K2639 liquid level controller

Dear, when i start my waterpump, the liquid level controller goes into alarm without any présence of a water leak. After resetting, everything is ok.
Ik think That there is a voltage drop when my pump starts which causes an instable voltage to the Velleman circuit board. The pump and the circuit board are on the same alectrical home circuit.
Can i make the voltage more stable to the circuitboard ?

Add an large capacitor in parralel with the power supply.
The should serve as a buffer.