the front end to drop to low mode. I was able to disconnect the bad one and drive around in Normal height (with the light flashing).
 
Awesome! I will go try all of these at daylight tomorrow.

Anyone point me to where the height sensor plugs are? Also Tbar bolt?

I am new to this vehicle...always been a jeep guy...can't wait to get this system gone and just put a proper simple system on this thing...love the engine though!

I will go do some research, but anything you can point me to will help.

Bundle of cigars shipping to whoever saved our day!
 
Plugs to height sensors:
For the rear it's easiest straight on the sensor, positioned over the rear axle, on the left side. Accessible by crawling in either in front of the left rear wheel, or from behind.
The fronts have a plug on top of the wheel well, under the hood. There is one plug for the abs sensor and one for the ahc sensor next to each other, you have to follow the cable down to the sensor to see which is which.

Torsion bar bolt:
The torsion bars are the front springs, shaped as a straight bar running front to back. Front end attached to the lower A-arm, rear end attached to an adjuster mechanism attached to the frame. This adjuster has a bolt you can turn in order to adjust the spring force. You can see them by looking up from under, about a foot behind the front wheels.
 
As your are new to AHC on the Land Cruiser 100, I thought I would give a few hints.
1 - intro:
The AHC hydro-pneumatic system is in addition to steel springs, taking roughly half of the load. The damping (shock absorber function) is through the AHC hydraulics, in the actuators placed on the frame close to each wheel. What looks like shock absorbers are hydraulic rams which have to be pressurized by the AHC pump in order to take their part of the weight of the vehicle.
2: - possible problems:
When it fails by staying in low all the time, it is most likely either because of to much load on the ahc, or because of a failed component.
If it is because of too much load, it will get worse over time as the steel springs weaken (front TBs and rear coils), until suddenly the ahc can not manage the ever increasing load.
If there is a failed component, the possibilities are many, but what I've seen most frequently are either a mechanical or electrical failure of the height sensors or connected components. When driving on forrest roads, it is possible that a stick or something can bend a link arm to a sensor, so that the sensor arm is in the wrong position. The sensor links can also wear out, rust, get stuck, or their adjusters get loose.
Electrically, the height sensors can have a broken (or corroded) cable; or fail internally, either because of corrosion or that they are worn out.
 
There was just another thread on here that you can maybe follow to get the height up. It involved diagnosing the pump and running it in short bursts in one direction to lift the truck, then disconnecting. If the pump runs, or squeals, try following that thread.

Are there any other indicators you can share? Like does pump run? Are the lights blinking or solid?

Found the thread

 
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Hi all! I am brand new here, first post.

2004 Toyota Land Cruiser Turbo Diesel

We are in Nicaragua and I found some good information of how to go through the system and debug the issues, but we are currently stuck in the middle of the jungle and the ride height is set to LOW and won't go back up.

Are there any hacks for fully disabling the entire system or locking it in HIGH? We can't make it out of here in LOW. We don't have a tester and minimal options for debugging and fixing.

Any ideas or hacks for just getting back to civilization?

The previous replies in this thread answer the question as best possible. Here are some pics which may help.

First, consider weight issues as previously described in this thread, by weight removal if possible, or, by torsion bar adjustment.

Torsion Bar Adjusters -- one on both sides of chassis, about half-way between front and rear wheels:
Turn the adjuster clockwise (looking up from below) -- multiple turns to shift weight off AHC system and onto torsion bars. The weight transfer of one full turn on both adjusters causes a reduction in Front AHC pressure of ~0.2Mpa, so try 5 full turns on both sides to make a real difference. Do this with engine OFF, make hub-to-fender heights both sides as equal as possible. This will be hard with vehicle at "LO" -- can try jacking vehicle to get weight off the suspension to make it a little easier while doing this BUT BE VERY CAREFUL -- you still will have to wrench hard, do not want vehicle coming off the jack on top of you!! Or see if you can start AHC pump to get vehicle to raise by (1) direct connection to the battery by suitable leads if you are carrying them, or (2) by using "Active Test" with a paperclip or similar as a connector per Section 5 at Page 6 of the attachment -- the latter method will not work if there are other faults.

You might succeed in getting the vehicle to "HI" height -- but the system is designed to drop the vehicle back to "N" height when the vehicle speed exceeds 30 kilometres per hour (19 miles per hour). Some Owners have devised circuit changes which cause "HI" height to be held but that is not a job to do on the trail -- and some Owners would say that this is not a good idea anyway.

So the aim is to re-establish "N" height as best possible.

Note that, unlike a conventional suspension, on an AHC-equipped vehicle the torsion bar adjusters are not used to vary the ride height of the vehicle. Ride height is adjusted at "N" by the Height Control Sensor adjusters. The Suspension ECU then determines "LO" height and "HI" height.

100 series Torsion Bar Adjuster.jpg


Second, if weight is not the issue, consider damage or internal failure of sensors or linkages or wiring harness or connectors in the Height Control Sensor circuits as previously described in this thread.

The idea of disconnecting Height Control Sensors is to try to induce faults ('fail safe function' for Height Control Sensors) which, theoretically, should cause the vehicle to adopt "N" height -- unless there are other faults disallowing AHC Pump operation or unless the vehicle is too heavy to raise.

Front Height Control Sensors:
Found in the Left and Right wheel arch as shown below. Follow harness rising to engine bay -- there is a connector there which can be pulled if it is difficult to pull the connector behind the sensor bracket in the wheel arch. Be sure that the correct three-wire harness has been followed. The harness connector sits in the engine bay above the wheel arch on each side of the vehicle, next to a two-wire connector and circuit which goes to the wheel speed sensor.

AHC Front Height Control Sensors.jpg


Rear Height Control Sensor -- there is only one, forward of Rear diff, attached to chassis cross-member and to LHS Rear Upper Control Arm:
Rear Sensor connector can be seen. It is easier to get at with the spare wheel out of the way.

AHC Rear Height Control Sensor.jpg
 

Attachments

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Gents! You guys are amazing. I have never seen a forum with so much help and detailed help. I am back in town on wifi and we are still stuck in L.OW. I disconnected all 3 sensors and still nothing. There are no options for jacking it up really. And don't have the tools for the torsion bars. But, we have a guy with a much more capable rig coming to hopefully get us up and out.

I am shocked this thing has this complicated of a system and doesn't have a FAIL SAFE mode that fails in NORMAL. Seems like a major flaw in engineering.

Either way, I appreciate it. Best replies and information I have ever seen on a forum of any type of vehicle I have owned.
 
Gents! You guys are amazing. I have never seen a forum with so much help and detailed help. I am back in town on wifi and we are still stuck in L.OW. I disconnected all 3 sensors and still nothing. There are no options for jacking it up really.
How much extra weight did you carry? May be overweight?
 
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Gents! You guys are amazing. I have never seen a forum with so much help and detailed help. I am back in town on wifi and we are still stuck in L.OW. I disconnected all 3 sensors and still nothing. There are no options for jacking it up really. And don't have the tools for the torsion bars. But, we have a guy with a much more capable rig coming to hopefully get us up and out.

I am shocked this thing has this complicated of a system and doesn't have a FAIL SAFE mode that fails in NORMAL. Seems like a major flaw in engineering.

Either way, I appreciate it. Best replies and information I have ever seen on a forum of any type of vehicle I have owned.

The fail safe is low mode. ;)

It can't necessarily "fail" to N because:

A) It may not know where "N" is. If the height sensor is wrong, the system doesn't want to indiscriminately raise. It might raise to max height which is arguably worse than low.
B) It might be (probably is if this has never been checked) over-pressurized and the hydraulic system isn't actually capable of lifting the car.
C) The system might have a fault that compromises pressure. If the system is leaking, there's no work around to lift the car.

AHC is complicated - sort of - but it's also very reliable if basic maintenance is done. If you've never adjusted neutral pressures, it's going to leave you stuck in L at some point. It's akin to never changing your oil and then getting stranded when the engine bearings finally fail.

On one hand, AHC is amazing in that it can run for nearly 20 years with a complete failure of maintenance. On the other, it doesn't exactly warn you before it finally reaches the "I give up" point.

Let us know how it goes! There's a bunch of AHC nerds here that want to see what led to the fault and make sure you get back on the road soon. :)
 
Either way, I appreciate it. Best replies and information I have ever seen on a forum of any type of vehicle I have owned.

So please advise what happened in the end @GreeceMonkey22 ?? Closing the loop and feedback on contributors' answers is what keeps this forum providing the quoted "best replies and information ever seen on a forum of any type of vehicle ever owned".
 
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