Hello,
Yes, 1762 is the ONLY code that I have from Techstream. I also think that a direct 12V to the pump to FORCE it to run is the solution, However, INDOCRUISE who is also a huge contributor summarized in very specific detail WHY running the pump directly with 12V, will NOT work to build pressure in the system ( The ECU won't allow the solenoid valves in the accumulator or globes open!!) read in post below;
"AHC Inop after fluid change 2007 lx C1751 and C1762 - post #312 from INDOCRUISE..>>> (ref.)"
Some notes on so-called ‘over-riding’ of the “fail-safe functions”:
- When the Height Control Operation Test (“Active Test”) does not work, it means a fault/DTC is preventing it from working and the fault and DTC must be cleared first before this test will work properly,
- This test is designed into the system to facilitate adjustments -- it is not designed to override fault conditions nor the "Fail Safe Function",
- When the Height Control Operation Test (“Active Test”) can work correctly, the ECU causes the AHC Pump to operate and also causes the Levelling Valves in the Control Valve Assembly to open as required to allow the vehicle to be raised or lowered,
- The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
- The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
- Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,
- The Solenoid Valve on the Height Control Accumulator is ‘normally closed’ -- the Solenoid Valve on a healthy AHC system only opens on a signal from the ECU to open (i) when the Height Control Accumulator is required to discharge at the start of a ‘raise’ to lift the vehicle, and (ii) when the Height Control Accumulator is being re-filled at the end of a ‘raise’ in readiness for the next raise,
- The Height Control Accumulator has no other function in the operation of the AHC system,
- The ECU will not signal to the Solenoid Valve to open at the Height Control Accumulator in the presence of a prohibitive fault/DTC which results in a “Fail Safe Function”,
- When the AHC Pump is caused to operate using a direct 12-volt supply against closed Levelling Valves and a closed Solenoid Valve on the Height Control Accumulator, then the AHC Fluid from the AHC Pump has nowhere to go other than some fluid forcibly bypassing/shortcircuiting internally within the AHC Pump sub-assembly, and with the pump pressure hammering on seals and valves. All that will happen is noisy cavitation within the AHC Pump. Unlike industrial gear pumps, there is no pressure relief valve to protect the AHC Pump. Instead, the system relies on ECU-controlled protections such as C1718, C1C1736, C1741, C1743, C1762, and C1763 to quickly shut down the AHC Pump under FSM-specified conditions."
"By the way, it is good to show the whole Techstream screenshot -- then it can be checked whether the "Main Relay Expectation" is "ON". This refers to the AHC Main Relay located at the ECU under the dashboard. If this relay is not working, operation of the AHC/TEMS is not possible. It also shows the number of Diagnostic Trouble Codes (DTCs) at the time of the test."
I think this is what broke my original pump and caused it to lock up... I was trying to force fluid, under extreme pressure against a CLOSED system, and no fluid left the tank and I was cavitating fluid inside the pump... Please let me know if this is wrong thinking on my part and INDOCRUISE..... But this does make sense. That's why I am hesitant to run the pump for more than 5 seconds to just check that it's working .( which it is, cause it's new!)
I think you're correct about the air, but I don't know how to force the pump to run AND get the solenoid valves to open so the system can actually pump oil to where I need it to go ?!????!!!
Apologies offered to
@suprasvobodea – my responses to requests on this Forum and in Personal Messages have been less than timely due to remote travels followed by illness, then some family issues and then other distractions.
Bottom line:
There are multiple issues here.
The fact that that the same erroneous low-voltage reading with two Pressure Sensors strongly suggests an open circuit somewhere -- meaning a broken wire.
If so, this will cause of a "fail safe mode" which is preventing the vehicle being raised in the usual way to enable bleeding to proceed.
Some futher notes a few other aspects of the AHC/TEMS system:
On air (or nitrogen) in the system ....
There is nothing to add to the long posts #307 and #312 (which also included parts lists of pump-related items and seals to replace) – see
AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-14364800
and
AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-15345982
Air in the system is, and remains, the primary cause of C1762 -- but not the only cause.
The concerns set out by
@suprasvobodea are noted at posts #313, #314 in the same thread which also added that a
new AHC Pump was installed.
Why is AHC Pump Replacement a Good Idea?
While opinions vary about the need for pump replacement, my opinion is definitely yes, just do it! -- and it has been done -- and it is smart when replacing 'globes' and/or 'shock absorbers' on an old vehicle.
Why bother?
It is suggested that because AHC Fluid is rarely changed in accordance with the Owner’s Manual, the fluid may be as old as these 1998-2007 vehicles (2002 LX470 in your case). If so, the fluid (which is a light mineral oil with proprietary additives)
- will be black, mainly due to oxidation,
- may contain air,
- inevitably picks up nitrogen which very slowly passes through the internal ‘globe’ membrane as ‘globes’ age,
- will have picked up atmospheric moisture (the system is not sealed -- open to air at the AHC Tank),
- will have picked up wear-and-tear debris from the around the AHC system over the years,
- will be sludgy and its properties as a hydraulic fluid will have degraded.
There are plenty of pictures of old AHC Fluid on IH8MUD showing the froth and bubbles which emerge (basically a phase change) when the fluid is released from high system pressure to atmospheric pressure in the bleeding tube and receiving container.
Air under operating pressures in the AHC system will be entrained or dissolved in the AHC Fluid and/or sludge rather than as 'bubbles'. The bubbles emerge when the fluid emerges to atmosphere at much lower pressure.
This is why, even though a great job may have been done of minimising air ingress during the parts change-over,
it cannot be assumed that there is no air in the system. It will be there -- and it remains one of the primary causes of C1762 -- and necessitates repetitive and persistent bleeding to cure.
Good IH8MUD pictures also show how sludge accumulates, for example, between the AHC Pump sub-assembly and the outer cover. There are other ‘sludge accumulation’ points around the system where fluid flow similarly is low, slow or intermittent, such as the ‘shock absorber chambers’ which are not fully stroked by the piston, as recently mentioned by
@2001LC .
Sludge accumulation is inevitable with age, and/or lack of good ‘hydraulic hygiene’ -- it was made worse by the ‘bad’ AHC Fluid batches in USA in one litre plastic bottles -- but these ‘bad’ batches are not the sole cause.
Inevitably, this degraded sludgy fluid arrives back at the AHC Tank -- and by the way, it goes
directly to the tank courtesy of the Return Valve. The returning fluid does not to through the Pump. Then it is drawn from the Tank and pushed back in to the system by the AHC Pump, until given enough time and neglect, the strainers inside the AHC Pump, especially the Inlet Strainer, finally block -- at which point C1762 is inevitable.
Meanwhile, the AHC Pump has been operating in this degraded fluid.
Wear increases –
- at the gear teeth, and,
- where the hard steel gears are pressed into the side-walls of the softer cast-aluminium housing, and,
- especially where the hard steel gears are pressed onto the ‘floor’ of the softer cast-aluminium housing by the designed internal pressure within the pump acting on hard steel gear cases -- the purpose of which is to reduce fluid bypass across the top and bottom surfaces of the gears -- which is a fine and effective idea until it is eventually defeated by wear and scoring in the pump housing below the gears.
In addition, the Pump will slowly and progressively be ‘starved’ of fluid as the Inlet strainer slowly blocks -- until as mentioned, eventually DTC C1762 is triggered – full title:
“Fluid Pressure Abnormality – Pump & Motor Does Not Supply Fluid”.
Long story short -- personal view: When opening and massively disrupting the AHC system and undertaking major, expensive replacements on these aged vehicle with unknown or doubtful AHC maintenance history, spend the additional USD100 and the additional half-hour replacing the AHC Pump as well, then close up the system, prime the new AHC Pump (or just let fluid settle into it) -- all in the interests of greater confidence in ongoing reliability and AHC ride comfort. So congratulations on getting this done already.
Further difficulties were reported by
@suprasvobodea at posts #315 and #317 in the same thread.
It is also clear from posts #318, #319 and #320 in the same thread that there was some response from the AHC system but that DTC C1762 quickly returned.
At least it is known that the system will work when DTC C1762 is resolved -- bleeding out air or resolving whatever other cause.
Posts #14 and onwards by
@suprasvobodea are noted, along with the very experienced and thoughtful replies which follow from
@2001LC and
@suprarx7nut at
LX470 AHC issues - pressure sensor 1998 Vs. 2002 - https://forum.ih8mud.com/threads/lx470-ahc-issues-pressure-sensor-1998-vs-2002.1267066/#post-15442867
On Levelling Valves ….
As
@IndroCruise, I certainly wrote at Post #312 at
AHC Inop after fluid change 2007 lx C1751 and C1762 - https://forum.ih8mud.com/threads/ahc-inop-after-fluid-change-2007-lx-c1751-and-c1762.1233088/page-16#post-15345982
- The “Levelling Valves” (one Front, one Rear) are ‘normally closed’,
- The vehicle cannot be raised or lowered unless the Levelling Valves are caused to open by the ECU – or unless the vehicle is lowered by dumping AHC Fluid from the bleeder screws,
- Operating the AHC Pump without opening the Levelling Valves (such as by direct 12 volt connection to the AHC Pump Motor) cannot raise the vehicle because there is no signal from the ECU to the Levelling Valves to cause them to open,
There is an error in the detail offered here in the first bullet point. There is a Levelling Valve for each Gas Chamber a.k.a. ‘globe’ -- so there are
four Levelling Valves in total in the Control Valve Assembly. I will edit the referenced post for that detail.
Not mentioned above but it is also worth noting that there are also four Gate Valves in the Control Valve Assembly -- one for each Gas Chamber a.k.a. ‘globe’. Relevantly, the Gate Valves are set “Normally Open” and remain so unless the vehicle is turning at the prescribed angle and at the prescribed speed. The effect is that when the vehicle is stationary -- such as in test conditions -- the fact that the Gate Valves are open means that the AHC pressures at Left Front and Right Front are equalised, and, the pressures at Left Rear and Right Rear are equalised. This means that the Front raises (or falls) equally on both sides. Ditto at the Rear. The LC100/LX470 system does not allow each corner to be raised or lowered independently, unlike some other vehicles. Also, there is no hydraulic connection at the Control Valve Assembly between Front and Rear -- so Front and Rear can and do operate independently (but not side to side).
The FSM extract below describes how the Levelling Valves are intended to work. In part, my opinion above is based on this extract.
I note that
@suprarx7nut had a similar opinion at his post #250, as did
@Moridinbg at his consecutive post #251, both here --
The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14466603
The FSM description is that each Levelling Valve is “Normally Closed”. That much is quite clear. However, is there a further error here??
This extract can be taken to mean that the ECU must transmit
EITHER an “Open” signal
OR a “Close” signal to cause the relevant action by each of the Levelling Valves.
My questions to self are: Is this assertion correct? Or is it the case that the “Normally Closed” position of the Levelling Valves is established by a spring pressure, and, the “Open” position of the Levelling Valves is caused by a signal from the ECU causing the solenoids to be energised, in turn causing the Levelling Valves to open?? Close inspection of the diagram could suggest this.
If the Levelling Valves are spring-loaded to be “Normally Closed”, then maybe it could be expected that sufficient AHC Pump pressure -- such as by powering the AHC Pump directly from a 12 volt battery -- would cause fluid to force open the Levelling Valves, enable AHC Fluid to flow to the ‘shock absorbers’ and cause the vehicle to rise. It also could be expected that if a bleeder valve was opened in this condition, then bleeding would occur under pump pressure.
However, the earlier posts referenced above by @suprarx7nut and @Moridinbg suggest that this cannot happen -- that it is not possible to use direct 12 volt battery operation of the AHC Pump Motor and cause the AHC Pump to push past the closed Levelling Valves.
‘My’ 2006 LC100 is now in the hands of my son-in-law in Perth on the coast of Western Australia, whereas I reside about 4,300 kilometres (2,700 miles) distant in Brisbane on the East coast of Australia. If it were not for this ‘inconvenience’, I would happily experiment further to see how to open the Levelling Valves remotely, somehow.
All that said, I am more than happy to have any mistakes I may have made pointed out by anyone -- and I will correct them -- and hopefully that may provide
@suprasvobodea with some of the reassurance he seeks.
On Pressure Sensor Test Voltages ….
I cannot contribute anything new on the dilemma of the consistently low reading of 0.505 volt reported by @suprasvobdea, equally when testing both the new and the old Pressure Sensor. I presume the tests were conducted exactly per FSM description,
including with the engine running (not just with the ignition “ON”).
The fact that that the same erroneous low-voltage reading with two Pressure Sensors strongly suggests an open circuit somewhere -- meaning a broken wire.
This seems to be a likely cause of the "fail safe mode" which is preventing the vehicle being raised in the usual way to enable bleeding to proceed.
The hunt can be challenging. The discussion between
@2001LC,
@suprarx7nut and
@Moridinbg provides a worthwhile review of their past experiences and gives some ideas on where to look -- starting at Post #249 through to Post #260 at this thread:
The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-13#post-14465654
A good Electrical Wiring Diagram (EWD) may be helpful -- a version can be found here at Post #420 by
@usedname . There may be some differences between model years – but generally the AHC/TEMS circuits are very consistent. See:
The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/page-21#post-14909889
The relevant sections of the Workshop Manual may be helpful – and the LX470 FSM may be on hand. If not, the equivalent on-line version for LC100 (with AHC where fitted) is found here:
LC100 Workshop Manual - https://lc100e.github.io/
Scroll to
+ Repair Manual
+ DIAGNOSTICS
+ ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS
giving the 1762 code every time I start the active test!! Running the pump directly…. Won’t pump oil into anything!!!!. So that’s why I am stuck… & in an endless “do loop” !! So if it’s not electrical short, not a bad PS , and I do have air in system…. WHY Can’t I get the active test to work ?? WHAT AM I MISSING 
