Views: 0 Author: Site Editor Publish Time: 2026-03-14 Origin: Site
A submersible pump that won't start, runs intermittently, or trips breakers isn't always a sign of a failed pump. More often than not, the culprit is the control box. Before calling a technician or pulling a pump from a deep well—a costly and time-consuming job—it pays to test the control box first.
This guide walks you through exactly how to do that. You'll learn what a submersible pump control box does, what tools you need, and how to systematically test each component to pinpoint the fault. Whether you're working with a standard deep well pump control box or a stainless steel pump control box built for heavy-duty use, the core testing process is the same.
The control box is the electrical brain of a submersible pump system. It houses the starting capacitors, relay, and overload protector that work together to start and run the pump motor. Without a functioning control box, the motor can't generate the torque it needs to start—especially under load.
Most residential and light commercial systems use a three-wire pump setup, which requires an external control box. Two-wire pumps have built-in starting components and don't need one. If your pump has three wires running from it (plus a ground), you have a control box in the circuit.
Testing electrical components requires care. Follow these steps before touching anything:
Turn off the power at the breaker panel and verify it's off with a non-contact voltage tester
Wait at least 60 seconds after shutting off power—capacitors can hold a charge and deliver a serious shock
Discharge capacitors by placing an insulated screwdriver across the terminals before handling them
Use a multimeter capable of reading resistance (ohms), AC voltage, and capacitance if possible
Refer to your pump's wiring diagram, usually printed on the inside of the control box lid
Having the pump's model number and horsepower rating on hand is also helpful, since capacitor specifications vary by motor size.
Start with a thorough visual inspection. Open the control box and look for:
Burn marks or discoloration on the wiring, terminals, or capacitors
Swollen or leaking capacitors—a bulging top is a clear sign of failure
Corroded terminals, especially in outdoor or high-humidity installations
Loose or disconnected wires
A deep well pump control box is often installed in a well house or utility room where moisture and temperature swings can accelerate corrosion. A stainless steel pump control box offers better resistance to these conditions, but even stainless enclosures can develop internal moisture issues over time.
If you spot obvious damage, the affected component almost certainly needs replacing. If everything looks intact, move on to electrical testing.
Capacitors are the most common point of failure in a submersible pump control box. There are typically two: a run capacitor and a start capacitor.
Discharge the capacitor first (see safety notes above)
Set your multimeter to the capacitance (µF) setting
Touch the probes to the capacitor terminals
Compare the reading to the value printed on the capacitor label
A healthy capacitor will read within roughly 5–10% of its rated value. A reading significantly below that—or a reading of zero—indicates a failed capacitor. Some multimeters don't have a capacitance setting; in that case, set the meter to resistance (ohms). A good capacitor will show an initial low resistance that rises steadily as it charges. No movement at all points to a dead capacitor.
Capacitors are inexpensive and straightforward to replace. Always match the replacement to the original's voltage and capacitance ratings.
The start relay switches the start capacitor out of the circuit once the motor reaches running speed. A faulty relay can prevent the motor from starting or cause it to burn out.
Remove the relay from the control box
Set your multimeter to continuity or resistance mode
Test across the relay's normally closed contacts (usually terminals 1 and 2, or as labeled on your wiring diagram)
A good relay will show continuity (low resistance) across those terminals at rest
Apply rated voltage to the coil terminals and check that the contacts open—resistance should increase significantly
If the relay fails to switch, or shows no continuity in its resting state, replace it.
The overload protector trips when the motor draws too much current, protecting it from burnout. If it trips frequently—or won't reset—it could mean the motor is overloaded, or the protector itself has failed.
Allow time for the overload to cool (thermal overloads need time to reset)
Press the manual reset button if present
With power off, check continuity across the overload protector terminals—there should be continuity when it's in the reset position
No continuity in the reset position means the protector has failed and needs replacing
Keep in mind that a tripping overload is often a symptom, not the root cause. If the protector keeps tripping, also check the motor windings (see Step 5).
You can test the pump motor's windings without pulling the pump from the well by measuring resistance at the control box terminals.
Disconnect all wires from the control box terminals
Using your multimeter set to resistance (ohms), measure between each pair of motor lead terminals
Compare your readings to the values in the pump's documentation
1
For a standard three-wire submersible pump:
Main winding (Black to Yellow): should show a specific resistance per the pump spec
Start winding (Red to Yellow): will typically show slightly higher resistance
Main + Start winding (Black to Red): should equal the sum of the two above readings
A reading of zero (short circuit) or infinity (open circuit) on any winding indicates motor winding failure. In that case, the issue is the pump motor itself—not the control box.
Also test between each motor lead and ground. Any continuity between a winding and ground signals a ground fault, and the motor will need to be inspected or replaced.
If all components tested within acceptable ranges, reconnect your wiring carefully according to the wiring diagram. Restore power and observe the pump as it starts:
Does it start promptly without hesitation?
Does the breaker hold?
Does the pressure gauge respond normally?
A healthy system will reach operating pressure within a reasonable time and run smoothly without tripping. If problems persist after replacing any failed components, the issue may lie with the pressure switch, the pump itself, or the well's water level.
Older control boxes—particularly those with plastic enclosures in demanding environments—can cause repeated failures due to moisture ingress and heat buildup. A stainless steel pump control box is a worthwhile upgrade for outdoor installations, coastal areas, or systems exposed to agricultural chemicals. Stainless enclosures resist corrosion far more effectively, extending service life and reducing maintenance intervals.
For deep well applications specifically, a properly rated deep well pump control box should match the pump's horsepower and voltage requirements exactly. Using an undersized or mismatched box strains the capacitors and relay, leading to premature failure.
Testing a submersible pump control box takes less than an hour with the right tools and a methodical approach. Working through each component—capacitors, relay, overload protector, and motor windings—lets you isolate faults quickly without unnecessary pump removal or service calls.
For high-quality submersible pumps, deep well pump systems, and stainless steel pump control solutions, Ruirong Pump offers a comprehensive range built for demanding applications. With over 30 years of manufacturing expertise and 60+ patents, their MASTRA-brand pumps and motors are trusted across more than 60 countries. Visit ruirongpump.com to explore their full product lineup or request technical support.
