4.8 Air Suspension issues, EHC 74 code and compressor won't switch on

[Leicesterboy15]

Hi All, I really need some help with my suspension, I've read loads of threads on this but I can't find a solution. The car is now slammed to the ground and running on the bump stops, I stopped using it as soon as this happened.

I have pulled the codes using INPA and DIS and I get the 74EHC Accumulator Pressure Remains Constant (accumulator filling requested).

When I searched for this I found that the compressor might need a rebuild so I rebuilt it with the kit and no difference. I could hear the top black part of the compressor clicking like it is trying to switch on but it then immediately throws the error again.

So I replaced the compressor with a new (aftermarket) unit and I am getting exactly the same thing. When I try to raise the car with INPA I can hear the top of the compressor clicking but the compressor doesn't kick in, when I try to lower the car I can hear a click from the rear of the car so it appears as though the signal is being sent but the compressor won't turn on. The first few times I could hear the compressor releasing air until now the pressure reads 0.5 and there is nothing left to release. The car was already slammed before this.

There is a back story:

I got the inactive warning on the dash and stupidly left it for a while, then I replaced one sensor and it turned out not to be the problem but I found that the rear sensor was seized, I bought a replacement and the found the bolt pattern was different when I went to fit the new one so I drove it for a week without the sensor, the suspension was fine.

I managed to fudge the bolt pattern and assumed the sensor was the same as the loom was identical, this new sensor didn't change anything. So I bought a genuine sensor from BMW and fitted that. I was still getting an error but the rocker light was now illuminated and I could raise and lower the car with INPA (I had always been able to do this).

Then one day as my wife was driving the car it slammed to the ground in one journey on all 4 corners at the same time which is when I stopped using it, this was about a week or so after the sensor was changed.

Since then I have been chasing the problem as above and now don't know where to look, I am not convinced that a BMW garage will know without replacing a load of parts. Oh and there is an LPG tank in the way so I can't easily get to the battery compartment.

The 2 air suspension fuses in the right rear are not connected to anything (I have 2 axle EHC) and I have checked the fuse in the glove box which is fine.

I am not sure where to go with this now, I would gladly take it to a garage and sink some money but it has to be within reason and I don't know where I am going to find the expertise in SW London. Is there anywhere else I can look? Should I get the LPG tank removed temporarily to get access to the battery compartment and the control module? Could it be that? Can this be repaired or only replaced by BMW?

I can see all the air lines and the loom disappear under the right boot cover, is there something under there that could have gone? I notice that the connectors to the top of the compressor (3 of them) are not connected to the power loom to the bottom of the compressor so I am guessing the top bit sends a reading somewhere which then sends power to the bottom part to switch on?

I am at a real loss here and I can't use the car until the is sorted, I am not sure where to go next!

[LVR]

Hi mate. Welcome to the air suspension hair pulling club!

Ok..I think you’re gonna have to get access to the pump/battery area.

I found when mine started playing up that the battery state of charge was affecting the pump operation so I would start by checking the voltage at the battery in cold state ie after systems idle for at least 20 mins. If it’s under 12.45 volts then it might need charging. Mine was down to 12.20 volts and died not long after. New battery and all was good again.

Then I’d check connections at the comp and check when it actually is running.

I don’t use INPA as it doesn’t allow a reset of the EHCII module from what I understand. I use ISTA which allows individual strut operation on a corner by corner basis, easy resets and a live check of the sensor operation.

It would probably be worth your while to see if an Indy has access to ISTA and can reset your module, and see where you go from there.

[Paulpen]

Simple things first - what is your accumulator pressure and compressor temp readings ?


From just your info it sounds like the ECU is requesting the compressor to run but there is a wiring issue preventing it from running correctly

[Leicesterboy15]

Thanks guys, I have all the readings, just trying to work how best to upload them...so you will have them son!

[Leicesterboy15]

This is how the car looks (also image test)

[Leicesterboy15]

Sorry guys I have been offline for a bit. This is what happens when you post after a few beers!

Got all the readings today from INPA and DIS / GT1, the car is going in tomorrow to get the LPG tank removed.

This is what the car looks like:



And these are all the readings from INPA and DIS / GT1, the first pic is of the software I have / use:











































I have a video of the compressor clicking as it tries to kick in:

https://photos.app.goo.gl/pNUVnvWvU8diMNsM9

Thanks in advance for your help! I will take some more pictures of the battery area after I get the LPG tank removed, I also have a new battery that I will fit before doing anything else.

[Leicesterboy15]

Originally Posted by LVR

Hi mate. Welcome to the air suspension hair pulling club!

Ok..I think you’re gonna have to get access to the pump/battery area.

I found when mine started playing up that the battery state of charge was affecting the pump operation so I would start by checking the voltage at the battery in cold state ie after systems idle for at least 20 mins. If it’s under 12.45 volts then it might need charging. Mine was down to 12.20 volts and died not long after. New battery and all was good again.

Then I’d check connections at the comp and check when it actually is running.

I don’t use INPA as it doesn’t allow a reset of the EHCII module from what I understand. I use ISTA which allows individual strut operation on a corner by corner basis, easy resets and a live check of the sensor operation.

It would probably be worth your while to see if an Indy has access to ISTA and can reset your module, and see where you go from there.

Thanks! Its a club I didn't want to join but I am definitely pulling my hair out, and my wife is bending my ear!

What is ISTA? Is that the same as DIS / GT1? I don't seem to have ISTA in my list of apps.

[LVR]

Hi

ISTA is the newer version of INPA (as I understand it). I use it to remove errors and reset EHCII module which is what controls the system. It allows diagnosis and test programmes etc.

I highly recommend it.

From those pics the first thing that stands out is that you have the same issue I did, which is a plausibility error on your front left sensor. If you look at the voltages on all of your other sensors, you'll see that they are within a similar range, except for that one.

If you use INPA, look at the Offset Values from the ECU screen. In my case the offset value was -5145 whilst the other wheels were all around 400 - 450.

The sensors when operating correctly put out a voltage depending on where the arm is (ie ride height). When you are setting the ride height you are telling the system which voltage you want and it goes to that point.

It has a limited range and the sensors are also prone to being knocked out of place or water ingress messing them up.


This is how I found my sensor


You can see the arm pointing down.

It should be like this



If it is in the right location I would be starting by replacing the sensor. They're pretty cheap and easy to install.

Then reset the ride heights etc.

If that doesn't work then I used a combo of INPA and ISTA to re-write the error in the sensor range to get it back.

[Paulpen]

From a quick look (im at work) the reservoir pressure screams you have a leak or a dead compressor to me.

[Leicesterboy15]

Originally Posted by LVR

Hi

ISTA is the newer version of INPA (as I understand it). I use it to remove errors and reset EHCII module which is what controls the system. It allows diagnosis and test programmes etc.

I highly recommend it.

From those pics the first thing that stands out is that you have the same issue I did, which is a plausibility error on your front left sensor. If you look at the voltages on all of your other sensors, you'll see that they are within a similar range, except for that one.

If you use INPA, look at the Offset Values from the ECU screen. In my case the offset value was -5145 whilst the other wheels were all around 400 - 450.

The sensors when operating correctly put out a voltage depending on where the arm is (ie ride height). When you are setting the ride height you are telling the system which voltage you want and it goes to that point.

It has a limited range and the sensors are also prone to being knocked out of place or water ingress messing them up.


This is how I found my sensor


You can see the arm pointing down.

It should be like this



If it is in the right location I would be starting by replacing the sensor. They're pretty cheap and easy to install.

Then reset the ride heights etc.

If that doesn't work then I used a combo of INPA and ISTA to re-write the error in the sensor range to get it back.

Thanks for this, I have checked the sensors and both front sensors are in the correct location, the front left was replaced a few months ago (aftermarket) as I thought this was the root cause of the problem but it didn't change the implausibility error I was getting at the time. I was still able to adjust the ride height using the laptop though.

Then I found the right rear sensor was seized so I replaced that, first with an aftermarket one but I found the bolt pattern was not the same as the one I removed. I fitted it anyway while the new one was on order from BMW and then I replaced the new aftermarket one with a new genuine BMW one, but this didn't clear the error - annoyingly I can't remember if I got a different error after this. At this point I was still able to adjust the suspension using the laptop.

The a few weeks later the car dropped to the ground and I got the ehc74 error.

The LPG tank is now out and I have bitten the bullet and booked it into a specialist on Tuesday as I am flummoxed! I will have another look this weekend but I don't really know where to look next. I will check the battery compartment and probably remove the side trim to see where the cabling and air pipe goes but I don't really know what I am looking at.

Will keep you updated and will post with the diagnosis from the garage.