1999 JLG 450a Steering and Boom Issues

Hi, I recently purchased a 1999 JLG 450a that has a few issues. First is the steering does not work. I replaced the joystick and verified that I am getting voltage down at the coil when I try to turn. The second issue is the boom intermittently does not want to raise or lower. Similar to the steering I am able to see voltage down at the coils when I move the controls from the basket. I am thinking it's a hydraulic issue but I'm new to hydraulics and don't know where to start. One other thing to note is the drive and swing functions work. Not sure if that helps but thought it might be useful. Any help would be greatly appreciated.

Maybe this will help: https://www.heavyequipmentforums.com/threads/jlg-450a-3-functions-not-working.57751/ Both symptoms point toward a common hydraulic/control issue on the main function manifold rather than the joysticks, since you have coil voltage and other functions (drive/swing) work. Given this model and your symptoms, the most common causes are: Main function “dump” / master solenoid not opening fully or sticking intermittently. Low system pressure or flow due to clogged return filter or suction strainer. Stuck or contaminated section spools in the lift and steer valve sections. Poor ground or weak power to coils under load, even though you see voltage at rest. Because drive and swing work, the pump and engine are likely OK, and the problem is local to the boom/steer control manifold or its supply. Step 1 – Verify hydraulic​ Check oil level and type in tank; top up if even slightly low. Inspect and, if unknown age, replace the return filter element. Look in the tank (through fill/inspection) while someone operates lift/steer; look for heavy aeration/cavitation. Listen for pump whine when trying to steer or lift; loud cavitation suggests suction blockage or low oil. Step 2 – Confirm the main dump / enable valve​ Locate the main control/dump solenoid on the valve block (refer to the 450A service/hydraulic manual for exact location and ID number). With an assistant commanding steer or lift, verify: You have battery voltage at the dump coil when any platform function is called. The coil is magnetizing (screwdriver test on the nut) and the valve body shows a slight change in sound/feel. If voltage and magnetism are present but steer/lift still dead/intermittent, pull that valve cartridge, inspect for debris or scoring, clean it and the bore, or replace if questionable. Step 3 – Test steer and lift​ Since you already see voltage at the coils, next check whether the valve sections are actually shifting and making pressure. Steer: While commanding left/right, feel the steer valve section; you should feel/see some reaction in the hoses and hear a slight change in tone. Check whether the steer cylinder tries to move or hoses go firm; if no reaction, crack a work-port line briefly (with proper PPE, engine at idle) and see if there is flow when you energize steer. If there is no flow under command but other functions work, the steer spool or its solenoid cartridge is likely stuck or blocked. Boom lift: Do the same crack-test on a lift cylinder port or use the test port for main lift pressure if provided in the manual. If pressure/flow is absent or erratic only when operating lift, pull and clean the lift section’s control valve cartridges. Contamination in older machines is very common and can cause exactly this intermittent behavior. Step 4 – checks electric under load​ Even though you see voltage at the coils, verify the circuit integrity under load. Measure voltage directly at the solenoid coil while someone fully strokes the joystick; ensure it stays close to battery voltage and does not collapse. Check the main ground point for the valve bank; clean and tighten all ground and power connections feeding the platform and base control logic. Inspect the harness plugs at the turntable junction/manifold for corrosion, broken pins, or pulled wires, especially for the lift and steer outputs. A weak ground or corroded connector can give a “nice” voltage reading with a meter but not allow enough current to shift the valve. If it is still, then the 450A service manuals contain good troubleshooting trees and the hydraulic schematic for lift/steer circuits. If steer and lift are solid from the base but not from the basket, focus on platform controls, wiring, or platform enable logic.

Thanks for the reply. I will check that out this week and see what I find.

Sorry for the delay in response. It's been a while since I've been able to get outside and work. Thank you for your earlier reply. I replaced the dump valve and things are working better. I can now steer and drive consistently. I am now trying to figure out some other issues. The basket doesn't rotate or level, the telescope doesn't go in or out, and the main boom doesn't go up or down. I'm wondering if it's issues with the cartridges for those functions. I tested the coils again and they all seemed fine. Any advice on where to look would be appreciated.

I'm also wondering if there's maybe another common hydraulic component that could be causing the issue since there are still multiple functions not working. Is there anything other than the dump valve that could affect those functions?

Sorry for all the follow ups but I was wondering if the part I have circled is the proportion control valve for the bang functions? I am going to test that out tomorrow if it is.

Umm, bang bang functions do not have a "proportional control". Being bang bang control, meaning, pretty much, full on or full off, is self defining. There in nothing proportional with bang controls. Is this an AI thread or something???

No sorry I'm just getting myself confused. I thought I saw a proportional valve mentioned in another thead.

chanman547 said: No sorry I'm just getting myself confused. I thought I saw a proportional valve mentioned in another thead. Click to expand... Point taken. And you may be legit, it's just hard to judge now days with AI everywhere. And I don't know the answer to your problem, and hope you find a resolution here.

Hi Willie59, I think where I was getting confused is here you mentioned a proportional flow control valve. willie59 said: I wish I could tell you the part #, unfortunately my head is full of over 50 yrs of numbers. Most hyd hose shops that have various fittings as well will have that quick coupler, of course you would have to remove the male part of the coupler from the manifold and take it with you so they could match the female to it. I might be able to get a part # off of one of mine when I get to work tomorrow. I think your sequencing valve is working proper because the steer is working. You can sort of verify that by plugging your test gauge into P2 (actually, the test port is G2) and powering up the dump valve. When you do that, if the sequencing valve is working correctly you should see somewhere around 230 PSI on the gauge. Yes, the steering and bang functions share the sequencing valve for those functions, but the steer does take a separate route, one all by itself, the bang functions follow a proportional flow control valve, and that's the next thing I'd look at. When you work a toggle for one of the bang functions at the lower controls, make sure the proportional valve is getting power and that it has a good ground. If it does, I'd pull the proportional valve coil and cartridge and swap them with the same proportional valve that works boom up/dwn or the one that works swing, then try one of the bang function toggles at lower controls. Click to expand... Is that located somewhere in the diagram I sent above?

chanman547 said: Hi Willie59, I think where I was getting confused is here you mentioned a proportional flow control valve. Is that located somewhere in the diagram I sent above? Click to expand... Umm, not in this thread, maybe in another as I respond to so many threads it's not possible for me to keep track. But for the sake of the discussion, bang functions are just that, full on or full off, typically toggle switch control. Proportional, that's control that works like the throttle pedal on a vehicle, function speed is determined by how much you move the throttle pedal. I hope this clears this up.

I had a good learning day today but no fix yet. I was able to get all the functions working when I tried them at the same time as a previously working function. Next I swapped the bang bang and swing flow control cartridges which didn't help. I then checked the voltage at the bang bang flow control coil and that looked good even for the functions that aren't working. Then I swapped the flow control coils which also didn't help. Next I swapped the sequence cartridges and that didn't work either. I'm thinking the only thing left is to try the load sensing shuttle valve. I didn't have the right size Allen wrench to try it so I'll get one tomorrow and try that next. If anyone has any other ideas or anything I missed feel free to let me know.

Something else I noticed today is the swing works both ways from the ground but only swings to the right from the platform. I'm not sure that it's related since everything else works the same from the ground and from the platform.

What's the machine serial number? What engine? What functions have proportional joysticks and which one's use toggle switch control? Is there a speed control knob at the platform for the toggle controls?

The serial number is 1000252 0300044220. It's got a Duetz engine. The only functions with a proportional joystick are the swing and lift/telescope. There is a speed control knob at the platform for toggle controls

Just to clarify, a 450A has two booms, upper and lower, are both of those booms proportional control?

It's for the upper lift. The lower lift is controlled by a toggle switch and that works all the time.

Got it. Assuming I'm looking at the correct service manual #3120750, drive/steer joystick is on the right and swing/upper boom joystick is on the left, lower boom (tower) is toggle switch control with speed controlled by proportional speed control knob.

The manual you sent is the correct one and the diagram and everything you said is accurate.

Ok, I see what you're working with. Not going to promise anything, but I'll noodle into this, but I am busy with other things. On top of that, the troubles you describe are those that are very hard to troubleshoot over the internet. But I will do a little digging and see if something jumps out at me. Maybe so, maybe not.