Back in the day, farmers filled a sprayer with 200 gallons of spray mix, used a simple chart that told them what speed to drive and what spray pressure to maintain when using a specific spray nozzle, and they sprayed their crops. For all the computerized complexity of modern self-propelled or pull-type sprayers, that's the same principle used today. Diagnosing problems with sprayers is theoretically simple even though modern sprayers can be mechanically complex.
Everything is related to how fast the sprayer is going, pressure or flow rate through the pump and nozzles, and the width of the spray boom. Any time something malfunctions, think of those three basic inputs and back-track the symptoms to diagnose the guilty component.
-If gallons per acre (rate) are inaccurate: what could be impeding or reducing flow? Low rate could be due to plugged strainers or filters restricting flow to the pump, or flow between the pump and nozzles. High flow rate hints of worn spray tips or inaccurate inputs from the pressure sensor or flowmeter. If you can switch the system from automatic rate control to manual rate control (where you designate a specific pressure or rate) and the rate is steady and accurate on manual control, that implies the pump, strainers, filters, etc. are okay, and that the problem is in a component in the automatic rate control system. That means either the ground speed sensor, the pressure/flowmeter sensor, or the actual computerized controller is the source of the problem.
-If spray pressure or spray rate fluctuates erratically: first step--switch the system to "manual" control and see if the problem goes away. If the sprayer operates correctly in "manual", the pump and basic system is okay. Look for problems in the flowmeter or pressure sensor or speed sensor to see if they are sending inaccurate signals that's causing the automated control system to perform erratically. Make sure you're showing ground speed--erratic ground speed, or no ground speed, indicates problems with the radar or GPS or mechanical wheel speed sensor or those sensor's wiring harnesses. Intermittent signals from flow meters or pressure sensors hint of problems with those components.
-If problems persist when the system is switched to "manual": keep diagnostics simple and easy at first. Check for closed or partially closed valves, from the main valve at the tank all the way to the last control valve on the boom. Check and clean ALL strainers and filters. If your owner's manual explains how to do a "deadhead" test of the spray pump, perform that check to see if the pump is up to specs. Spray pumps leaking water from any part of their housing hints it's time to rebuild or replace the pump.
-If boom sections refuse to turn "on" or "off", be suspicious of control valves. The old push-pull plunger control valves are notorious for sticking after exposure to chemicals. Newer "ball" valves rarely stick, but are mechanical and therefore not immune to damage from time, caustic chemicals and other villains. Use a small screwdriver or feeler guage to see if the valve's electric magnet is magnetized when its switch is activated in the cab. If magnetism is absent, be suspicious of problems with the wiring harness between the valve and the cab. If the valve's solenoid magnetizes when the switch in the cab is activated, then disassemble the valve and see if there's an internal mechanical problem.
Modern sprayers have fancy stuff like hydraulic proportional valves, compensators, swath control, automatic boom shutoffs, GPS mapping and dozens of other complicated systems that all play off each other. There are times when diagnosing problems with modern sprayers is a mechanic's nightmare.
But...85 percent of the time problems with sprayers are related to the basic concepts and components that Grandpa used on his sprayer. Don't let all the fancy bells and whistles mislead you when diagnosing sprayer problems. Keep it simple, think things through logically, and most of the time the problem is simpler than it first appears.
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