LX470 AHC issues - pressure sensor 1998 Vs. 2002 (2 Viewers)

[84bj60]

Good morning all,

2002 LX470 CDN Spec 335000km. Rig is stuck in low, stiff as a board and has the flashing "off" . The system went from fully functional to dead over a period of a couple days, slowly going down to the Low position (or lower)

Vehicle was in for AC repairs at the local Toyota dealership and I asked them to look into some codes I pulled from the AHC system. 1718, 1732, 1763 are the 3 codes. I cannot get it into test mode to manually raise the vehicle.

Essentially they pulled the same codes I did and told me they needed to replace the $600 pressure sensor before anything else could be done.

After probing the sensor and testing per the FSM excerpts I found on some threads here myself, the voltage is out of range (5 volts from ground to one pin and .5 volts on ground to the other pin when pressing the raise button to neutral position. It is likely the sensor is in fact failed. The pump runs with 12 volts applied to it and the AHC Fuse is intact, Relay is functional. when the pump runs it sounds smooth for a few seconds then it sounds like it is dead heading which I would assume is due to the levelling valves being closed.

I have extensively read the AHC threads on the forum, some people say to unplug the pressure sensor and you should be able to raise the vehicle up if that is the problem. This does not work.

I found a complete used pump, pressure sensor, temp sensor and reservoir assembly a couple hours from where I live from a fellow that converted his rig to conventional suspension (1998 LX) purchased this and 3 the three level sensors for $200.00. Only problem is the pressure sensor is a "flat" connector 3 pin Vs the one only my '02 LX is the triangular one.

My first question: Are there any other things I can do to test out the system to get it out of fail-safe/lockout

Second : Can I use the 1998 pressure sensor if I use the old harness from the 98 truck and splice to the 3 wire harness on my truck. If so are the wire colors the same, if not does anyone have a pinout for the 2 sensors.



Thanks in advance


-Mitch

 

[suprarx7nut]

Good morning all,

2002 LX470 CDN Spec 335000km. Rig is stuck in low, stiff as a board and has the flashing "off" . The system went from fully functional to dead over a period of a couple days, slowly going down to the Low position (or lower)

Vehicle was in for AC repairs at the local Toyota dealership and I asked them to look into some codes I pulled from the AHC system. 1718, 1732, 1763 are the 3 codes. I cannot get it into test mode to manually raise the vehicle.

Essentially they pulled the same codes I did and told me they needed to replace the $600 pressure sensor before anything else could be done.

After probing the sensor and testing per the FSM excerpts I found on some threads here myself, the voltage is out of range (5 volts from ground to one pin and .5 volts on ground to the other pin when pressing the raise button to neutral position. It is likely the sensor is in fact failed. The pump runs with 12 volts applied to it and the AHC Fuse is intact, Relay is functional. when the pump runs it sounds smooth for a few seconds then it sounds like it is dead heading which I would assume is due to the levelling valves being closed.

I have extensively read the AHC threads on the forum, some people say to unplug the pressure sensor and you should be able to raise the vehicle up if that is the problem. This does not work.

I found a complete used pump, pressure sensor, temp sensor and reservoir assembly a couple hours from where I live from a fellow that converted his rig to conventional suspension (1998 LX) purchased this and 3 the three level sensors for $200.00. Only problem is the pressure sensor is a "flat" connector 3 pin Vs the one only my '02 LX is the triangular one.

My first question: Are there any other things I can do to test out the system to get it out of fail-safe/lockout

Second : Can I use the 1998 pressure sensor if I use the old harness from the 98 truck and splice to the 3 wire harness on my truck. If so are the wire colors the same, if not does anyone have a pinout for the 2 sensors.



Thanks in advance


-Mitch

Pressure sensor appears to be <$250 from Partsouq if you want a new one.

SENSOR, PRESSURE ACCUMULATOR 8919260020 | Toyota Parts | PartSouq

Quality parts for Toyota auto with fast delivery here! For a fast search using the part number 8919260020 - SENSOR, PRESSURE ACCUMULATOR

partsouq.com

I would start your work at the ECU. Wiring harness faults are more likely than component failures, IMO. If you have a DMM and skinny probes, you can measure the pins and see if the ECU sees the same component resistance as the component itself. If not, the harness is probably your fault. I like starting with this because it can avoid a lot of parts swapping work. Taking the trim off is a little annoying, but much better than electrical connector surgery.

 

[84bj60]

Pressure sensor appears to be <$250 from Partsouq if you want a new one.

SENSOR, PRESSURE ACCUMULATOR 8919260020 | Toyota Parts | PartSouq

Quality parts for Toyota auto with fast delivery here! For a fast search using the part number 8919260020 - SENSOR, PRESSURE ACCUMULATOR

partsouq.com

I would start your work at the ECU. Wiring harness faults are more likely than component failures, IMO. If you have a DMM and skinny probes, you can measure the pins and see if the ECU sees the same component resistance as the component itself. If not, the harness is probably your fault. I like starting with this because it can avoid a lot of parts swapping work. Taking the trim off is a little annoying, but much better than electrical connector surgery.

Nice find on the pressure sensor. I have always been hesitant to purchase parts from overseas and from sites I am not familiar with however would be happy to with a referral like this.

I will buy a new one if I can confirm the sensor is shot, just didn't want to spend big bucks throwing parts at it. Are you aware of if the 98 and 02 sensors are the same in terms of electrical values?

 

[suprarx7nut]

Nice find on the pressure sensor. I have always been hesitant to purchase parts from overseas and from sites I am not familiar with however would be happy to with a referral like this.

I will buy a new one if I can confirm the sensor is shot, just didn't want to spend big bucks throwing parts at it. Are you aware of if the 98 and 02 sensors are the same in terms of electrical values?

Partsouq is pretty well loved here. Legit parts from a mega-seller that supplies the globe. They often ship faster than domestic dealers.

Sensor electrical values. Let me go check my manuals. I have an electrical wiring manual from an 06 and 99 and can compare the two.

 

[suprarx7nut]

Alright, so in these more recent manuals the electrical values aren't all published in the wiring manual (most values were published in the early 90's ones). Anyways, here's the connector comparison. The wiring diagrams of the pressure circuits look identical between the 98 and 06. 06 has the triangle. 98 has the flat connector. Both F1 and both pinouts used for the same conductors.

For the 06, here's what the manual says:
Approx 5V at pin 1 of the pressure sensor to ground (pin 3).
1.48 to 1.85V signal from the pressure sensor (pins 2 and 3).

I assume you should get those ballpark values across all years for the 100, but don't have the FSM for anything but the 06 to verify.

 

[84bj60]

Damn you're good.

I was getting .5v on the pin 2 and 3 when running without the jumper installed on the diagnostics plug with or without pressing the raise button...

Im going to try changing the sensor with the users 98 sensor and make a quick harness adapter for it for testing...


Any idea how difficult it will be to bleed the air out when I change the sensor? will I be able to just crack the flare nut on the discharge of the pump?

-M

 

[84bj60]

Last question,

how does one clear the codes from the AHC ECU?

 

[suprarx7nut]

Damn you're good.

I was getting .5v on the pin 2 and 3 when running without the jumper installed on the diagnostics plug with or without pressing the raise button...

Im going to try changing the sensor with the users 98 sensor and make a quick harness adapter for it for testing...


Any idea how difficult it will be to bleed the air out when I change the sensor? will I be able to just crack the flare nut on the discharge of the pump?

-M

No idea honestly. I've never had an issue with air in any part of the system, but plenty have. Bleeding *should* be simple, albeit tedious. There are multiple ways to run the pump during the bleed process to make it go quickly. Most folks just cycle the system over and over and over.

Last question,

how does one clear the codes from the AHC ECU?

I've always cleared via Techstream. I think there's a way to manually do it with jumpers and or pulling fuses. I don't have the fsm for the early year LX, so can't look up the manual process, sorry!

 

[84bj60]

I replaced the sensor with the 98 one. hooked it up per the pinout and the AHC lights would not illuminate at all. I reversed pin 1 and 3 as those are the only one that could be screwed up and I had the same problem as before.

There is no way to get the pump to run when it is in fail safe mode.

Anyone out there know how to clear the codes on the 02 lx AHC computer?

 

[ramangain]

Techstream. If you're gonna mess with AHC, get it.

 

[84bj60]

Can you link a reputable source for the cable and software? is the software free and you just need to pay for the cable or is it a kit?


I just ordered a new sensor from Partsouq hope to see it within a couple weeks. $320 bucks with shipping in our worthless CAD.

Techstream. If you're gonna mess with AHC, get it.

 

[suprarx7nut]

Can you link a reputable source for the cable and software? is the software free and you just need to pay for the cable or is it a kit?


I just ordered a new sensor from Partsouq hope to see it within a couple weeks. $320 bucks with shipping in our worthless CAD.

So here's the deal with Techstream.

The "reputable source" is Toyota and the software and hardware is $1600 for one year. $1300 each year after that to keep the license active. See below.

https://techinfo.toyota.com/techInfoPortal/appmanager/t3/ti?_pageLabel=ti_ts_lite&_nfpb=true

OR if you're not a professional shop:

Any number of ebay/amazon offers where the cable is $30-$100 and the software is included or available via many sources in the enthusiast community.


There's a massive gap in this market because Toyota never meant for you (a normal car owner that doesn't own a full garage shop) to purchase Techstream. Toyota then completely failed their customers when they failed to properly train and equip technicians for the most basic AHC upkeep success thereby making authorized dealers a non-viable option for AHC.

 

[84bj60]

Reviving my AHC thread. Finally was able to find a pressure sensor online for a reasonable amount when I had the money, ordered and installed to no avail.

I will be digging further into this issue this weekend. Is anyone able to provide EWD for a 2002 lx along with excerpts from the FSM for testing the above listed codes? I need to start from the beginning on this one.

Thanks in advance

 

[suprasvobodea]

Damn you're good.

I was getting .5v on the pin 2 and 3 when running without the jumper installed on the diagnostics plug with or without pressing the raise button...

Im going to try changing the sensor with the users 98 sensor and make a quick harness adapter for it for testing...


Any idea how difficult it will be to bleed the air out when I change the sensor? will I be able to just crack the flare nut on the discharge of the pump?

-M

HI

I have the same problem and after 60 hrs + of research, I will share my nightmare with you and everyone to help ease the pain and frustration. Suprarx7nut is correct on the Voltage specs for the pressure switch leads
Approx 5V at pin 1 of the pressure sensor to ground (pin 3).
1.48 to 1.85V signal from the pressure sensor (pins 2 and 3).

I get 5V at the ECU wire harness ( located by the driver's RH leg behind that metal plate under the steering wheel. I checked the V both at the pressure switch connected with a sharp probe (quite difficult to pierce the 18ga wire and not poke your finger holding it back but I did it! AND GOT 0.505 V!. This is NOT within the range of 1.48-1.85V when you depress the switch on the console,... so YOUR PRESSURE SWITCH IS SHOT AND YOU WILL NEED A NEW ONE! There is NO WAY to run the active test with this faulty pressure switch Voltage value at 0.5V !!

Here is the secret that has NOT been published in the TSM or by anyone on this forum before now, so you are the first one to get and hear this knowledge, please repost it to help every and anyone else with an AHC Lexus or LC that struggles with this issue.

<<<< If YOU RUN THE PUMP DRY WHILE FLUSHING OR CHANGING THE PUMP, REPLACING THE SHOCK BUSHING or REFILL THEM... anything that empties the reservoir while it's running,... IT WILL DAMAGE THE PRESSURE SWITCH! - AND YOU WILL HAVE TO BUY A NEW ONE. The cheapest I found it IMPEX out of Japan. They are USD 229 + shipping but that is a lot less than any US Lexus online parts shop. It takes a week or so to get but you will save $80-150 dollars on this one part ( Ask me how I know ..? )


I have gone through the REAR Hydraulic strut removal process and decided to cut access holes in the back cargo area to gain access to the 2 10mm bolts that are always rusted that hold the bracket over the top of the piston where the 2 gaskets are and it was THE best time-saving idea I had. I will be writing up a detailed, photo-supplied procedure that should help everyone who is too scared to tackle this job due to the other nightmare of accessing the top of those hydraulic "shocks " BTW >> ( they really are NOT shocks but actuators)

Feel free to contact me directly with any questions about this, or the Accumulator replacement, as I have done EVERYTHING now on my 2002 LX system and I want to help others!

 

[2001LC]

HI

I have the same problem and after 60 hrs + of research, I will share my nightmare with you and everyone to help ease the pain and frustration. Suprarx7nut is correct on the Voltage specs for the pressure switch leads
Approx 5V at pin 1 of the pressure sensor to ground (pin 3).
1.48 to 1.85V signal from the pressure sensor (pins 2 and 3).

I get 5V at the ECU wire harness ( located by the driver's RH leg behind that metal plate under the steering wheel. I checked the V both at the pressure switch connected with a sharp probe (quite difficult to pierce the 18ga wire and not poke your finger holding it back but I did it! AND GOT 0.505 V!. This is NOT within the range of 1.48-1.85V when you depress the switch on the console,... so YOUR PRESSURE SWITCH IS SHOT AND YOU WILL NEED A NEW ONE! There is NO WAY to run the active test with this faulty pressure switch Voltage value at 0.5V !!

Here is the secret that has NOT been published in the TSM or by anyone on this forum before now, so you are the first one to get and hear this knowledge, please repost it to help every and anyone else with an AHC Lexus or LC that struggles with this issue.

<<<< If YOU RUN THE PUMP DRY WHILE FLUSHING OR CHANGING THE PUMP, REPLACING THE SHOCK BUSHING or REFILL THEM... anything that empties the reservoir while it's running,... IT WILL DAMAGE THE PRESSURE SWITCH! - AND YOU WILL HAVE TO BUY A NEW ONE. The cheapest I found it IMPEX out of Japan. They are USD 229 + shipping but that is a lot less than any US Lexus online parts shop. It takes a week or so to get but you will save $80-150 dollars on this one part ( Ask me how I know ..? )


I have gone through the REAR Hydraulic strut removal process and decided to cut access holes in the back cargo area to gain access to the 2 10mm bolts that are always rusted that hold the bracket over the top of the piston where the 2 gaskets are and it was THE best time-saving idea I had. I will be writing up a detailed, photo-supplied procedure that should help everyone who is too scared to tackle this job due to the other nightmare of accessing the top of those hydraulic "shocks " BTW >> ( they really are NOT shocks but actuators)

Feel free to contact me directly with any questions about this, or the Accumulator replacement, as I have done EVERYTHING now on my 2002 LX system and I want to help others!

It's great you want to help others. Kudos to you!

But I disagree, that running pump dry will damage the pressure switch.

I've seen more than a few reservoirs & pumps run dry. For instance when globes rupture, accidental underfilling during a flush or leak from O-ring damage during globe or shock replacement. Which I just recently had one of each, come to me to correct those condition.

I correct the issue(s). Fill reservoir. Then keep flushing fluid through the system, until no air (foam) from bleeders. Usually 3 full flushes.

Tip: I use a transmission inline filter, to filter new/good AHC fluid as I repeatedly flush. Before cycling back into the reservoir.

 

[suprasvobodea]

It's great you want to help others. Kudos to you!

But I disagree, that running pump dry will damage the pressure switch.

I've seen more than a few reservoirs & pumps run dry. For instance when globes rupture, accidental underfilling during a flush or leak from O-ring damage during globe or shock replacement. Which I just recently had one of each, come to me to correct those condition.

I correct the issue(s). Fill reservoir. Then keep flushing fluid through the system, until no air (foam) from bleeders. Usually 3 full flushes.

Tip: I use a transmission inline filter, to filter new/good AHC fluid as I repeatedly flush. Before cycling back into the reservoir.

I know all these things now after struggling for 3 months with trying to get the active test to work. It DID WORK after I bought a new pressure switch. but I didn't expect it to work and it drained a full tank of fluid. !!! I did NOT see it in time as I was in the vehicle ( big mistake! ) so I didn't know. I've checked the relays and fuses in the engine bay and the sidekick panel and all are working. The 4 step to check for code 1762 #4 is checking the pressure switch voltage and I get 0.5V when I press the UP-DOWN switch.
To bleed air from the Accumulator and globes, the PUMP MUST WORK first! I cannot manually get the pump to run due to the 1762 error! So how can I bleed if I cannot get fluid (and air) to travel to the accumulator and globes ....? This is my dilemma, so the only thing I know to do,it correct the only known problem ( out of spec V on the PS) and replace it again with a new one! If you know of ANYTHING else that could prevent an "ACTIVE TEST" from being performed, please offer any suggestions!


This is not the spec 1.48- 1.85V ... so by process of elimination, and by the TSM book ( not feelings or intuition...) that HAS to be the only reason it's still not working.! I have placed another order for a NEW PRESSURE switch from IMPEX of Japan and next week I will install it and, by God's GRACE and prayer..... it will work again. But I will be standing next to the reservoir and make sure IT DOES NOT EMPTY WHILE RUNNING!

 

[2001LC]

Your PS SW may or may not be bad, IDK!

If I understand you correctly. You R&R the AHC PS SW with new OEM. So you had determined, the original had failed. Then NEW OEM PS SW, also went bad, after running a full reservoir dry. Not sure how you'd run a full reservoir dry. But I assume you meant full, after filling with 2.5L (to the top/cap). Not just to max line. But no matter! Running dry is running dry.

Keep in mind when you ran reservoir & pump dry. This introduce air into pump & valve chambers. Which air, must then be bleed.
FSM talks about times the reservoir tank empty or when pump/motor replaced. It does not give any warning this will or even may damages PS SW or state need to replace the PS SW. That I've seen. It just talks about need to bleed air from system, after these dry events.

I'm not trying to shame you in any way. But understand, I've worked on many of these AHC system in both 100 & 200 series. I've not seen a bad PS SW yet.

One of the most difficult cases I worked on. Had multiple issues, which it is not unusual to have multiple issues at the same time. But what was unusual, AHC system going into failsafe (OFF flashings) intermittently, seemly randomly doing so. It to was indicating a pressure issue. Turned-out to be a wire harness housing block, under rear DS 1/4. Moisture was getting in and changing resistance. Sometime when wet it would work. Other times when wet, wouldn't work.

I've seen also seen, as have others have. Broken undercarriage wires. Some no explanation as to why. Other rodent chew.

BTW:
In case you'd not heard. Toyota had a batch of bad AHC fluid. Which was only found in the plastic bottles. Fluid was jelling. Makes a mess of pump & valves.

 

[suprasvobodea]

Your PS SW may or may not be bad, IDK!

If I understand you correctly. You R&R the AHC PS SW with new OEM. So you had determined, the original had failed. Then NEW OEM PS SW, also went bad, after running a full reservoir dry. Not sure how you'd run a full reservoir dry. But I assume you meant full, after filling with 2.5L (to the top/cap). Not just to max line. But no matter! Running dry is running dry.

Keep in mind when you ran reservoir & pump dry. This introduce air into pump & valve chambers. Which air, must then be bleed.
FSM talks about times the reservoir tank empty or when pump/motor replaced. It does not give any warning this will or even may damages PS SW or state need to replace the PS SW. That I've seen. It just talks about need to bleed air from system, after these dry events.

I'm not trying to shame you in any way. But understand, I've worked on many of these AHC system in both 100 & 200 series. I've not seen a bad PS SW yet.

One of the most difficult cases I worked on. Had multiple issues, which it is not unusual to have multiple issues at the same time. But what was unusual, AHC system going into failsafe (OFF flashings) intermittently, seemly randomly doing so. It to was indicating a pressure issue. Turned-out to be a wire harness housing block, under rear DS 1/4. Moisture was getting in and changing resistance. Sometime when wet it would work. Other times when wet, wouldn't work.

I've seen also seen, as have others have. Broken undercarriage wires. Some no explanation as to why. Other rodent chew.

BTW:
In case you'd not heard. Toyota had a batch of bad AHC fluid. Which was only found in the plastic bottles. Fluid was jelling. Makes a mess of pump & valves.

Hello again!

Thanks for the prompt reply. I am puzzled like "The Grinch Who Stole Christmas "that the townspeople were signing in the morning after he just taken all their presents!
To answer your questions, you may make more suggestions that have worked for you in the past ( since you said you have a lot of experience on several models - I'm WILLING TO LISTEN & LEARN! I filled the reservoir with fluid to 1" from the top. After installing a brand new pressure switch, the pump started working when I pushed the up switch. I was inside the vehicle ( with the door closed!) so I couldn't see the reservoir. I used new fluid in the metal can and even poured it in an Elynmeyer Flask with a vacuum nipple and pulled the vacuum on the new fluid to remove all /any dissolved air! ( THERE WAS ALOT!! -believe it or not!) I recycle the fluid after purging air from the system as well to ensure COMPLETELY air-free oil.

The pump started working immediately after I pressed the UP switch and the tank went down steadily and emptied, I stopped the engine and pump. After it started making the cavitation sound, the 1762 code returned on my tech stream screen, and then the truck started falling and all the fluid that was pumped into the system ( plus a quart of additional) returned to the pump and overflowed out onto the ground. My LX is on the stops ... again!! As I indicated in my previous entry, I checked both relays, -they work and the fuses are good. Sooooooooooooo, what's up??! I even applied direct power to the pump with my automotive tester and it works and makes a nice quiet hum , however, I know that it will not pump any oil since the leveling and Accumulator solenoids are NOT open and it will just destroy the pump if you run it more than 10-15 seconds

My question is this,., Assuming that the pump gears may have air trapped in them... How do I remove any air IN the pump gears ... crack open the outlet fitting and let it drip for a few seconds ??

Next Question... Since I already know that the Pressure switch voltage is NOT IN SPEC (0.505) V, and the TSM says to REPLACE it if it is NOT in the range posted ... I need to start with this only parameter that is not per the TSM, so I just ordered ANOTHER PW and 1.5L can of Oil.

WHAT ELSE SHOULD I CHECK THAT IS PREVENTING THIS SYSTEM FROM DOING THE

 

[2001LC]

You wrote:
"My question is this,., Assuming that the pump gears may have air trapped in them... How do I remove any air IN the pump gears ... crack open the outlet fitting and let it drip for a few seconds ??"

Since the fluid came back/pushed into reservoir from system, as vehicle lowered. There shouldn't be air in the pump, at least not enough to matter much. But the FSM does have a procedure, to run pump to bleed air: in Diagnostics of C1762 DTC, step # 3. I've never used it. I've found just, filling reservoir and let it sit a few minutes pump will run. Your, DTC will likely need clearing first. If it comes right back, without pump running at all. It's likely a short.

You wrote:

" I filled the reservoir with fluid to 1" from the top. After installing a brand new pressure switch, the pump started working when I pushed the up switch. I was inside the vehicle ( with the door closed!) so I couldn't see the reservoir. I used new fluid in the metal can" "The pump started working immediately after I pressed the UP switch and the tank went down steadily and emptied, I stopped the engine and pump. After it started making the cavitation sound"

First, you couldn't see the reservoir.
So you must have assumed it emptied. It's not likely it did. Unless you somehow went into ultra high mode and beyond that.

Second, you heard pump "started making the cavitation sound".
You may have heard high pitch or groan sound, we sometimes get. It's believed this to be fluid passing a seal/valve under high pressure. We hear this now and then, in normal operation. Mostly just before pump stops.

After a full drain of system and I've fill the reservoir. I start engine with door open, set AHC SW to L, standing outside as I close door and watch reservoir. Pump starts and runs for a bit. But nothing seems to happen, vehicle just sit there as reservoir fluid levels begins to lower. Than after a bit of time, vehicle raise to L. Then I select N, and again watch from outside as vehicle raised to N height. All good, I select H.
Note:
If bleeding air and or filter fluid. I flush/bleed from L, without going to N or H at first. As I want to reduce chance, of pushing air and or particles into in chamber of shocks. Second bleed from N, third bleed from H. Then a forth bleed from H, to make sure I see no air (foaming or bubbles)

You wrote:

"After it started making the cavitation sound, the 1762 code returned on my tech stream screen, and then the truck started falling and all the fluid that was pumped into the system ( plus a quart of additional) returned to the pump and overflowed out onto the ground.

Where there is about 1L more AHC fluid, in the system after getting out ~2.5L. We can not get it out. It's deep in the system. Most in the inner chamber of shock, some from lines (globe membranes to pump).

~1qt overflowing out the reservoir. Happens if reservoir filled to top, without system being drained first.
When we flush the system, by draining all 3 circuits when AHC in high (I do from H). By opening first circuit which is the rear globe bleeder we get ~1.0L, accumulator ~0.4L, than front globe (~0.8L) and suck remaining fluid from reservoir ~0.3L. We get out a total of, ~2.5L of AHC fluid. At this point, vehicle is as low as it can get (on the stops). This is provided, stock configuration (rear coils, T-bars and high sensors factory setup) and good globes. If you then fill reservoir, with all bleeders close. It take 2.5L to fill reservoir (one tin). The after running the pump and raise to high (H). Reservoir shouldn't go much below minimum line. Even if vehicle, goes into ultra high mode some remains in reservoir.

You wrote:
"and then the truck started falling

This shouldn't happen. There's basically a check valve in the return valve. Also see this and the Control valve assembly in PDF I attached.

You wrote:

"I get 5V at the ECU wire harness ( located by the driver's RH leg behind that metal plate under the steering wheel. I checked the V both at the pressure switch connected with a sharp probe (quite difficult to pierce the 18ga wire and not poke your finger holding it back but I did it! AND GOT 0.505 V!. This is NOT within the range of 1.48-1.85V when you depress the switch on the console,... so YOUR PRESSURE SWITCH IS SHOT AND YOU WILL NEED A NEW ONE! There is NO WAY to run the active test with this faulty pressure switch Voltage value at 0.5V !!"

"Voltage specs for the pressure switch leads
Approx 5V at pin 1 of the pressure sensor to ground (pin 3).
1.48 to 1.85V signal from the pressure sensor (pins 2 and 3)"

Perhaps you've a typo "(pins 2 and 3)" FSM has this as pins 1 and 2 at ~4.5volts, when ~4.5volts applied to pins 1 and 3.
Perhaps you didn't included the whole procedure (three 1.5V batteries (4.5V)).
See #5 of C1762 check fluid PS sensor.

I don't know why you first started working on AHC, that resulted in first PS sensor R&R. But system failing again, it's more likely the issues was elsewhere and or multiple issues!

The number one area other than failed globe & accumulator membranes and failed height sensors. Is issues arise from a short in wire harness. Which is become more common with years/age.

This PDF may help you: