Since it has the Deutz engine, should be the F4L912. With that application, engine will never be at an "idle" speed, it will only have two engine speeds; low speed (for operating boom functions) = 1800 rpm, and high speed (used during drive) = 2500 rpm.
You are required to test the overload system on the machine each time you put it in service, or begin each work shift with machine. With any brand aerial, whenever you get over 80 foot of boom length, they are unstable because the boom is so long. Even though that machine weighs 44,000 lb, that boom still creates a lot of leverage. If they made it heavy enough to provide sufficient counterweight for boom the machine would be too heavy to be practical in operating on jobsites as well as transporting. So the trade off of workablility and stability is to design a system that prevents machine from being tipped.
Compared to some other brands that use very complex electronic gizmos and CPU controllers (which go nuts on occasion and shut down operations), the 120HX has a very simple system to monitor overload. The primary system/operation monitor is mounted in the rear of the boom underneath the rear boom cover. It's a box that contains a cable reel with an attached potentiometer to monitor boom length (extension) and an angle sensor (boom angle). These two components send varying voltage inputs to controller inside lower control box. Whenever the controller gets inputs from the two sensors that would be an area of instablility (boom length and boom angle) it lights the warning lights on upper/lower control panels and will ultimately prevent boom extension and boom down, preventing tip over.
In the case that primary system fails, there's a double redundant system to "back up" the primary system. It consists of two switches in the lower turntable base. One is a mechanical switch, the other is a proximity switch. When boom is in low angle positions, and the primary system failed, as you extend boom the weight of the boom will cause lower house to tilt on pivots. At a certain point, it will activate the two switches. If the mechanical switch is faulty, the proximity switch will be back up, or vice versa. The chances that all three systems fail at the same time, primary, mechanical, and proximity switches, is nearly not possible.
Now that you know the nuts and bolts of the system, the overload test is the procedure to test these components to make sure they're working properly. The operators manual gives you the procedure. It's very simple, and only takes a few minutes. If you don't have the operators manual (shame on rental company if they didn't provide one), you can download manuals from the JLG website.
Also, at the back of operators manual is a "Transfer of Ownership" form. You should fill out this form and mail it to JLG. Then if there's any issues with the machine, any updates that need to be performed, they will forward you the information.
