JLG 50HT idle problem

[mdnelson86]

I've got an old JLG 50HT boom lift I use around the farm and it's started having some issues I'd like to resolve. Biggest problem right now is that sometimes it gets stuck in its' high idle position even when I take my foot off the dead mans switch in the basket.

Normally when you press on the switch it revs the engine up and allows hydraulic control. Let up off the switch and it idles back down and you none of the controls work. When it sticks it just stays in high idle and doesn't allow any of the control functions so that would sound to me like it is working alright between the basket and the main frame. This doesn't seem to be consistent as sometimes it will work and other times it won't. also doesn't seem to be related to any certain activity. Sounds to me like possibly a sticking solenoid somewhere that controls the throttle setting. any ideas where that is? I've tried tapping on everything I can find to see if something is sticking but haven't had any luck. it has the 4cyl ford engine.

 

[willie59]

I'm glad you added that it has the LRG Ford engine. That engine should have a Precision Governor control box and actuator to control carb throttle. You could have a faulty Precision Governor controller, but I wouldn't go there just yet. It's gonna be hard for you to troubleshoot this if it only does it occasionally and at random. Every time it messes up, and you get mad and go to do some checks, the darn thing will start working proper again.

Does it have a high engine switch at platform controls? I doubt it will make a difference, but when engine remains at high throttle, does it change if you flip high engine switch?

Problem could be with high engine relays in lower control box, there should be two high engine relays. If you could catch it acting up and test the terminals of those relays would be nice, see if they have no voltage input on terminal 85 but are putting out voltage output on terminal 87A of relay. With no voltage input to 85, there should not be voltage output on 87A.

 

[mdnelson86]

This machine does have the high engine switch for when driving the machine. While on the subject it also has, if I remember them right, a "drive speed high/low" switch and a "drive motor" high low switch. how exactly do these three switches work? sometimes I get in some soft ground conditions and have trouble spinning a wheel out and have found that certain positions of the switches seem to work better in tough situations. Obviously the engine speed controls the throttle, seems like the drive high/low acts like a 2 speed gear switch but I'm not sure how it works, and the motor high low acts like a high or low range for the motors. when in the high position it tends to bog the engine down a little bit unless I'm on perfectly flat and solid ground.

back to the original topic, when I use the high engine switch while I'm having the problem with it staying at high throttle it actually seems like the RPM drops slightly when I start to move. Sounds like the high engine relays might be sticking (engine at high throttle) and when I start to move the machine it pulls the engine down slightly under load.

on another note, I read in a different post when I tried searching about this subject about a different model jlg lift that sounded like it is supposed to stay at the lower idle setting except when it is being driven. Is mine supposed to be that way as well or is it working properly when it throttles up as soon as you step on the deadman's switch? When working properly it currently idles (quite low but smooth) when off the switch, when you step on the switch it throttles up pretty high, when you flip on the high engine switch and move the machine it throttles up just a hair more. The reason I ask is when I'm up in the air I have some trouble with my swing control moving to fast and it would be nice if the engine would be at a lower RPM so it wouldn't be so sensative. It had a new PQ circuit board from hindley electronics just before I bought it a few years ago. I tried adjusting it myself and it helped a lot but it's still not very good. seems like the feather position between all or nothing is too small. It also doesn't help this this machine has been around a while and has some slop in the turntable gears. Currently I work around this by shutting the engine down and using the auxiliary motor to slowly swing myself into a tight position.

that just reminded me of another symptom of the throttle issue. when I flip with switch off in the basket shouldn't it shut off power to the engine and kill the spark so the engine dies? when I'm having the high throttle problem and try to shut it down after it's been running (at high throttle) for a while the engine will rev down like it's about to die but then surge up and down almost indefinitely. At first I thought it was just running on without spark for a little while but I found that if I go to the main control box and flip the master switch off it will die right away.

Sorry for such a long response. I really appreciate your help and am just trying to get a better feel for how this machine is supposed to work.

 

[willie59]

There will be three switches that control speed of drive motors. You have the creep switch, which limits the output of the PQ controller to give very slow drive operation. Pump output switch is another. In normal mode, there is one hyd pump that feeds a certain GPM of oil to drive motors, when you flip the pump output switch, it add an additional pump and its GPM output to the drive circuit, which increases drive speed since there's more oil going to the drive motors now. And lastly the drive motor speed switch. The drive motors are variable displacement axil piston motors. When you flip the drive motor speed switch to high speed, it sends oil pressure on a small line to drive motors that changes position of the swash plate of each drive motor and increases speed of motors.

As far as throttle operation, JLG did it differently on their various machines. On most of the diesel machines the engine remained at low rpm until drive was engaged and then engine would throttle up. On some, but not all, of the gas or dual fuel engines, the engine would throttle up when high engine switch is selected and foot switch is depressed. And on others still, JLG installed a "mid range board" on the throttle circuit. With engine running it would remain at idle until you depressed the foot switch and then it would throttle up to 1800 rpm (mid range) for boom operations, then when you flip the high engine switch it would throttle up to 3000 rpm.

The problem your having when making gentle movements in tight places are not so much related to engine speed, whether high or low, but more to do with you're either not using the creep switch for slow movements or the creep position of PQ controller is not adjusted properly. I will make a separate post explaining how to adjust the PQ controllers.

On the Ford LSG and LRG engines, JLG didn't not shut down ignition when you flipped the engine stop switch at the platform controls. Instead, when you flip that switch, it takes away power from the anti-dieseling solenoid mounted in base of carburetor. This would stop fuel flow into the engine and kill it. After engine dies, an oil pressure switch would open from lack of oil pressure and break power to ignition circuit of engine. If engine does not die after you flip engine kill switch at platform, then something is either keeping anti-dieseling solenoid powered, or the solenoid spool is stuck not blocking fuel flow through carburetor.

 

[willie59]

The PQ controllers have operation speed adjustments. There should be 4 trim pots on the circuit card of controller with adjustments screws labeled something like Lo, Hi, Mid (or Threshold), and Ramp. It takes a very small flat head screwdriver to adjust trim pots. And always wear an approved safety harness when operating an aerial lift.

The first thing you do is back off all trim pot screws by turning them CCW. Those are 32 turn trim screws for full adjustment range, but most likely each trim adjustment will be somewhere in between 0 and 32 turns already. You need to be in a real quite place when backing off screws.

Turn each screw CCW until you hear a faint "click click" when rotating each screw. That sound is the adjustment at it's end. Don't continue turning much after hearing those clicks. Do this to each trim pot adjustment and you're ready to adjust controller.

Let's start with boom up/dwn controller. Have boom fully retracted, start engine. Depress foot switch and move boom up control lever about 15 degrees, what's called "notch off center", and hold the control lever in that position. Adjust Lo trim pot CW until boom just starts to get a boom up movement, no more.

Then, with boom up lever fully stroked, adjust Hi trim pot CW. At a certain point of adjustment, boom operation will speed up as screw is turned. Adjust screw CW until boom speed no longer gets faster, stop there.

Now, bring the basket back near ground and extend the boom a good bit, long boom. Flip the creep switch to creep position. Fully stroke boom up control. Adjust Mid (Threshold) screw CW until you get a desired slow boom operation speed for when you're working in tight spots.

Lastly, turn off creep switch, fully retract the boom, and turn the Ramp screw in about 10 turns for pre-adjustment. Fully stroke boom up controller to make boom move at full speed, then simply let it go so it returns to center. If boom abruptly stops, adjust Ramp screw a few more turns CW, try again. At some point of adjustment, boom up will no longer stop abruptly, but instead will drift for a moment and come to a slow stop when controller is released. Once you reach that point, adjust Ramp in or out to get desired power down that doesn't cause abrupt stop of boom movement nor does it drift excessively.

This adjustment procedure applies to each function controlled by PQ controllers. If the adjustments won't work out, PQ controller is likely faulty and needs replacement.

 

[mdnelson86]

Thanks for your explanations! I've got a few more things to look into now. the only thing I'm confused about is the "creep switch" you refer to. Your saying there should be a switch that limits the output of the PQ controllers to allow for slower operation? I don't recall there being anything like that on my machine. certainly not any switch with the word "creep" on it. Is it possible it's under a different name? As I said in my post above I have 3 switches (all related to drive I believe) - engine speed high/low, drive speed high/low, and drive motor high/low. I'm thinking my machine doesn't have this creep option though. I know it won't work on the swing pq controller as, last time I tried to adjust it, I noticed it (being a newly replaced circuit card) had only 3 trim pots compared to the others have 4. It only has hi/low/ramp. Is it possible to add a creep switch in (knowing I'm sure I'd have to replace the swing PQ control with a 4 pot version) to allow for slower operation? that sounds exactly like what I need. Lastly, are all 3 PQ controllers (swing, lift, and drive) supposedly the same? if so I could swap out the drive controller for the swing controller. I'd much rather be able to gently feather my way around swinging with the boom extended than be able to feather driving the machine around.

 

[willie59]

Well I'll be damned, I have to admit I've never worked on a 50HT, and, likewise, I've never seen a PQ controller that only has Lo Hi and Ramp only. But I just checked the parts manual PDF and sure enough, that model doesn't have the creep switch. Well butter my biscuits, didn't see that coming. Drat. As for adjusting them, I'm not sure now as I have never done those. The PQ controller, whether 3 trim pot or 4, is still a Pulse Width Modulation (PWM) controller, they are set up and adjusted with an oscilloscope, something most folks, even I, don't have. The adjustments is a method I have worked out by working with the machines. But I'm now curious, the method may still apply, just omit the creep switch adjustment as it's not there. Notch off the controller and adjust Lo trim pot until you just get movement, and I mean just get a reaction, no more than that. That's about the best you can get without creep mode. And the controllers should be all the same, even if you're seeing different part numbers on them, one should interchange with the others.

 

[mdnelson86]

thanks that helps explain some stuff to me. do you think there is a way I could wire in my own creep switch? As I said earlier my lift and drive PQ controls have 4 pots but the one that replaced the swing control only has 3 so that one wouldn't work. I don't imagine it would be as simple as wiring in a switch that, when switched on, put the appropriate voltage (I also don't imagine it would be as simple as a standard 12v signal) to pin "x" on the controller harnesses? sounds to me from your description that it's all done electrically within the PQ controls. so I would think the ones I have that have the creep trim pot would have the ability to go into creep mode if I just knew what it needed to see to activate that.

 

[mdnelson86]

Oh and by the way, I must say I'm extremely disappointed in you that you haven't worked on a 50ht and don't have instant answers to all my questions (please insert a healthy dose of sarcasm to the previous statement) Leave it to me to find the one lift around for the right price, but it sends everyone away scratching their heads. All in all i'm very very happy with it even though it's old and somewhat wore out and has plenty of flaws. It sure beats climbing ladders and working from front loader buckets like I've been doing for years. All these little things are just minor annoyances that would be nice to clear up, but more importantly, I like learning how it is supposed to work so when something else pops up down the road I have a better idea of what it's actually doing electrically or hydraulically to accomplish this or that.

Again I appreciate your help. I'm not a drinking man and don't know if you are or not either, but I tell you, with all the help you've given me and others on this forum I'd gladly buy you a beer!

 

[willie59]

First, sarcasm taken as appropriate humor, no worries. Second, just 'cause I ain't worked on a 50HT don't mean we can't fix it, I have worked on a 60HT and the machines are very similar. JLG made a pile of models, and have a number of variations within the models. Heck, they even offered a steering wheel in the platform on some models. I'm only saying that JLG made so many different things, it's not unusual to miss one of their variations. The big problem I'm having is JLG decided, for some crazy reason, to have practically non-existent wiring diagrams in the manuals for the 50HT, which is odd because they've always been good at having diagrams in their manuals, although not the easiest to read and work with.

And yes, if your drive and boom controllers have the 4 pot circuit cards, one pot labled mid or threshold, then you could activate creep on those controllers. All that is required is a voltage input from a toggle switch to the controller, that input causes the controller to limit output. And that being the case, all you would need to do is get a PQ card that has the 4 pots and fit it to your swing controller. Does your PQ controllers have a terminal strip or a harness with a plug connection in the harness?

And lastly, yes, send beer. Bah ha ha hahahaha

 

[mdnelson86]

the PQ controllers on this machine all have plug connections into the main harness.

 

[willie59]

Ok, this is the harness connector for a controller on a 40H. Check and see if yours is something like this. On yours, if it does not have a creep switch, and if it originally came with 3 trim pot controllers, then you shouldn't have a white wire connected to pin #7 on platform harness side of plug. But, look at the other side of your plug on your 4 trim pot controllers, see if there is a wire connected to pin #7 of controller side of harness. If there is, all you need to do is connect a voltage input to pin #7 to activate the creep function of controller.

 

[mdnelson86]

Thanks! that should get me to where I'd like to be if it all works right.