No need to run a wire to the BeCM, behind the LH rear light there's a white, 4 way, plug intended to connect to if you have twin towing sockets so is usually just dangling around doing nothing. That has a permanent live, ignition switched live, ground and more importantly, a reversing light feed. Simply find the plug, connect to the Green/Black wire (12v when reverse lights are on) and Black (permanent ground) and job done.
It kills the power completely to the EAS system when the ignition is off so nothing will change with doors being opened or closed. All the inhibit switch does is stop it from dropping to motorway height at speed or staying at motorway height when you slow down.
You've got a leak somewhere. Pull fuse 44 (pre-99) or 29 (99 or later) and leave it overnight. That will show you which corner is leaking and dropping. Then you will need to get the soapy water spray out and check the air springs, connections to the air springs and where the pipe comes out of the valve block for bubbles.
After standing for a week, it should run for anything up to a minute. Once it is up to pressure, you should be able to press the brake pedal 3 or 4 times before the pump cuts in again. That shows the accumulator is good. If it takes a long time to pressurise but cuts in after every press of the brake pedal, that shows the accumulator may be OK but the pump is weak or the brakes need bleeding. Air in the power circuit doesn't give a spongy pedal, it gives a slight delay between pressing the pedal and the brakes coming on (as the fluid has to compress the air in there before it can cause anything to move). If the warning light comes on when you press the brake pedal, the accumulator is completely shot.
When I had the dash out on a car last week, I used this https://www.diy.com/departments/diall-black-self-adhesive-draught-seal-l-6m-w-9mm-t-5mm/1802898_BQ.prd in the duct joints when it went back together. Could have done with it being a bit wider but it did the job nicely.
A further update. Phil changed the alternator thinking that was the only thing left but it still flattened the battery after a couple of days. So I've been there today and started from scratch. Clamp on ammeter showed it was drawing 2.8A on the cable to the fusebox, after 2 minutes the BeCM went to sleep and the draw dropped to 1.8A. Pulling Maxi fuse 1 and it dropped to 1.4A, pulling maxi fuses 2 and 3 had no effect but pulling 4 and it dropped down to 0.4A. Definitely getting somewhere.
Set the drivers door latch to closed so the BeCM would go to sleep and continued. Maxi fuse 1 feeds fuses 1, 2, 3 and 4 in the BeCM, pulling them one at a time and fuse 1 saw it drop to 1.4A, so it is something on fuse 1 drawing current. Maxi fuse 4 feeds fuses 12, 13, 14 and 15 so pull them one at a time and with fuse 15 pulled, current draw dropped, so it is something on fuse 15 too. Checking the ETM and we started to get worried as the only thing common to fuses 1 and 15 is the DSP amp. Then when I put fuse 15 back in I noticed two things, the first was a small spark as I plugged the fuse in and the other, as I was sitting next to the drivers door speaker, I heard a pop from the speaker as I plugged the fuse in. It shouldn't do that, the ignition is off and so is the radio, so the DSP amp should be turned off, not drawing any current at all and certainly not causing a pop through the speakers. That points towards the radio head unit itself as it sends a wake up signal to the DSP amp. Out with the Alpine head unit, unplug the grey plug from the back and the current draw drops away completely. Plug it back in, the speakers pop and the current draw goes up again. Whip an identical Alpine head unit out of one of Phil's other cars, fit that and all is sweetness and light. Current draw sitting at 1.4A, 2 minutes later the BeCM sleeps and it drops to 0.5A and 30 seconds later when (I assume) the alarm settles and it drops to 0.024A, or 24mA, just what you would expect. So, it appears there were two separate problems all along, the door outstation stopping the BeCM from sleeping and a faulty Alpine stereo that was keeping the DSP amp permanently on. Not surprising a previous owner had fitted a battery isolating switch.
Don't totally discount the AA though. I've got full AA cover as an included benefit on my Lloyds bank account which covers me irrespective of the vehicle or even if I am only a passenger. I've used it three times. First was on the Classic LSE with a loaded car transporter on the back and the serpentine belt tensioner bearing gave way. They came out with a recovery truck, put the car on that, hitched the trailer up to the back and took car, trailer and me home. Second time was on the P38 when the water pump bearing exploded but we weren't going directly home. They arranged a hire car for us to continue our journey (and were very apologetic that they couldn't give us a Range Rover and the best they could offer was a Zafira) and recovery to home for the car. We got home in the hire car about 9pm that night and the car arrived at 11pm. Third time was while the P38 was being resprayed and I was using the Maserati. Parked it for about 6 hours only to find the battery had gone flat, got a jump start and it appeared to be charging (interior light got brighter when I revved it anyway) so set off for home. First roundabout I came to and had to stop, being an auto, I couldn't keep the revs up and it died on me. AA man came out, confirmed what I thought (short circuit rectifier in the alternator), disconnected the alternator, put a fully charged battery on it then followed me home in case it died again. I couldn't fault the service at all on any occasion.
Continental cover is a different story. None of the usual specialists will cover anything over 16 years old and don't do personal cover. Up until 2000, for 84 Euros a year, ADAC, the German equivalent to the AA would give a similar level of personal cover for anywhere in Europe but they stopped doing that unless you have a German address. I didn't realise until earlier this year when I realised I hadn't renewed it for this year. Considering I must have done at least 15,000 miles on the other side of the Channel thinking I was covered when I wasn't. Good job I didn't need them.... I haven't spoken to the AA to see how many arms and legs they want to extend the cover I already have to cover all of Europe, but the RAC will do personal European cover for around £350 a year which seems about the only option these days.
Try charging it and see what it drops to when cranking then, it shouldn't go below 11.5V. If it does, it's faulty so time for a warranty claim.
@Harv, your car is a GEMS so has the main power cables going from the battery to starter and starter to alternator. A poor contact at the starter causes the voltage drop and there was a TSB that recommended exactly what you have done, an additional cable from the alternator to the battery. However, this won't be StrangeRover's problem as his car is a Thor which has a cable directly from the alternator to the battery as standard.
Sounds more like a starter problem that a battery problem. What does it drop down to when cranking?
What are you doing to kill the Hankook's? I've got one in each P38, have supplied 3 more for other owners and one in a boat with a 4.3 litre V6 and not had a problem with any of them.
I've only ever seen a couple that have been badly rusted (donmacn's being one of them) but I can't see how it would be an MoT fail unless it is actually leaking.
Defect Category
(a) A gas, air or fluid suspension system inoperative Dangerous
(b) A gas, air or fluid suspension system component damaged, modified or deteriorated in a way that:
(i) it would adversely affect the functioning of the system Major
(ii) its function is seriously affected Dangerous
(c) An obvious leak from any part of the system Major
577 is a thread sealant (https://www.henkel-adhesives.com/jp/en/product/thread-sealants/loctite_577.html) so not sure why they recommend that. For metal to metal faces I've always used Hylomar Blue.
No, that's a RH door latch and isn't the same, the MG only has one microswitch in it (door ajar), it doesn't have a CDL switch so won't work in the passenger door on a P38. The LHD, LH latch https://rimmerbros.com/Item--i-FQJ102292PMA is a direct replacement for the drivers door latch on a pre-99 LHD car and can be used on a later one if you chop the plug off and solder your original 8 way plug onto it.
If you got it from battery megastore, it's got a 4 year warranty (see para 17 here https://www.batterymegastore.co.uk/terms-and-conditions/). Charge it fully and see what it drops to after it has been left but if it drops below 10.6V it is fubar.
In theory you just replace the sensor on the board but, as you say, the brushes spring out and are a real pain to get back in. I suspect that is why the one I saw had a replacement sensor mounted using one of the screws on the outside. I've only tried to take one compressor motor apart to replace a bearing and gave up on it.
The Viair is only slightly bigger than the original (which was a Thomas pump and not Dunlop as some seem to think) so could probably be fitted if the box was slightly modified but Marty has used the vacant space next to the box. Wouldn't be an option on a LHD car but plenty of space on ours.
Have you fitted one before? Getting the motor apart can be a real pita. I've seen one, on a Classic, where the sensor was attached to the outside of the back plate of the motor and connected to the Orange and Black wires externally. It was done very neatly and almost looked like it should have been there.
This one I suspect https://uk.rs-online.com/web/p/thermostatic-switches/2532773/?sra=pmpn, or you could go for this https://www.ebay.co.uk/itm/264871750482 which claims to be the correct one.
Cold pump with thermal sensor showing Hot says the sensor is fubar. I'd be inclined to short the Orange and Black and run it a bit longer (while keeping a hand on it to prove it isn't getting too hot).
I've seen mention of a car that didn't want to play while sitting on the bumpstops, but once it was jacked slightly so the height sensors saw movement, it all burst into life. Jumping the relay, or turning the compressor on with the Nano, should build pressure. If you monitor the pressure switch while it is running, you'll see when it is filled (although you'll probably hear the pump starting to struggle anyway).