The regulating valve (also called the control valve or pressure regulator) is the brain of the pneumatic control system. It converts output pressure into control air pressure that tells the inlet valve and engine speed what to do. If it fails, the compressor cannot regulate itself.
What you'll see
If the control air lines are disconnected or cracked, fix those first -- they are more common than regulating valve failure. See: No Control Air to Valve Actuator.
How to diagnose
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Measure control air at the regulating valve output
Connect a pressure gauge to the control air output of the regulating valve. With the compressor stopped, control air should be zero. As the compressor runs and builds pressure, control air should increase proportionally. When output reaches the rated pressure, control air should be high enough to close the inlet valve and reduce engine speed to idle.Result: Control air tracks output pressure = regulating valve OK. No correlation or stuck = valve faulty. -
Try adjusting the regulating valve
Most regulating valves have an adjustment screw or knob. With the outlet valves closed, adjust the pressure setting. If the pressure does not change as you turn the adjustment, the valve may be stuck internally. Compare the current setting to the nameplate rated pressure.Result: Pressure responds to adjustment = valve working, just maladjusted. No response = internal failure.
How to fix it
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Overhaul or replace the regulating valve
The regulating valve contains a diaphragm, spring, and valve seat that can wear or become contaminated. Some can be cleaned and overhauled with a repair kit. Others need complete replacement. If replacing, use the correct valve for your compressor model -- the spring rate and pressure range are specific to the machine.
Do not adjust the regulating valve to set a pressure higher than the compressor is rated for. This overloads the engine and the screw element. The adjustment screw sets the working pressure -- check the nameplate for the maximum. Turning it all the way up does not mean more air, it means more load on everything. Also: when adjusting, always do it with the outlet valves closed so you can see the actual unload pressure. Open the valves after to check working pressure under load.
Pressure gauge. Screwdriver for adjustment. Replacement regulating valve or repair kit. Compressor nameplate data for correct pressure setting.
A failed regulating valve can cause the compressor to overpressurize if it gets stuck in the low-control-air position (inlet fully open, engine at max). The safety valve should protect against this, but do not rely on it exclusively.
This issue can also cause
- Engine Won't Rev Up Under Load Engine stays at idle RPM when compressor should be loading—speed actuator or control air issue.