TLDR: I bought a used AMK compressor to replace my ebay hitachi. I'm curious to know if there are any parts on the AMK compressor I should rebuild or replace before upgrading.

I keep discovering issues on this used LR3 I bought last December. Suspension gave me a "normal height only" message. I've known there is some sort of slow leak somewhere in the system as it lowers to the bump stops after sitting for a few days. Always had a pretty responsive lift from the bumps though and so I thought I'd have time to fix the leak eventually.

Taking things apart and investigating, I found a slight leak at the air compressor's desiccant cover (a common failure point from what I'm seeing around forums). Ordered a replacement cap and found out today that the compressor installed has a slightly smaller pipe than the air fitting for the desiccant cover. The pipe on the compressor is a 4mm, it looks like the replacement caps are for a 4.7 or 5mm hose.

I'm really tempted to just convert to springs, but I've already rebuilt and replaced the three valve blocks and spent a lot of time fixing some bad air line repairs from the previous owner. To get a little more time with EAS for my labor and money, I decided to try and find a cheap, used, legit OEM compressor. Got lucky and found a junker on ebay selling an AMK. The fools have the middle valve block, bunch of partial air lines, air compressor, compressor bracket, and the top and bottom covers all for $150 shipped. Besides some corrosion on some of the compressor screws and cylinder/piston cover, it looks like it's in good shape (came off a 2012 L320 per the vin).

I have found a large amount of Hitachi compressor rebuild kits on all the usual sites, but nothing for the AMK except for fresh desiccant and what looks like the exhaust valve solenoid. Anyone have any advice on cheap/easy rebuilds for the AMK compressors?

I am aware I'll have to change the ECU from hitachi to AMK, I've got a GAP for that.

If this doesn't work, I'll be throwing $1500 shocks at it. Honestly, if an air spring has damaged to the point of leaking, I may have to convert anyways. The price for a decent quality air spring replacement strut is pretty pricey for just one corner of the rover.

 

Not an upgrade. A leaking cap would not cause any corner to drop overnight but certainly would cause performance issues when lift or filling reservoir. I thought all the pipes were 6mm. I never have seen any different with OEM or any aftermarket compressor. And I have seen quite a few. The AMKs are not generally rebuildable but at minimum I would replace the desiccant or dryer. I am not sure if the dryer can be opened to replace just the desiccant, never tried it yet.

 

I understand that opinions have shifted over the years and the Hitachi vs AMK is still discussed quite a bit. For my situation, a known factory part is going to be an upgrade over whatever generic knock-off is installed in mine. I did see at least one post where they made a comment about the programming for the AMK being different than the hitachi and the programming made a difference in longevity. Something about lowering the pressure of the system. I was under the impression the only difference was in the temperature reporting method being different between the two versions.

The tube I'm talking about is the smaller diameter tube coming from the compressor cylinder and going in the red fitting on the compressor. It is slightly smaller than the regular 6mm lines everywhere else.

I'll definitely replace the desiccant and get some new filters to throw in it. I know you can replace seals in the hitachi, but doesn't seem like a common rebuild for an AMK

 

AMK is not a direct swap with a Hitachi or knock off hitachi so you'll need to make up some new lines, as DT mentioned get the drier service kit with the temp sensor at a minimum.

 

I'll look into the temp sensor, thanks for that info. This part should have some lines included if the listing pics are accurate which should give me some extra material to swap things over. Should be here later this week. I'm hoping the lines with the almost 90 degree bends will be included.

 

Update: New compressor is fitted and tested out ok. Built pressure up to 220 psi when the reservoir hose popped out of the middle valve block (probably an installation error, it was a new fitting). Once I'm confident that hose is seated and not damaged, I'll get things back on all four wheels and really test things out.

Once I've eliminated valve blocks, compressor, and general hose leaks, I guess I'll find out if a strut is leaking. I think the previous owner said he replaced the front drivers side, It certainly doesn't look new, but maybe he swapped for a used part. If he did one and not the other side, it may just be time for a new front passenger strut.

If I raise to off road height and pull the fuses and nothing drops, but it drops when the fuses are in, does that indicate a height level sensor is acting up?

I appreciate everyone's help and advice, bunch of legends in this forum

 

Well if one line can pop out, others may not be fitting well such as at the struts that the previous owner worked on. That certainly could cause a leak. Hence why a soapy water test is always a good idea to help locate leaks - not that is can always locate em. 220 PSI is good working pressure but that seems a touch high. The new software revisions should have reduced it closer to 200. Unless that was just for Hitachi compressors, could be. But not matter, that is a strong pressure.

When you turn the truck off it will make occasional adjustments to keep it level. So a leak in the front will cause the rear to be lowered during an adjustment. Likewise if there is a small leak up front and it does an adjustment, the struts may balance out if one is low on air and not the other. And one reason for the fuse/non-fuse test is a bit pointless in a way. The struts can crack/leak at the folds in the air bags. Since the most common height is standard, that is the areas on the rubber that flexes the most a will crack the most. So the off-road height test is just one way to see if that can be an issue. By raising up that crack prone area is moved and can sorta reseal itself. Best way to describe this is if you have a belt and fold it back on itself. Imagine how the outside of the bend would be and how any cracks would be more open. But if you roll the belt just a little that bent area is now flat and any cracks would be sorta closed up. So if it drops at standard, but not off-road then the leak it probably on the air bag portion of the strut. And the idea of the fuse pulling really is more to see which end is dropping because like I said, it wants to level. So if the front drops front a leak, it will in turn drop the rear even tho there is no leak back there - just to keep it level.

So if it it dropping, you need to check for leaks at the struts, fitting and even spray the valve blocks and the body itself can leak. If you are certain there are no external leaks, then the valve blocks are suspect as the will let the struts leak back into the gallery and through the compressor and exhaust since the gallery, by design, is not air tight.

 

This time it popped at 240psi. Pretty sure the tech manual said the working pressure is near 280psi. This time I removed the voss connector and fitted it per Atlantic British's guides on reusing them. This was a brand new voss connector and I had thought I just didn't get it pushed in far enough the first time. Now I'm thinking the hose may have been damaged. The previous owner had tried to repair it with a barb type fitting and piece of rubber hose. I had assumed it was because the voss connector had failed on them and they didn't know where to get a new one. The ebay compressor came with the section of hose from the middle valve to the reservoir, I'll swap it out if it pops again.

I've done soapy water tests on all fittings and valve blocks, but if the bag is leaking behind the metal sleeve, its harder area to get soapy water to bubble. I suppose i could remove it and submerge it but that seems a little involved and would probably require equipment I don't have (in assuming the working psi in the strut is above 200psi).

 

The manual is wrong though because of software updated. I had to look it up, there was some conflicting into with some saying 240psi was old max and 200 is new. But based on a more in depth thread with actual testing by the GAP team, 258 is the new max PSI for Hitachi and AMK. So 280 seems oddly high but the gallery should be able to handle at least 300psi and maybe even a bit more. The tank itself rated. I would first get the software updated, sooner than later. But with the max pressure rating, no lines should be popping. What should happen, and I have seen this a few times when tinkering, is the compressor itself will hit its max pressure and blow the bypass in the exhaust. Basically a huge spring that eventually can not fight the high pressure and it vents. That would happen before any line would blow. You are not mixing 6mm and 1/4" fitting or anything like that by chance?

The bag PSI is much lower, probably 140 ish. The truck will raise with as little as 120 PSI depending on load. Done it before in the trail with a high end tire compressor. The reason of the higher pressures is just to fill the reservoir.

For the struts, just spray a heavier mix of soapy water up in em. Lay under the strut and you can see up inside most of it. If bubbles form, it is obvious.

Another diagnostic trick would be a little DIY. Get a 6mm push connector or adapter for a regular Schrader valve like on a car tire. Amazon has em. Use the system to get the air ride up to normal. Then remove the main air line at the compressor - the one that goes to the center block. Attach the adapter. At that point you will have zero PSI because you dumped the gallery air by disconnection but the valve blocks will be closed. Pull the fuse for the EAS computer (26, engine bay) and give it some time. Hours or overnight. Then take a reading. You should have zero to a very low reading as I can see that maybe some leakage is normal on an older system. But if it is higher, like over 20psi I would have to think a valve block is leaking back into the gallery. But keep in mind the truck may not lower much at all because you have the gallery sealed now, before the air would slowly bleed back out through the compressor.

 

Well now I'm stumped. I haven't gotten the truck to pressurize. Here's what it's doing based on what I'm seeing in the Gap tool:
1. I have it on a hydraulic lift to keep the tires close to "normal" height.
2. I turn the truck on, and the pump starts bringing things up to pressure.
3. As the pump reaches max temp, it stops.
4. As soon as it stops, it closes the reservoir valve, and opens the exhaust valve.
5. A huge amount of air comes out of the exhaust valve, more than I would assume is in the gallery alone (lines between the pump and front/rear valve blocks should be the only thing with pressure if all solenoids are closed, right?).
6, After it cools down, it starts up and opens reservoir valve. But opening the reservoir valve isn't bringing things back up to the temp it stopped at (example: it will stop at 180psi, and when it turns back on, the reservoir psi seems to be just as low as the gallery pressure ~60psi)

To try and diagnose, I swapped the middle valve block (was a new amazon block) with the block that came with this 2012 AMK compressor. I did disassemble the junkyard valve block and its clean. No powder inside, no corrosion on solenoid, O-rings aren't flattened or cracking. The same thing occurs.

I'm assuming this means that one of my valve blocks is stuck open? I replaced all three with the compressor. Maybe there are one or more that are stuck. I can hear each valve block click when I use the GAP tool to manually test the valves.

I did flash the AMK software from the Gap tool to the truck, so it should be using the latest firmware, or whatever firmware was loaded into the Gap tool which I bought new Christmas of 24

Am I missing something? The behavior I'm expecting is it fills until the compressor reaches temp, and closes off the reservoir. I understand that it is likely relieving pressure from the gallery to take the strain off the pump when it kicks back on, but it shouldn't be emptying the reservoir at the same time, right?