Constant-pressure (variable-speed) well systems: field service manual

Covers: Residential variable-frequency or variable-speed drives that run a submersible (or above-ground) pump to hold a pressure setpoint, with a pressure transducer or sensor and a small diaphragm tank. Fault codes listed here are Franklin SubDrive/MonoDrive Connect; other brands use their own codes.

Cross-checkedBrands: Franklin Electric (SubDrive / MonoDrive), Grundfos (SQE / CU 301), Pentair (PENTEK IntelliDrive), Goulds (Aquavar), Others6 components

Instead of switching the pump fully on and off between cut-in and cut-out, the drive changes motor speed to hold a steady pressure. Techs recognize it by a wall-mounted drive near a small tank and a transducer on the tee. Most calls are fault codes, wrong tank precharge, transducer or wiring faults, and incorrect DIP-switch setup.

Safety first

Drive and motor circuits are 230 V with stored DC bus energy: lock out and wait the time printed on the drive before opening
Live tests are caution work
Downhole work by a licensed well contractor

How it works

A pressure transducer (4-20 mA, e.g. 100 psi on SubDrive Connect) or a pressure sensor reports system pressure to the drive. When water use drops pressure below the setpoint the drive ramps the motor up; as demand falls it slows and finally stops.

Soft start ramps voltage and frequency, cutting inrush current and heat. Some drives run 3-phase motors from a single-phase supply.

The diaphragm tank is smaller than on a switch system, but precharge matters more: Grundfos and Pentair both specify about 70% of the setpoint (never above). A precharge above the setpoint prevents the drive from controlling pressure.

Setup lives in DIP switches or an app: motor type, phase, HP, sensor type, underload sensitivity. A wrong setting causes nuisance faults such as underload or overcurrent.

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Vertical pressure tank connected to a water pump outdoors by W.carter, CC BY-SA 4.0

Diagnostic guides for this appliance

Guide · Sump / Well Pump

Well pressure switch chattering or clicking rapidly

Chatter happens when the switch sees pressure bouncing around its set point. The usual causes are a plugged sensing port or nipple, a waterlogged or low-precharge tank, a switch mounted far from the tank, worn contacts or a differential set too narrow. Chatter burns contacts fast and stresses the motor, so fix it the same day.

8 steps ~45 minmoderate

Guide · Sump / Well Pump

Jet pump loses prime or runs without building pressure

A jet pump only works when its housing and suction line are full of water. Lost prime almost always traces to an air leak on the suction side, a leaking foot valve or check valve, a clogged foot-valve screen, or a well that has drawn down. Fix the cause, then re-prime; repeated priming alone just burns pumps.

7 steps ~60 minmoderate

Guide · Sump / Well Pump

Well pump hums, won't start or trips overload: control box test (3-wire)

On a 3-wire submersible, the start capacitor and relay in the control box are the most common surface failures and can be tested with a meter. First confirm power through the pressure switch, then test the box. If the box is good or the system is 2-wire, the problem is down the well and goes to a well contractor.

8 steps ~60 minadvanced

Other sump & well pumps manuals

Manual · Sump / Well Pump

Zoeller M53 Mighty-Mate and M98 Flow-Mate sump pumps: quick sheet

Zoeller cast-iron submersible sump pumps with the integral float in a switch case: M53 (3/10 HP) and M98 (1/2 HP), 115 V automatic versions; the same switch case family appears on M57/M63/D-series and some N (effluent) variants. Cord length and LED plug vary by SKU.

7 components5 common failuresZoeller

Manual · Sump / Well Pump

Liberty Pumps 250 and 280 series sump pumps: quick sheet

Liberty 250-series (1/3 HP, e.g. 257 with vertical magnetic float, 250-P piggyback) and 280-series (1/2 HP, e.g. 287 with vertical magnetic float, 281-3/283 variants with other switches). HV = high-voltage-rated variants; -2 suffix = longer cord. Confirm switch type from the model number.

7 components5 common failuresLiberty Pumps

Manual · Sump / Well Pump

Battery backup sump pump systems: field service manual

12 V DC standby pumps installed in the same pit as a 115 V primary, with a charger/controller, float switch(es), alarm and a deep-cycle lead-acid (wet or AGM) battery. Examples: Basement Watchdog Special / Big Dog, Zoeller Aquanot 508 / Aquanot Fit ProPaks. Water-powered backups are not covered here.

7 components5 common failuresBasement Watchdog, Zoeller (Aquanot), Wayne, Others