Installation Guide
Low-Pressure Cutoff Switch
MPSM4 / MPSHDM4
Automatically shuts off the pump when pressure falls too low or drops quickly. Manual restart after a trip.
Which Switch Do You Have?
MPS | Standard
Confirm the part number printed on your switch cover.
MPSHD | Heavy Duty
Same function. Heavy-duty electrical design.
Before You Start
Verify model, voltage/HP ratings, and pressure range. Confirm tank precharge and adequate water supply before using low-pressure protection.
Installation | 4 Simple Steps
Disconnect Power. Relieve Pressure.
A qualified installer locks out the correct breaker, verifies zero voltage, then opens a drain until the gauge reads 0 PSI.
Check the Tank and Pressure Connection.
Check precharge with the water side drained. Remove sediment from the pressure sensing port; replace damaged or restricted fittings.
Install the Pressure Switch.
Use suitable thread sealant on compatible male pipe threads. Keep the pressure opening clear and avoid twisting or damaging the switch housing.
Connect Wiring. Replace the Cover.
A qualified installer follows the markings and model wiring instructions, verifies grounding and ratings, then secures the cover before restoring power.
Operation + Safe Startup
MPSM4 / MPSHDM4 | Follow the lever markings on your exact model.
How the Lever Works
M4 | Automatic Low-Pressure Cutoff
Under normal pressure, the switch turns the pump on and off automatically. If system pressure drops abnormally low (typically around 10 PSI below the cut-in setting) or drops quickly, the switch trips OFF to help protect the pump.
A trip signals a possible problem. NEVER repeatedly reset the switch without diagnosing the reason pressure fell.
How to Restart After a Trip
First, a qualified person checks the water supply, loss of prime, leaks, closed valves and tank precharge. After fixing the cause, follow the lever markings on this exact switch to manually start and return to AUTO when pressure has recovered. Stop if it trips again.
Startup Check | Qualified Installer
1. Restore
Confirm cover is secured. Restore power under qualified supervision.
2. Observe
Using a working gauge, verify pump starts and stops at correct pressures.
3. Stop If
Stop for unsafe pressure, repeat trips or abnormal cycling.
Bladder Tank Precharge — Water Side Empty
| Pressure Setting | Precharge |
|---|---|
| 20/40 PSI | 18 PSI |
| 30/50 PSI | 28 PSI |
| 40/60 PSI | 38 PSI |
Important Safety + Legal Notice
Before any work: disconnect and lock out power, relieve system pressure, and verify the circuit is de-energized. Use only marked voltage, current, and pressure ratings, and confirm all wiring and settings match specifications before restoring power.
M4 LOW-PRESSURE CUT-OFF: Trip threshold is approximate and model dependent. Never bypass protection or leave a tripped pump unattended during reset.
For use by qualified professionals only. The installer and end user are solely responsible for proper selection, installation, operation, and compliance with all applicable codes and standards. Merrill Manufacturing Company assumes no liability for injury, death, or property damage arising from misuse, improper installation, unauthorized modification, or failure to follow applicable codes and these instructions.