AHC C1751 & C1762 Help/Recommendations/Suggestions (1 Viewer)

[NC-Grayson]

@YvesNL thanks for all your help, as well as @PADDO

I managed to get through all of the topics you described, and I'm still at a roadblock unfortunately.

Got my step values back to 8 somehow, (not sure what I did, but they're there at 8).

I still CANNOT get the system to "bleed air of AHC & AVS hydraulics system" or step 3 on the C1762 page in the FSM.

I can get the truck's AHC light to flash after pressing down 5x on the switch w/in 5 seconds of turning the key to on, but the truck will NOT raise.

it says, "Hint, at this time the AHC motor relay comes ON in order to raise vehicle height of the front wheels and air inside the AHC/AVS hydraulic system start to bleed with the pump motor operating"

HOWEVER, NONE OF THIS HAPPENS. It's like the ECU is not sending a signal to the motor relay to turn the pump on, because when I do the steps, the pump does not turn on!

12V power direct to pump - pump pumps
jumping AHC relay - pump pumps
bleed air of AHC/AVS hydraulic system - pump DOES NOT TURN ON, pump does not pump!

I'm so lost.

 

[Julian Stead]

I know it sounds daft but all doors are shut and are showing shut in the AHC ECU. Check the fuse, even if it looks good.

 

[Julian Stead]

Ps, disconnect the battery for five minutes as well.

 

[Julian Stead]

just looked at your screen shot, your voltage looks low, was the vehicle running

 

[NC-Grayson]

just looked at your screen shot, your voltage looks low, was the vehicle running

I know it sounds daft but all doors are shut and are showing shut in the AHC ECU. Check the fuse, even if it looks good.

Ps, disconnect the battery for five minutes as well.

Thanks for your advice. The original techstream shot the vehicle was off, Idle voltage is around 13.8-14.1 so we're good in that respect.
I have REPLACED every fuse already, even the ones that tested good. All doors are shut, and I have disconnected the battery, and AHC ECU multiple times, sometimes over night even.

Puzzled.

 

[PADDO]

The engine must be running (and the ECU must know it's running via the GEN signal on the AHC ECU REG terminal) for the damping system to function. The damper step values are reported as zero with just the ignition on. On starting the engine the damper step motors do a self check shuffle and set at mid point (8) then start shifting through their ranges as a function of speed, damping setting and wheel height displacement, otherwise the step value fields aren't populated. Just IG is handy for checking height sensor and steering angle feedback but generally the engine must be running with normalized inputs [as best you can] to ensure valid fault finding. The FSM (some versions) actually fails to mention the engine must be running for the 16 step damper test and if you follow the incorrect FSM verbatim you get nowhere but frustrated.

 

[NC-Grayson]

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.

how did you manage to test the fluid pressure sensor by measuring on the ECU? are your DMM test leads small enough to fit into the back of the ECU connectors leaving the wires in place per the FSM?

 

[NC-Grayson]

I replaced the fluid pressure sensor today, still no luck. very frustrating.

Looking at the power source circuit of the ECU.......when I test for continuity of the GROUND terminals of the AHC ECU and the body ground, I do NOT get continuity.

So I replace the ECU?

 

[YvesNL]

Nope if you are not 100% sure the ECU is broken I would not replace it.
The ECU is an expensive part.

Measure the different ground pins to ground. If there's no contact between the pins and the ground something in the wiring harness must be wrong.

Do the AHC light on your dash light up?
Is it possible to communicate with the ECU through Techstream?

If the answer to one of those questions is Yes there must me some sort of ground in your system.

 

[NC-Grayson]

Nope if you are not 100% sure the ECU is broken I would not replace it.
The ECU is an expensive part.

Measure the different ground pins to ground. If there's no contact between the pins and the ground something in the wiring harness must be wrong.

Do the AHC light on your dash light up?
Is it possible to communicate with the ECU through Techstream?

If the answer to one of those questions is Yes there must me some sort of ground in your system.

AHC light does light up - as well as the flashing OFF light on the dash. Both are functioning.

I believe I've been communicating with the ECU through techstream....I can do the tests for the main relay, motor relay, solenoids, etc. and get results.

The only thing I cannot do is step 3 in the FSM, "Bleed Air of AHC & skyhook TEMS hydraulic system"

I jump the two terminals in the DLC3, push down 5x w/in 5 seconds after starting the engine, change damping mode switch to comfort, press and hold up on the height select switch.......and nothing. radio silence.

The AHC OFF and ABS lights blink rapidly per the fsm, but there's no movement out of the pump.

I can manually activate the pump by supplying 12V directly or jumping the relay. just nothing during the active test.

 

[Julian Stead]

Are you in comfort mode?

 

[NC-Grayson]

Are you in comfort mode?

yes I am....I wish it was that simple!

 

[Julian Stead]

Sorry got to check, comfort is all the way to the left. Does it operate under its own steam, you know engine running, all doors shut. I recently lost all my fluid, drove 100 miles with the fluid pumping out of a leaking pipe and I managed to bleed the system after repair without techstream.

 

[NC-Grayson]

Sorry got to check, comfort is all the way to the left. Does it operate under its own steam, you know engine running, all doors shut. I recently lost all my fluid, drove 100 miles with the fluid pumping out of a leaking pipe and I managed to bleed the system after repair without techstream.

no, it doesn't operate at all. stuck in low

 

[Julian Stead]

There is a pump relay mentioned, have you tested or bypassed this to see if pump will operate.

 

[Julian Stead]

Without a wiring diagram I cannot say for certain but I would image that if the ECU is happy, it will send a feed or ground to the relay coil, this intern will close the switch to send power to the pump, have you tested the ECU signal to the relay with a test light when you are activating bleed procedure. you could test this signal direct on the ecu then follow it downstream to the relay.

 

[Julian Stead]

Sorry reread your original post regarding the relay, I would test the signal at the ECU, the one that activates the relay, do this with the relay in and out.

 

[NC-Grayson]

Without a wiring diagram I cannot say for certain but I would image that if the ECU is happy, it will send a feed or ground to the relay coil, this intern will close the switch to send power to the pump, have you tested the ECU signal to the relay with a test light when you are activating bleed procedure. you could test this signal direct on the ecu then follow it downstream to the relay.

can you elaborate on this procedure?

 

[Julian Stead]

if you have the wiring diagram you will be able to find the signal wire that goes from the ECU to the relay, when you activate the system you need to test this connector pin on the ECU, IF the signal is there then test it at the relay. If it isn't there then the either the ECU isn't happy with something or the ECU itself is faulty, You might have to clear your fault codes before the system will try to operate.

 

[Julian Stead]

My best guess to your problem is a weak pump or fluid contamination or both. The pump isn't activating at present to protect the system, the pump is unable to move the vehicle to the selected height in a given time so flags up the C1751, this pressure abnormality also brings up C1762. The system might need a reboot so clear fault codes and then disconnect the battery and even the AHC ECU, refit, if the pump tries to operate but doesn't succeed in moving the height and there are no leaks present then I would guess the pump. You could try cranking up your torsion bars temporarily for testing to help the AHC system along.