Yes, I was writing predominantly about the L322 and not the even older P38, but also the Disco's that use the same Wabco style suspension compressor.
But, you write "Inlet air and exhaust air are 2 separate circuits". I don't believe that is so, certainly not with any of the several RR compressors I've had to fix.
Consider this. The dryer is in the air intake circuit so it can absorb moisture from air being drawn in and dry it before it is pumped around the closed (hopefully) pneumatic system that operates the suspension. That moisture accumulates inside the dryer desiccant until it is expelled through the exhaust. Hopefully.
The exhaust filter is screwed into the dryer so that air being vented will help expel moisture captured in the desiccant inside the dryer chamber. This is where the moisture comes from that ends up clogging the exhaust filter.
There is no other vent on the dryer for air to enter so it has to be sucked in through the 'exhaust' filter. If air flow becomes restricted the pump has to work harder, possibly much harder, and will eventually over heat. The cylinder in the compressor has no oil lubrication. The soft iron rings will wear more when over heated, or as happened on one of my Land Rovers, the piston will seize, break the aluminium connecting rod and make a hell of a lot of noise!
Of course slow leaks will generate suspension pressure related fault codes and messages but the controllers only have limited number of messages and the processor does not have the processing logic to handle the AI (artificial Intelligence) needed to deduce which combination of errors is causing it to have a tizzy. This is common to the many microprocessors used in vehicles of this vintage.
Being a computer geek for the past 60 years I am very aware that modern microprocessors make errors if the available voltage gets too low. Most require a reliable and 'clean' 5V supply. When we start our Range Rovers the battery demand is high, not just from the starter motor drain, but all the other gadgets, such as heated steering wheel (I love mine), heated seats, heated mirrors, fans, radios, DVDs and CD players, lighting etc., etc. All the microprocessors can make a lot of errors and dump a lot of messages on the CANBUS. When trouble shooting faults codes, make sure you have cleared out the junk and that the battery is fully charged. If you don't have an IID tool or similar, disconnect both cables to the battery and hold them together for 10 - 15 seconds to ground all the microprocessor memories to clear 'out' old codes. Then reconnect them and use the vehicle to regenerate fresh codes. They will be meaningful ones. Any that have disappeared were probably the result of low voltage at some time.
While I think of it, low system voltage after cranking the motor can result in transmission faults. Cold mornings thicken the transmission fluid so the shift solenoids need full power. If the battery is low the solenoids may not function quick enough and hydraulic pressure sensors will trigger a fault. I have found that when it's cold and my battery is being loaded heating my bum, hands, mirrors, windshield and rear defogger, I manually shift through the gears for the first mile or two and never get transmission faults from early morning low voltage and high load issues. I'm looking forward to my next vehicle being purely electric. At least electrons don't leave a stain on the floor when they leak out. ;-)
Believe it or not, I got into this topic trouble shooting a parasitic battery drain problem. Our RR may only get driven once or twice a week but the battery sometimes drained low enough between uses that I started to look for electrical problems One evening I happened to hear the compressor while the RR was parked for the night. Normally I'd not hear it but happened to have the tailgate open unloading some tools. Thinking about it made me realize that if the compressor woke up a few times when the vehicle was parked it could draw down the battery pretty quickly. Opening up the compressor noise baffle let me see the exhaust vent filter was pretty clogged and wet, so the compressor was probably running long enough to really run the battery down fast.
I leave you with this thought. In one of the "Pirate of the Caribbean" movies the pirate Captain Barbossa points out the Pirate Code is "not so much as set of rules, but more like guidelines". I suggest we think of the error codes and messages our 'Pirate' Range Rover vehicles generate be treated as 'guidelines', not hard and fast indicators of what is wrong. We have to consider for ourselves what is possibly going on. A sometimes a little luck goes a long way.
. It seems that 06?/07 was when they changed to the Hitach pump.
