AHC active test (1 Viewer)

[Chris T]

So having completed a replacement burst pipe I've now run into another problem the car refuses to pick itself back up.
I know from running tech stream that the rear ride height sensor is throwing a DTC I plan to replace it at some point but getting the car drivable was then goal.
No problem I thought let me do an active test and see if I can't force the front end back up to ride height.
From my understanding you should be able to jump the TS and E1 pins and then using the controller raise the front and rear independently of each other.
Unless I'm an idiot I'm getting sweet nothing, no pump activity at all.

So the question is does the failed ride height sensor prevent the active test from running the pump?

 

[TXLXAG]

I don’t know for sure the relationship between the ride height sensor and the operation, but I just had the active test also not work.
I replaced my globes about 6 months ago, and did not bleed enough. I had a decent amount of air in the system and it started the pogo again in the front. I bled the system, all 4 globes and the center height accumulator. Went ok for 2 raises and then the back end fell to the bumpstops.
It took me 2 days of repeatedly trying active test(never worked), clearing codes, applying 12v direct from the battery, and clearing codes before it finally worked.
I rocked the car back and forth after clearing codes and starting the engine and noticed the “abnormal pressure” DTC didn’t come back on as fast when I kept rocking it back and forth. I jacked up the back end by the pumpkin until tires were about 2-3” in the air, started the engine and rocked it repeatedly, and it eventually caught and lifted. I bled the system and cycled it L-N-H probably 10 times after that. So far it’s been working great.

 

[IndroCruise]

So the question is does the failed ride height sensor prevent the active test from running the pump?

Yes -- see detailed responses to @Chris T in other threads. Where a Height Control Sensor Circuit failure (Sensor or Connector or Harness) causes "Fail Safe Function" to be initiated by the Suspension Electronic Control Unit (ECU), this will not be over-ridden by Active Test.

Conversely, if Active Test will not work, it is being prohibited by a defined fault somewhere in the AHC/TEMS systems.

The task then is to find the fault that is preventing the Active Test operation.

 

[IndroCruise]

I don’t know for sure the relationship between the ride height sensor and the operation, but I just had the active test also not work.
I replaced my globes about 6 months ago, and did not bleed enough. I had a decent amount of air in the system and it started the pogo again in the front. I bled the system, all 4 globes and the center height accumulator. Went ok for 2 raises and then the back end fell to the bumpstops.
It took me 2 days of repeatedly trying active test(never worked), clearing codes, applying 12v direct from the battery, and clearing codes before it finally worked.
I rocked the car back and forth after clearing codes and starting the engine and noticed the “abnormal pressure” DTC didn’t come back on as fast when I kept rocking it back and forth. I jacked up the back end by the pumpkin until tires were about 2-3” in the air, started the engine and rocked it repeatedly, and it eventually caught and lifted. I bled the system and cycled it L-N-H probably 10 times after that. So far it’s been working great.

There can be various causes of the "Fluid Pressure Abnormality" Diagnostic Trouble Code (DTC) C1762. By far the most is common cause is when there are large quantities of air in the AHC/TEMS system. Other causes can include partial blockages of the strainers inside the AHC Pump or incorrect operation of the Return Valve.`

The point is that the response time of DTC C1762 is very quick -- about 0.6 seconds according to the FSM extract immediately below -- at which time the Suspension ECU initiates the "Fail Safe Function" defined below. This prohibits the Height Control function, meaning the ECU prevents operation of the AHC Pump, and, meaning the ECU may cause the vehicle to drop to the level of the lowest wheel.

The fix procdedure described on the second page of the full FSM extract for DTC C1762 attached below is similar to the Active Test, except that the instruction is to start to bleed with the pump motor operating. This would mean that the ECU causes opening of the Levelling Valves in the Control Valve Assembly to allow AHC Fluid to pass to the open bleeding screw(s).

However, experience shows that this procedure does not always work and the defined "Fail Safe Function" repeatedly stops the AHC Pump after ~0.6 seconds.

When this happens, the repetitive effort described by @TXLXAG in Post #2 is the practical workaround.

 

[Chris T]

Thank you for the repeated help, I feel like I'm loosing it.
Am I being a dunce?
Worked my way through steps 1 and 2 of DI-9 all fine no issues got to step 3 "bleed air out of ahc and tems system"
Looking at the DLC3 port on the top row working left to right the TS pin should be the third along? However there's no pin in the third position from the left. What am I missing here?

Skipping past that step and onto step 4 I get 69ohms over the relay, is that to high?
I currently do have a good way to apply battery power to the relay so unsure if it's operating correctly when power is applied.

Step 5 fluid pressure sensor.
How do I go about this with sensor on or off? Or do I need to be able to probe the backside? Does that mean revealing a section of the conductors.