The only remaining section of the electronics package is the actual solenoid itself. The solenoid of a paintball marker is the physical connection between the electronics that fire the marker, and the pneumatics that do the actual firing operation. The solenoid is an electromagnet-based valve that is used to switch airflow within the marker, simultaneously pressurizing and depressurizing different parts to actuate the valve system. Every electronic marker uses a solenoid, although there are many differences in some of the valves used in other applications.
The solenoid in an Ion is located on the rear of the circuit board, with a number of microline hoses leading out.
When the solenoid bracket is removed, the head section comes off. Within the solenoid housing is the only part of the valve itself, the armature. Here is a more detailed view of the pneumatics:
The solenoid is soldered directly to the circuit board. During normal conditions there should be no need to remove it. Should it need to be removed for any reason, though, it must be desoldered from the circuit board.
Solenoid Operation: 
The purpose of the solenoid is to take in pressurized air and output it into the front of the firing assembly. The Ion solenoid is a three-way valve, meaning it has one input, one output, and one exhaust. The output air is carried to the front of the bolt, where it pushes and holds the bolt in the open position. Because the solenoid is open while idle, it's also characterized as a normally open valve.
To recap the Ion's firing cycle, while the marker rests idle pressure is forced into the front of the bolt, to hold the bolt open. When the solenoid fires, its internal plunger (also known as the armature) vents the air holding the bolt open, and allows it to close forward. Once it reaches the full forward position the ion releases the air stored in its dump chamber and the marker fires the ball. After the dwell time expires, the solenoid will switch back and air will again move through its output to the bolt section, to push the bolt into the open position once more. After this time, the marker is ready to fire once more.
If you wish to learn more about how the solenoid works, please refer to the How solenoids work link in the Related Links section. Detailed information and diagrams can be found there on the Ion solenoid as well as other types of solenoids too. Please also refer to the Ion design & theory page.
Internal Hoses & Microline Fittings:
The hose system within the frame is what delivers pressurized air to the various parts of the markers. There are two types of hose used within the Ion:
(two lengths) main hose: 4-mm outside diameter; 2.5-mm inside diameter.
(one length) solenoid output hose: 1/8" outside diameter, 1/16" inside diameter (or 0.060" inside diameter, if possible). This is the same type of hose that is used on Autocockers, Impulses with LPR, and other markers.
The hoses attach to the different parts of the marker by 10-32 threaded microline fittings (commonly referred to as banjo fittings or simply banjos for short). These are types of push-to-connect fittings, which operate by placing a small crimp on the outside of the hose in order to seal it. The hose is released from the fitting by pushing in on the button around the hose, at which point it will be loosened and the hose can be removed. This is the same principal behind the macroline fittings that come on the Ion's regulator and ASA.
Since the marker uses two different sizes of microline, it is obvious that it also uses two different sizes of microline fittings. The front and rear hoses (which are 4-mm) attach using 5/32" microline fittings. The middle hose (1/8") attaches using a 1/8" fitting. All three of these fittings are removable from the marker by using a 1/8" allen wrench to unscrew.
Microline Barbs & the Solenoid Head:
The two lengths of 4-mm hose attach to the top of the solenoid at a brass intersection called the solenoid head. This is a threaded piece that allows some air to fork off and enter the solenoid, located directly under the head section.
The solenoid head uses a pair of microline barbs (aka. microline nipple) to seal the 4-mm hoses on either side. The barbs then have their own o-ring that seals them with the head section. The head section itself then has its own o-ring to seal with the solenoid housing. All these seals are unnecessary to service, however any leaking from the solenoid head section can be caused by any of these causes. Refer to the troubleshooting page for more details.
Hose Removal: 
Removal of the hoses requires a bit of patience. The 4-mm hoses are installed very snug onto their barbs, and can only be removed by gently prying each end up using a razor, knife, or other tool. Eventually enough of the hose will be off the barb so you can pull it using your hand.
Newer Ions come with a much more stiff hose (usually not clear-colored) which is much more stiff and difficult to remove. For this hose the only thing I can suggest is that you use a knife to split the hose laterally, down it's long axis.
The solenoid barb is a different story. Since the solenoid is made from a composite plastic, it is much more fragile than the brass head fittings and parts. During normal conditions you should leave the hose on the barb and avoid removing it. If it must be removed, you must take care; if you pull too hard in any direction on the hose leading from the solenoid barb, the barb will most likely break off. The hose must be removed by pulling it in the same direction as the barb, which is angled slightly up. In other words, don't pull the hose straight away from the solenoid, because this will break the barb. Splitting the solenoid hose is the best way to avoid breaking the barb.
A broken solenoid barb will require the purchase of a whole new solenoid, which is $35 by itself, or $70 with a board soldered and ready to go. If you're unable to solder then you'll have to buy the $70 assembly.
Quick Exhaust Valves (QEV):
Quick exhaust valves are used in many other applications where a hose is used to carry pressure to another part of the device. What they do is allow air pressure traveling back through the hose to vent out right at the QEV, instead of having to travel all the way back to the solenoid in this situation. What this allows is a much decreased dwell time due to the increased venting speed, which also helps to increase efficiency and lengthen battery life. An Ion using a QEV will also be able to cycle faster than one without, although the real limiting factor in the marker's rate of fire is the speed of the hopper.
There are many QEVs available that are made for other markers, such as Autocockers, Rainmakers, Sovereigns, etc etc. The only QEV that is made specifically for the Ion is the Smart Parts 360º QEV, which uses an allen wrench for installation and removal just like the stock middle banjo fitting. Other QEVs don't swivel around like the 360º QEV, and thus can be troublesome to install and tune. Pictured below is the 360º QEV.
Other than the 360, most regular 10-32 threaded QEVs will also work, however as said installation and future disassembly may be more troublesome. The only 10-32 QEVs that won't work in the Ion are ones that are too tall to fit in the frame. With regular QEVs, there is no way to control where the QEV will stop when you're tightening it onto the body. Becuase of this, you are limited to using loctite or experimenting with different size/durometer o-rings to seal the fitting (I don't recommend using teflon tape, lest it get blown into the solenoid).
· Clippard QEV kits are available from a number of sources (Clippard is the actual manufacturer of the QEV), including Airsoldier.com, AdvantagePB, Trinity Paintball, New Designz, and others. These consist of a brass QEV with either a microline barb or a push-to-connect fitting installed, depending on the seller. below is a picture of the Clippard QEV with a PTT fitting.
· Eclipse QEVs will also work however they require cutting to the Ion exoskeleton to fit correctly. Honestly this is more trouble than it's worth in comparison to the other QEVs out there so I won't go into detail about body mods. This QEV has its own barb fitting so no additional part is needed.
· Palmers/ANS QEV works however the "swivel" type is too tall and won't fit in the frame.
Most "regular" QEVs will seal against the Ion using a 8/90 o-ring. If this doesn't work, often a 7/90 o-ring will do the job. If that doesn't work, try a 6/70 or 7/70. Loctite can also be used, or teflon tape, however I suggest using an o-ring since they are easiest to deal with and don't require time to cure (unlike Loctite).
Solenoid Maintenance:
Occasional maintenance is required on the part of the solenoid to keep it clean and moving freely. This is detailed in further depth on the solenoid maintenance page in the Related Links section below.
Overview:
· The solenoid is a pneumatic valve that switches airflow within the marker, when energized by electricity from the circuit board.
· The solenoid is soldered directly to the main circuit board; it can't be removed without soldering.
· Air pressure is carried to and from the solenoid by a system of microline hoses, 4-mm and 1/8" diameters.
· The metal section on top of the solenoid is the head section; 4-mm microline hose attaches here at two places.
· The solenoid has only one internal part (armature) and requires occasional maintenance.
· Don't remove the hose from the solenoid barb fitting unless absolutely necessary.
Related Links:
· Leaking troubleshooting
· Solenoid maintenance
· How solenoids work
· Main circuit board
· Ion design & theory