Hey guys! I know air suspension posts from new members are probably your least favorite thing on this board, but i've spent all morning reading different threads and can't really find anything that applies!

So I bought a 2008 XJ8 the other day, only one previous owner and 123k miles. Very, very clean and runs beautifully but when I bought it the suspension was stuck all the way down and the battery was completely flat and needed replaced. I suspect the car had been sitting for some time. I installed a new battery and scanned the car, finding communication codes everywhere. The air suspension module alone had:

C2780 ECU in manufacturer sub-state
C2302 Leveling plausibility error
B1671 Battery module voltage out of range
U1900 CAN Communications bus fault
U2521 CAN Signal fault ABS
U2518 CAN signal fault dual air temperature control
U2523 CAN signal fault petrol engine control module
C2303 Reservoir plausibility error

I cleared all the codes, restarted the car, and drove it around the block a few times to see if everything would correct itself with cleared fault codes and a new battery. No dice. Made it a few blocks before "Vehicle too low" and "Air Suspension Fault" returned on the dash. C2303 was the only code that reappeared. At this point I was thinking compressor. I swapped the relay with a known working one. Nothing. Then I jumped relay pins 5 and 3 together to engage the compressor. I can hear it running, but nothing happens. While the compressor was running I tried to listen for air escaping from the struts, but didn't hear anything.

I'm wondering if I can just use a shop compressor to try and air up the reservoir in the trunk, and even if it's wise to do so. Not sure if anyone might have some helpful suggestions to shoot my way! Thanks in advance

 

Your compressor could well be shot, if it runs with no results at all.
Sometimes a new piston ring will help, if the bore is not all scored up.
But you could well have other issues to deal with before the compressor.
Others will know more than i do, and should give you some ideas

 

The C2303 code indicates the compressor is failing to charge the reservoir in the time allowed by the software. This is either (1) the compressor is knackered, or (2) you have a leak somewhere that is letting air out as the compressor pumps it in. At the mileage you quote, if the compressor is the original, it is probably that rather than leakage.

 

R M T had new compressors for just under $200 delivered.

Put one in the 04, it works well. with a lifetime warranty.

I bought one for each car at that price, one still on the shelf.

Chances are, a pair of front shocks, along with a compressor, will be needed now.

Rears down the road, a few more miles.

 

I’m working on a similar issue. Car was fine, noticed right front was dropping quickly (maybe an hour after shut off). Replaced strut, now it won’t raise. The pressure reading off of the obd reader was showing like 5 bar (then dropped) after cycling the key and getting the compressor to run a few times. I did notice the compressor sound changed from a louder pffffft noise to a less noisy hum. I’m hoping a new ring cures this. I checked all of the usual connections for leaks. My rear struts are original.

This is all while chasing a trans shifting issue (waiting on a map sensor).

 

Interesting. I was thinking the same thing. I have a 'T' in the hose to my left front shock and ran a line into the car where I have a pressure gauge. I've seen the errors when the pressure is down around 100PSI. Normally it seems to be closer to 115-120 although this evening I noticed it was up to around 135. My home compressor shuts off around 110 (would have to see if the regulator will let me run it a bit higher). Based on what I saw on the gauge, as long as you are not pressuring it beyond 130 or so, I would think there' would be no problem.

I got the rebuild kit a couple of years ago hoping it would fix the problem, but no such luck. I'm thinking I need need new front air shocks, but haven't figured out yet where the leak is coming from.

 

The compressor doesn't usually direct air directly to the struts, it pressurizes the reservoir, and that pressure is then directed to the struts as needed to raise the height. If the reservoir is below a threshold of pressure, the valve body can direct the compressor's flow into the struts, but that's only when the reservoir can't raise the struts by itself.

One of the items in the scan tools list of monitored systems in the air suspension should be reservoir pressure. That should be at least 7 or 8 bar, and should rise well with the compressor running. Again, the compressor's primary function is to pressurize the tank.

I just got the rebuild kit for my compressor and found a very worn piston ring and completely used up desiccant beads in the dryer cylinder, there was water pooling in the dryer. My system is working much better now, although I now have several leaks to track down and fix. Didn't know about the leaks when it wouldn't raise, before. I also had some electrical issues, including my ASM not even powering up. Yours powers up, though, or you wouldn't be able to read the codes.