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We just got a used VN42 and aren't using it yet...doing some minor parts replacements so I haven't really looked at the manual. I see that it is quite a bit different than our VN2+1 that wins more of our mutual battles than I do, but I sure have studied the pneumatic diagram and replaced a bunch of parts...so I am at least familiar with it if not skilled at fixing it.
I see a lot different in HOW the VN42 is plumbed so I have to do some looking.
I have seen two different manuals for the VN42 and they were even different...like there was a left-handed and right-handed (in appearance) version.
I am employed full time elsewhere and end up doing the difficult maintenance because I seem to figure out what to bang my head on better than the framers. I have a time problem presently, but don't mind being reminded to keep your question in mind. I will be working on the VN42 shortly so that will give me the opportunity to compare to the VN2+1 and maybe seeing different ways to do the same function (putting nails into the frame) may help me understand the stubborn problem on the VN2+1.
Have you tried to put a little air tool oil in the lines? Internal parts of the ITW AMP underpinners need to be oiled to operate properly. Make sure you use the Air Tool Lubricant, I use Ingersoll-Rand Air Tool Lubricant, and squirt it into the air intake line. A few drops of oil should be added every couple of weeks. It is worth a try anyway.
VN42 (at least my generation) has some overlap in the foot pedal and hand-actuator functions.
I think you can use foot-pedal alone in two steps; halfway down to hold the frame, and the rest of the way down to drive the nail,
foot pedal halfway down to clamp the frame, then press the hand valve (metal plunger in the vertical assembly between the front and rear hand lever clamps along the ruler tape) to drive the nail.
Both nail-driver methods rely on the foot pedal to hold the frame in place...from what I am gathering so far. That is peculiar but I guess we'll get used to it after 20+ years with a VN2+1.
Both our v-nailers have red, black and white (4mm ?) airlines between the foot pedal and the machine. I just looked under VN42 and see the same (visually, anyway) kind of pneumatic valve (Pneumax gold anodized aluminum) under that metal hand actuator, so the foot and hand valves are interconnected...can't say much intelligent about how yet, so I won't try.
I just looked at the VN2+1 which is harder to see (everything is covered and inaccessible) but simpler and easier to observe the pedal functions. Bear with me as this explanation of the pedal function may help understand what may not be happening on the VN42.
On the VN2+1 the red, black and white (clear) air lines have push-in fittings on the pedal and on the side of the v-nailer. Unfortunately the VN42 has the lines entering the open-construction machine and they go directly to the pneumatic valves that control the air.
I don't recommend disconnecting anything yet from the VN42 until you have taken notes, photos, and/or studied the manual. My manual doesn't seem to have a pneumatic schematic like the VN2+1 does...
Assuming the red, black and clear lines have the same function (ignoring for a moment the more complex foot/hand option on VN42), the red one on the side of the VN2+1 sends air out to the foot pedal.
If you hold the red air line, and push the plastic ring at the side, and pull the red line out of the v-nailer air rushes out of the v-nailer fitting you just removed the red air line from. I think in a CMC I was looking at a red fitting was where air flow was supplied, so that may be a convention in the pneumatic world.
If you were to push the red line back in, and disconnect the black air line, nothing happens until you press the foot pedal halfway. Air will exit the black air line coming from the pedal instead of being fed back into the black fitting on the v-nailer. No valve actuation occurs on the v-nailer as one would expect. Re-connected, the black line on the VN2+1 controls the pneumatic valves that clamp the sliding fence (VN42 doesn't have that), the rubber or felt padded vertical clamp that comes down from above the frame (if present), and pushes in the pointy rabbet pusher that will crush your thumb the first time you don't watch out for it. (You will probably only do that once, if ever, speaking from personal experience). With the black air line removed, those three clamping functions do not happen...only air for the clamping begins to exit the black airline from the pedal.
If you press the foot pedal the rest of the way down, with the black line disconnected from the side of the machine, the nail driver will push a v-nail out. With no clamp functions, the v-nail will just fall a short distance to one side of the v-nail block. In your troubleshooting , always remember to make sure you didn't inadvertently drive a v-nail into one of your frame clamping 'pads'...it will stay there and get hammered into the top of an expensive frame if you leave it there. (Not sure how many decades ago, but yes, personal experience bragging again).
Say you reinstalled the black air line into the machine, and then remove the white/clear one. This is the nail cylinder feed line. With it disconnected, air will come out of the clear line when the pedal is depressed from half-way to completely down. If air exits the line with the pedal fully depressed, assuming it is of proper pressure/flow, the foot pedal is at least routing the air feed to the ports on the side that expect air for their respective functions.
If you remove all the nails from the machine (magazine, feed tray, whatever it's called), reversing the black and white lines on the side of a VN2+1 will reverse the order of the multi-location clamp functions and the nail driving function. Only sane reason for doing this is to study how it works.
As alluded to previously by less-rambling people, something may be stuck in the clamping pneumatics if they receive sufficient air but don't move.
Since the VN42 has no easily identifiable side air line ports like the VN2+1, you could do those experiments at the foot pedal. I don't remember if you can reach the air line release rings from the rear of the foot pedal valve without removing the foot pedal cover (two screws underneath on ours). Don't disassemble the foot pedal unless you really need to. There is a spring or two to watch out for and where I got burned was disassembling the foot pedal pneumatic valve per obsolete instructions sent to me by ITW AMP...wrong construction inside. I think most people just replace the component, but you could get tired of throwing money at replacing components...I did.
So, if you are still awake, I think the foot pedal black air line controls the clamping. If your rabbet pusher works but the top-side frame clamp doesn't, there may be a clogged valve or a related exhaust port (usually have plastic or brass 'mufflers/silencers' threaded into them. The hand-valve on our VN42 does not, for some reason. The hand valve also has red, back & clear air lines like the foot pedal, so I assume the black & white ones have the same functions on the hand valve.
Some of the functions are sequential due to multiple air-controlled air valves being connected (in series, I guess, trying to make electrical thoughts explain pneumatic functions, but it hasn't helped me solve very many problems, despite my ability to put a frame-joining audience asleep with my tales). By the way, the foot and hand -actuated valves are the only ones that are not AIR-activated. If a sluggish valve doesn't respond in a pneumatic function at its opportunity in a sequenced operation, if may simply not respond or get stuck in a wrong position and not reset until that operation is cycled again. In my progressive troubleshooting, I have observed weird things like that, or valves released seconds later than they should. Some I have not figured out, to my frustration. Some machine functions may work with higher air pressure setting but not lower.
Our VN42 has blue lines with numbered clip-on labels. I haven't figured them out yet. The VN2+1 has no markings but at least came with a pneumatic schematic. That is still mostly cryptic to me, but I can at least get things re-connected properly by marking the air lines and the schematic identically.
If someone knows of a pneumatic drawing for the VN42, I would be happy to look at it.
Be extremely cautious when experimenting with pneumatic valves. Injuries are possible from high pressure air (eyes, ears), and air cylinders don't care if your fingers are in the way.
I once tried disconnecting lines to observe functions working/not working, and had some bad near-accidents. Almost got hit in the face with some part of the nail air cylinder I was reassembling and tried to use air to push it back in...the wrong way.
The nail feed on our VN42 is spring-fed. The VN2+1 has a pneumatic feed. I had something disconnected from that and air feed with no exhaust port connection moved so hard the end of the cast brass nail tray/magazine snapped off, and I had to pay for a replacement one (ugly steel instead of the handsome brass one).