AHC Inop after fluid change 2007 lx C1751 and C1762 (1 Viewer)

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HELP!!! I just did the PADDO method of AHC fluid change today while wrapping up baselining a new to me 07 lx. 160k no documented AHC maintenance. Things went more or less fine but now AHC is inoperable!!! I turned the truck on the first time after the left side blead and it took about 2 min or so to go from l to n. Turned it back off to do the passenger side quick bleed and when I got back in the truck it would not go from l to n. Previous to this procedure AHC was working and I had 9 grads with just barely out of spec rear pressures and spot on front pressures. I am at a loss. The pump was initially louder than usual but now I can't hear it...😭

I'm getting this code:
C1751 - Continuous current to compressor motor

I've tried the "bleed air of AHC" aka "Active Test" in case it is suffering from air lock. I can hear the ?pump? motor running but the vehicle does not move. The ahc does not seem to respond to this procedure. The truck isn't moving when I try to force the pump on. After the "bleed air of AHC" procedure I am getting additional codes / lights:

C1762 - Abnormal Oil Pressure for Pump
and
AHC "off" flashing or staying lit

I'm happy to dig in techstream for some other data if it would help.

I also tried to re-bleed the height accumulator (the 12 inch cylinder on the driver frame rail) because when I first blead it I almost blead it dry and thought maybe I let some air in there and now that it is pressurized (per techstream the pressure there is 6.5)... I don't understand how this is the case but when I tried to re-bleed nothing came out...

I also manually activated the pump it does run! So what is the deal?

Next step I tested for Motor Relay Continuity... I get about 62 Ohms out of terminals 3 and 4 but my multimeter isn't beeping to indicate there is continuity... what does this mean??? is the relay bad??? terminals 1 and 2 are open and when a 12v load is applied to terminals 3 and 4 terminals 1 and 2 have continuity (with the beep from my multimeter). I swapped the ABS relay for the AHC relay also as a test but still no dice. I'd test the AHC Pressure sensor on the pump since that is the last thing to test in the flow chart for c1762 but I can't see how I would do this without taking it out of the truck and I'm worried I'm going to spill all my fluid if I take it out??? Will all the fluid leak out where the pressure sensor is??? I might just risk it to get to the bottom of this. The pump is definitely pumping but the suspension is going nowhere...

I checked the pressure sensor. I think??? its the one on the pump assembly right next to the temperature sensor right??? reason I'm not sure is the connector is three pins in a triangle orientation whereas the fsm shows them in a line... I am only getting .66 volts, should be getting 4.5 volts, I'm just not sure about my testing because the diagram shows 3 pins in a row but I have three pins in a triangle arrangement so I could be testing the wrong pins. Do I have the wrong sensor???

After messing with it all day I now have no pressure at the front or rear sensor... see second photo for current techstream data...

1120201742.jpg


1120202159.jpg
 
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ramangain

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Ok. Deep breath.

If you are very sure the reservoir never went dry during the bleeding procedure, check the pressure sensor again. The fact that you can't build proper pressure in the tube accumulator tells me you either got quite a bit of air in there, or your pressure sensor is suspect. If you can apply 12V to the pump (motor) and it works, it may also mean the pump section itself could be bad or so gunked up it isn't working properly anymore.
 

suprarx7nut

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Agreed on the deep breath. You're ok, and it's incredibly unlikely components just started failing right after your bleed.

Your event sounds exactly like what I'd expect some air in the lines to do. I think you're the second person to have this sort of thing happen in just the last week.

Air in the lines would make the pump unable to build proper pressure. The motor would run and run and run, but be unable to build pressure because it would have to move an enormous amount of fluid to compress a big air bubble. If the system is bled, the pump moves relatively little liquid, but can build a large amount of pressure. If there's air, the pump will run out of fluid before the air is compressed. The result would be that the pump runs seemingly forever without a buildup in pressure. If I were designing a hydraulic system like that, I'd build in a warning when no pressure was seen after a certain pump run time. Sounds like that's what you're seeing.

I would run through a very careful and thorough bleeding process, making absolutely sure the reservoir never gets close to low. I bet you've got a nice big air bubble somewhere in there. I'm also not the paddo method works to clear out all bubbles. I think it works to clear old fluid, but you might need to crack each bleeder (be careful) to get all possible air pockets out.
 
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Agreed on the deep breath. You're ok, and it's incredibly unlikely components just started failing right after your bleed.

Your event sounds exactly like what I'd expect some air in the lines to do. I think you're the second person to have this sort of thing happen in just the last week.

Air in the lines would make the pump unable to build proper pressure. The motor would run and run and run, but be unable to build pressure because it would have to move an enormous amount of fluid to compress a big air bubble. If the system is bled, the pump moves relatively little liquid, but can build a large amount of pressure. If there's air, the pump will run out of fluid before the air is compressed. The result would be that the pump runs seemingly forever without a buildup in pressure. If I were designing a hydraulic system like that, I'd build in a warning when no pressure was seen after a certain pump run time. Sounds like that's what you're seeing.

I would run through a very careful and thorough bleeding process, making absolutely sure the reservoir never gets close to low. I bet you've got a nice big air bubble somewhere in there. I'm also not the paddo method works to clear out all bubbles. I think it works to clear old fluid, but you might need to crack each bleeder (be careful) to get all possible air pockets out.

I agree that it is acting like there is air in the lines. I will re-bleed first thing in the morning.

I would think the only way air got in the lines was from the bleed nipple particularly at the height accumulator, it ran dry but the pump did not... I took out 1 quart, put 2.5 quarts in, and blead between 1 and 1.5 quarts out... after the blead there was a little over a quart in the reservoir...

If the air is in the height accumulator, and it has been blead dry, how do I blead it when nothing is flowing?
 
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Ok. Deep breath.

If you are very sure the reservoir never went dry during the bleeding procedure, check the pressure sensor again. The fact that you can't build proper pressure in the tube accumulator tells me you either got quite a bit of air in there, or your pressure sensor is suspect. If you can apply 12V to the pump (motor) and it works, it may also mean the pump section itself could be bad or so gunked up it isn't working properly anymore.

I am very sure the reservoir never went dry. The only time it was empty was after I finished emptying it before I refilled it and the truck was off at that point. I sucked about 1 L of fluid out, put 2.5 liters in, and blead between 1 and 1.5 liters through the 5 blead points (height accumulator, LF, LR, RF, RR).

12V fires the pump (motor) right up.

I tested the pressure sensor again today applying voltage in reverse of the fsm (because the fsm is a little unclear) and got 3.06 volts and 1.14 volts depending on which leads I tested. Testing with voltage in the direction indicated by the fsm yields .62 volts. I tested in this fashion 4.5 volts (three 1.5v watch batteries in series) positive applied to the top left lead, negative to the top right lead, and multimeter between the top right and bottom lead. For posterity sake I have attached the flow chart I am using.

I am a little unsure about which leads are the correct ones to test since the diagram in the fsm I was able to get shows the leads in a line whereas the actual leads are in a triangle formation. I'm a bit confused about this because the part numbers for the pressure sensor are the same for all years of lx and the two years the lc had the option. Regardless I'm not getting 4.5 volts so this means my sensor is bad right???
 

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Starting to think it is my pressure sensor... anyone in Central Virginia have a working one I could swap in? I really don't want to go down the rabbit hole of throwing parts at it unless I am as sure as I can be...
 
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I found a ?newer? version of the AHC dtc 1762 on another post on which the correct triangular shaped connector is shown. The procedure has you check the voltage at the connector of the pressure sensor while it is installed. I get a voltage of 1.48-1.50 which is within fsm spec of 1.48-1.85... now what??? new pump??? I guess I bleed LR, LF, FR, LR (height accumulator does not yield anything when I crack it) if that doesn't do it new pump???
 

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I agree that it is acting like there is air in the lines. I will re-bleed first thing in the morning.

I would think the only way air got in the lines was from the bleed nipple particularly at the height accumulator, it ran dry but the pump did not... I took out 1 quart, put 2.5 quarts in, and blead between 1 and 1.5 quarts out... after the blead there was a little over a quart in the reservoir...

If the air is in the height accumulator, and it has been blead dry, how do I blead it when nothing is flowing?
I blead about 6 ounces out of each line. I was able to get maybe 2 ounces out of the height accumulator. I did notice in testing the system that the front and rear are going up some, more the rear than the front and when I blead the system the vehicle did drop some, much more the rear than the front. A couple times in testing the vehicle would turn on in the n setting and the light would stay lit. Not sure it was actually in n. I would then try to go from n to h and it wouldn't go nor would it go back to n after trying to go to h. It would only stay lit in l...

Also I noticed the pump is sometimes running loud... it is intermittent so I have not caught it but will post up the video when I get one.

I got the vehicle to go into n once... It really does feel like sometimes when it moves the front is not moving only the back... thinking about cranking the torsion bars to make it easier on the front but not sure because my current front pressures are so low and when I checked them last week when the AHC was working the front was spot on at 6.9...
 
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ramangain

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Another deep breath.

There is a thread by @BullElk where he had to diagnose a dead pump assembly too. It was within the last month or two. In that thread, another generous MUDDER had offered to send him a spare working AHC pump for testing.

Perhaps locate that thread and reach out. With any luck, someone cam send you a spare pump for testing purposes; maybe a sensor too.

The only spares i have are lugnuts and rear wheel studs, sorry.
 
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At 6.9 It would not hurt to crank the TB'S. Each clockwise turn on each tb will lower pressures only by .2 Mpa . Being on the lower end is fine and may be beneficial for longevity of the globes.
 
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At 6.9 It would not hurt to crank the TB'S. Each clockwise turn on each tb will lower pressures only by .02. Being on the lower end is fine and may be beneficial for longevity of the globes.
I'll do this here shortly... why not... I doubt it will fix the problem but I'll try anything at this point.
 
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Another deep breath.

There is a thread by @BullElk where he had to diagnose a dead pump assembly too. It was within the last month or two. In that thread, another generous MUDDER had offered to send him a spare working AHC pump for testing.

Perhaps locate that thread and reach out. With any luck, someone cam send you a spare pump for testing purposes; maybe a sensor too.

The only spares i have are lugnuts and rear wheel studs, sorry.
Deep breath, ok :) I'll reach out and see if someone can send me a spare pump and sensor for testing purposes... not sure what else to do at this point...

Is it possible that it is a height sensor that got fouled when the truck was dropped all the way to the bumps stops in the bleed process???
 
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I'd keep bleeding too. Fluid is much cheaper than pumps. Search ebay for used pump an other parts. seller: okautopartsms lx470 has pump accumulators globes etc. Off a really clean 07lx.

And since you used Paddo's method maybe time to ask @PADDO for help.
About to pull the trigger on a pump if I don't have a breakthrough soon. I could run another liter through it but I have my doubts... Paddo has been inactive for a couple months so I'm not sure he'll respond...
 
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I'll do this here shortly... why not... I doubt it will fix the problem but I'll try anything at this point.
Tried it, no change... I did get it to go to n once after this confirmed by techstream (also all my height sensors were giving me appropriate data) however wouldn't go to h and when I took it back down to l it wouldn't go back up to n
 

ramangain

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So, bleeding is just manual labor since you can recycle the fluid.

Also, there is a way to "reset" the AHC system by jumpering specific pins. I know it can be done, it has been done, but I haven't done it myself so I don't know which pins where to jumper.
 

ramangain

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I don't think bleeding is a bad idea since a guy in the 200 forum experienced similar symptoms to yours in the last month or so. He diagnosed the pump as actually OK in the end, and through a process of cleaning the pump (I think) and multiple bleeding cycles, everything went back to normal for him.
 
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So, bleeding is just manual labor since you can recycle the fluid.

Also, there is a way to "reset" the AHC system by jumpering specific pins. I know it can be done, it has been done, but I haven't done it myself so I don't know which pins where to jumper.
E1 to TC key to on position hit brake pedal 9 times..
Or something like that... I guess I'll bleed a couple liters through and see...
 
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I don't think bleeding is a bad idea since a guy in the 200 forum experienced similar symptoms to yours in the last month or so. He diagnosed the pump as actually OK in the end, and through a process of cleaning the pump (I think) and multiple bleeding cycles, everything went back to normal for him.
I put 2 more liters through with no change, actually now on techstream I'm not getting any pressure in the front or in the accumulator. I guess I'll look into cleaning the thing...

1121201631.jpg
 

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