JLG 40 RTS issue

[Eric Fritz]

The JLG 40 foot rough terrain scissor lift that I use has developed a strange issue. Often times when the operate/deadman switch is pressed, the machine jerks back hard about 6 inches. This happens frequently, not every time though. All the rest of the functions work as they should. It is a late 90s model with 4x4 and outriggers. Any ideas what it may be? Thanks!

 

[Grestil]

That's quite an issue to have. Lots of wiring on that machine and many possible causes given its age. Using appropriate safety measures, I would check the output from the potentiometer in the drive controller first. The last time I saw an old machine of similar vintage and symptoms it was wiring damage in the platform to ground harness where it had been crushed between the scissor arms and shorted the enable and drive circuit wires.When I energized the enable circuit by pressing the enable switch the machine drove without moving the joystick until the timer cut power to the enable circuit. Nearly hit my workvan. Be super careful because when the engine is running if that circuit is powered up for any reason the machine will move. Unintended machine movement is very bad. Service manuals are available for free from the manufacturers website.

 

[Eric Fritz]

Appreciate the reply! I will check the output up at the controller, and down at the ground end. One more thing to note, once the machine warms up and the hydraulic fluid gets warm, the problem goes away. Once the machine cools down again, the problem comes back. However, not every time. For example, I just ran it today and it never did it once, even from cold start. Makes me wonder if there is some sort of intermittent hydraulic cross leak or wiring issue between the brake release and drive hydraulic circuits. When you press the operate button, the brakes release as they should, and that fluid (or voltage) could be leaking into a different hydraulic circuit potentially. There is a lot of wiring and hydraulics on these machines!

 

[Grestil]

Safety issues aside, it would be nice (as far as troubleshooting is concerned) if it did it consistently. Would make the problem easier to find. If you cant find any electrical issues then I would go straight to the pump. If memory serves, the swash plate angle is controlled by 2 valves that sit on top of the pump. 1 for forward and another for reverse. These valves control the swash plate angle by using charge pump pressure to change the angle of the plate. If one were to prop up the machine and remove the connectors on these valves and the wheels still moved, you would know for sure the problem is in the pump. Intermittent problems are the worst.

 

[Eric Fritz]

That’s very good information, I did not realize this was a swashplate style pump, I thought that it simply used variable bleed solenoids to operate the wheel motors. Now that you bring that up, that might be another place to check. Once the issue starts occurring, it always sounds as though the pump is slightly being loaded even if the wheels are not turning and the brakes are locked. You can hear a little bit of a whine as if the pump is pushing pressure, and when it does that, it will always jerk back. All sorts of places to check!