AHC C1751 & C1762 Help/Recommendations/Suggestions

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Joined
Jul 18, 2015
Threads
15
Messages
184
Location
Wilmington, NC
Greetings All,

98LX, 117K miles, all original AHC components. Since buying it, I have flushed all damping actuators and the accumulator 4 months ago until fresh red fluid came through. Ride quality improved for about a week, and has since gone way down hill worse than when before I flushed fluid.

Fast forward to present day. The LX has been parked in the driveway and hasn't moved an inch in over two months. It's a constant source of disappointment every time I walk out into the driveway and see it sitting there, looking neglected, needing some love that I just can't wrap my head around where to begin now.

I took it out to the 4x4 beach the day before I parked it. At the beach, I found a tiny mouse in the engine compartment hiding out near the battery. It does sit idle for days on end so this mouse wasn't too much of a surprise. He had definitely NOT taken up residence when I found him....he was just looking for a place to escape the heat I think. I'm rambling on here about any detail leading up to the LX getting parked that I can think of that may end up relevant or not. That said, I haven't found any chewed up wiring or any damage that the tiny mouse may have caused.

When I got home from the beach, I noticed the AHC light was blinking OFF, and the fronts had settled and parked itself in the lowest position.I drove it to work the very next day. about halfway to work, AHC operation returned to normal, the flashing light went away, and I could cycle through L-N-H and back down. On my way home from work however, it defaulted back to limp mode, and the front is back down on the bumps with the flashing OFF light and no up/down control of the AHC

I pulled the DTC's with my amazon techstream, and got DTC C1751 (continuous electric current to compressor) and C1762 (abnormal oil pressure for pump). I've read every one of the VERY few threads here on mud regarding these two DTCs, (which point to the pump sub-assembly), as well as reviewing the FSMs that I own. In the past I've considered myself a pretty handy fellow around a vehicle...but EVERYTHING to do with the AHC completely confuses me to the point that my frustrations with the suspension are so frustrating that I let it sit parked and try to ignore it. But the time has come to fix the damn thing and get back to loving every minute of driving it.

First and foremost, I'm not even sure how to clear the DTCs in my copy of techstream. I figured that if i cleared the codes, I could at least get it back to N height to drive it to a indepentent shop or the dealership to get their eyes on it. But if I managed to clear the DTC's, that may not even work to get me back to N height.

I can say that I have somewhat visually confirmed that the height sensors are still connected...but it's difficult short of removing the front wheels when the suspension is stuck so low on the bump stops.

I've read several instances of manually activating the pump with a jumper wire in the DLC1, but I'm not sure which posts to jump, (even with my FSM I feel blind), and I'm not positive this would even get my height back up to N.

Basically, I'm looking for ideas on what to do next. I'm pretty sure at this point its probably more cost effective to replace the AHC rather than throw parts at it, just to find out I need more parts. But I could just be fed up enough that I've convinced myself the whole system is too troublesome and needs to go. My logic tells me that right now my issues are just with the pump itself - but once I get that fixed I still have to deal with the bouncy harsh ride - and we all know how much those dampeners and accumulators run.

I'm definitely handy enough to swap out standard suspensions for standard suspensions, but I'm not sure what to do with the AHC. I'd prefer to do it the right way and rip the whole system out if I have to, but that may be above my pay grade and I may be better off just capping the lines and swapping out to a standard suspension.

I'd replace with an OME medium kit (~$1,700 w/ UCA's), or something Icon/Fox/King designed for the LC ($$$$$). At any rate, I'm confident a nice lift with aftermarket goodies would still be light years cheaper than to replace my AHC components at this stage.

I guess what I'm looking for is recommendations from those experienced on what I should do next. I'm down to tinker with the AHC and codes.....but I'm so confused at this point I'm not sure where I'd start first.

Can anyone recommend an experienced LX/LC shop somewhere in NC or close that has done conversions? I believe it's slee that will do a full conversion and removal of all AHC components even including the bulb on the dash and the "pocket" coin holder....Are there any companies that sell kits for capping off the lines if I were to do the conversion myself? Can anyone comment on the level of difficulty of completely removing all the AHC components and lines myself?

So confused on what to do......thank you ALL for your input.
@PADDO
 
You're describing a failed front height sensor/linkage. I wouldn't get hung up on the pump codes at this stage as they can lead you down a rabbit hole. Pretty straight forward I suspect, as long as the mouse hasn't done a number on a harness which would need a bit more effort to track down. Bring up Techstream AHC page and read off what your front height sensor heights are reporting, you can jack the front up to where it should be at N ~ 19.75in center hub to fender bottom and read off Techstreams height numbers. Do this just with Ignition on (not engine running) as you just want to see and monitor the feedback and don't want the thing to come alive and auto adjust on a jack. The sensor that's reporting the outlier number when it should be ~ 0.0 at your N height is the suspect.
To clear codes there is a tab box on the lower left (on my version anyway) of the AHC screen that has an image of an engine with an eraser - click it and agree to the confirmation.
If it is a failed height sensor you can score an Aisin replacement with the bracket/lever from Rock Auto for $150 or just the potentiometer made by Dorman (part 924-755) for $55 off Amazon. You graft the Dorman potentiometer on to your existing bracket/lever mechanism.
 
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THANK YOU @PADDO !

All I needed was a nudge in the right direction to get the ball rolling. You have provided just that and more. I'll be in Charlotte in two weeks, and if you're Up Over then I'll deliver some beers.

I'll get on this tomorrow and let you know where things are.

THANK YOU


You're describing a failed front height sensor/linkage. I wouldn't get hung up on the pump codes at this stage as they can lead you down a rabbit hole. Pretty straight forward I suspect, as long as the mouse hasn't done a number on a harness which would need a bit more effort to track down. Bring up Techstream AHC page and read off what your front height sensor heights are reporting, you can jack the front up to where it should be at N ~ 19.75in center hub to fender bottom and read off Techstreams height numbers. Do this just with Ignition on (not engine running) as you just want to see and monitor the feedback and don't want the thing to come alive and auto adjust on a jack. The sensor that's reporting the outlier number when it should be ~ 0.0 at your N height is the suspect.
To clear codes there is a tab box on the lower left (on my version anyway) of the AHC screen that has an image of an engine with an eraser - click it and agree to the confirmation.
If it is a failed height sensor you can score an Aisin replacement with the bracket/lever from Rock Auto for $150 or just the potentiometer made by Dorman for $55 off Amazon. You graft the Dorman potentiometer on to your existing bracket/lever mechanism.
 
@PADDO

here's the data. Note: I have 285/75/16's so 33's, not sure if that matters.

As it sits stuck in L Position
~16" Hub to Fender
FR -2.9
FL -2.8

At 19" Hub to Fender
FR -0.1
FL -0.1

At 19.5" Hub to Fender
FR 0.4
FL 0.5

I was able to clear the DTC's per your instructions. I cleared the codes at least five times, but every time after clearing, in a matter of seconds the C1762 code reappears. The C1751 has not returned.

I was able to get some audible movement/noise out of the pump when I had it still up on the jack at 19.5". I cleared the codes, started the truck, and pressed the up on the AHC switch to get it to N. I heard noise, and saw a bit of upwards movement from the front of the hood, but then it stopped and the OFF light kept flashing.

Back down on the ground at ~16" hub to fender, I get nothing. No sound, no movement, just stuck in low with C1762 continually reappearing.

I take it my sensors are in ok shape based on this data? Should I be looking at replacing the pump sub-assembly?
 
Yes the front sensors seem to be fine, if one's gone it'll have some crazy value, or no value, when the other correlates with the actual height i.e. 0.0 @ 19.5 inches for stock.

Here's the fsm guide for 1762, after confirming fluid level is ok in the reservoir I'd go straight to step 3, manually raising the front/bleed air and see what the outcome is. Use the the sheet that has you shorting the connectors in dlc1 (under the hood, passenger fender) because it's easier then trying to short terminals upside down on the DLC3 in the driver footwell as per the fsm guide. You'll get the same outcome, just follow the instructions: IG off, fit short, start, push DOWN 5+ times in less then 5 seconds after starting, switch the damper setting from position 2 to 1 (softest comfort) then hold in the the UP rocker and fingers crossed it kicks into life and this can take 10 seconds or so.

bleed AHC pump at dlc1.PNG
 

Attachments

Thanks for the info @PADDO

I'm at work for the day so I can't dig in at the moment. But yesterday after I found that the height sensors seemed to be intact, I went down the FSM for C1762. In your post you say you'd skip to step 3 after verifying that the reservoir was full (which it's actually above the full line, to be expected with the fronts as low as they are).

I tried step 2 in the FSM, which is the same as what you attached in a .pdf. Not sure what voltage battery I should have used on the pump motor connector, but I had a 9V handy and tested it that way. I'm not sure what I expected to happen, but I got a VERY faint sound out of the pump motor. I don't know much about electronics and pumps and such, but what I heard did not sound like a healthy functioning pump.

I'll skip to step 3 when I get home and see what happens.

Thank you!!!
 
The pump motor is 12v (actually operates in the 13.5-14.2v range, the alternators output voltage) and its fused at 50A so you won't get much activity out of a 9v battery - not one of those little 9v alkalines for smoke detectors and energizer bunny toys was it? DC motors like the AHC pump need current at rated voltage to produce torque so if you were to test it you'd either connect it to a 12v auto battery or a regulated power supply. Grab a paper clip and try the active test and if that doesn't do anything it could be procedural so then you'd connect 12v to the motor to rule it in or rule it out. Blue terminal is + on the motor.
 
The pump motor is 12v (actually operates in the 13.5-14.2v range, the alternators output voltage) and its fused at 50A so you won't get much activity out of a 9v battery - not one of those little 9v alkalines for smoke detectors and energizer bunny toys was it? DC motors like the AHC pump need current at rated voltage to produce torque so if you were to test it you'd either connect it to a 12v auto battery or a regulated power supply. Grab a paper clip and try the active test and if that doesn't do anything it could be procedural so then you'd connect 12v to the motor to rule it in or rule it out. Blue terminal is + on the motor.

Thanks @PADDO for the clarity. Question though....and maybe I'm over thinking things here:

On The attached .pdf & as well as my paper copy of the FSM....they differ ever so slightly from your personal notes and directions. For instance, your directions say to switch the damper from 2 to 1, and the FSM says keep it at 2. You say to push and hold the up rocker, FSM says just press it for the front wheels.

Am I missing something ?
 
Starting from the center position 2 you switch it to 1 after doing the 5 push sequence - that's what my hard copy FSM says (and my notes) and you push and hold the up rocker. When you release the rocker switch it stops the up command. Holding the up rocker in let's you drive up beyond H or holding the down will let you drive it all the way to the bumps. I've done it this way many times.
 
@PADDO So I tried step 3 of the FSM and your .pdf for the active test and I got nothing. . I was able to get the OFF light to blink at .25 second intervals after starting and pressing down 5 times within 5 seconds...but I get nothing. No movement up whatsoever, and no noise.
I'm assuming my fancy paper clip short worked just fine in the DLC1, because when I removed it after no movement I tried again and was unable to recreate the .25 second flashing interval of the OFF light.

Does shorting Ts & E1 of the DLC1 accomplish the same thing as shorting the Ts & Cg of the DLC3?

As in did I only try the active test by shorting DLC1 or did I also "bleed air of AHC & skyhook TEMS hydraulic System"?

What's next? 12v at the pump motor?

Also, is the DLC3 the same thing as what I plug my Scanguage into? newb here.
 
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DLC3 is the obd11 connector in the ds footwell. The "active test" and the "bleed air procedure" are the same initial steps to energize the motor/pump and raise the front. These steps were suppose to be quick and easy checks - gone a bit off track it seems. You can apply 12v directly to the motor too, if it's air locked then that should clear it. Can you post up a screen shot of techstream AHC page please because a 1762 fault code shouldn't drag on like this.

image.jpg


image.jpg
 
thanks @PADDO.

Laptop with techstream is at home, I'll get on the screenshots when I get off work.

Earlier you mentioned using the 12v auto battery to test the AHC pump.....can I just run some wires from my battery terminals to the AHC pump & motor connector to test?
 
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Yes you can run 12v directly to the motor terminals - you can either run + and - via twin wires from the motor terminals back to your battery or ground the top motor terminal (the one with the white/black cable into the connector and just run a single wire (from the terminal that had the blue wire to it) back to the + post on your battery. Regardless, be careful to avoid shorts, sparks etc. If you've supplied power correctly you should hear the motor energize.
 
Yes you can run 12v directly to the motor terminals - you can either run + and - via twin wires from the motor terminals back to your battery or ground the top motor terminal (the one with the white/black cable into the connector and just run a single wire (from the terminal that had the blue wire to it) back to the + post on your battery. Regardless, be careful to avoid shorts, sparks etc. If you've supplied power correctly you should hear the motor energize.


@PADDO thank you for all of your help thus far. I am just getting a chance to dig back into this.

I rigged up a 12V source via my portable jump-start battery pack, and two leads from my multimeter. I struggled at first, but I DEFINITELY have noise coming out of the pump this time when I hooked it up.

I have however noticed, (not sure if it's relevant or not), that by shorting Ts & E1 in the DCL1, that the OFF light, (and the abs light), automatically start flashing at 0.25 second intervals immediately upon startup, BEFORE I have a chance to complete the "push the down button 5 times within 5 seconds after starting". Not sure if it matters or not, but the way Im reading the FSM is that it shouldn't flash at .25 second intervals until AFTER pressing down 5 times. Maybe I'm over thinking it.


full


this is the screenshot from techstream. Take a look and lmk what you see. Thank you!!!
 
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@PADDO.....I could really use your masterful experience here.

I seemed to have run through all the steps in the FSM regarding C1762 and can't seem to isolate the problem.

Is it time to pony up and replace the pump, the pump-sub assembly, or just wash the whole AHC system for something standard?
 
bump. does anybody know how many volts I should be getting on my dmm when connected to the pump motor connector?

I've gone through every step of the C1762 diagnosis, and no go.

I can operate the pump manually connected to 12V source....but it's as if once the truck is on the pump isn't getting a signal.

Very frustrated with this.
 
I would:

- Check the connection from AHC ECU to relay (signal side);
- Check the ground of relay (signal side);
- Jump the relay with a wire to rule out wiring to the pump (large terminals), pump should run;
- Check fluid pressure sensor (measure on ECU, then you also know that all the wiring and connections are good);
- Check your height sensors with a multimeter (Ohms), or with 3 AA betteries as per FSM.

If you need help or don't know where to measure, let us know.
 
I wouldn't just rip out the AHC system. It drives smoother than stock and you also have some adjustments to play with.
Swapping out costs quite some money and the problem in the AHC could be cheap if you troubleshoot yourself.

Parts should be easy to get and cheap as everyone rips out the AHC.

The system itself isn't that hard to understand there are quite a few but very understandable components.

The thing that makes the AHC a bit of a PITA is that we don't know the software that is in the ECU.
No software or flowchart is present. That means we have no idea what is going on in the ECU when something is out of spec and not in the error handling of the ECU. So the only thing we can do is make sure all components work like they should, you can check all the individual parts of the AHC system with basic tools.

I was reading you posts again and it seems we have very similar problems like:
- Active test not working;
- (sometimes) strange trouble codes;
- Pump does nothing... ever, but will run when power is applied manually (check if pump is running by jumping the relay);
- STEP VALUES OF 0!

Just noticed that ^^. Your step values are also zero. Front wheel step and rear wheel step that is.

Have a look at my thread: Hi From The Netherlands & AHC Issues

I would perform the checks I did too. Read everything to gain a bit more knowledge about the system. Pay special attention to post #29.

Do the checks I did, try the "INPUT SIGNAL CHECK" in techstream and also get the multimeter out.

Check your steering angle sensor. First in techstream, wheels straight ahead should be around 0 degrees. See if the values changes like they should (lock to lock).

Also check the values with your multimeter like described in post #29. See if the value is changing between 0 and 5 volts, and check resistance.

If you have any questions or measurement results, post here I'm subscribed to your thread and I'm sure others also want to help you too.
 

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