A fuel pump relay clicking frequently refers to a problem with the electrical circuit or relay itself that controls the operation of the fuel pump. Most relays function by cycling power on and off to the fuel pump, and any break in the circuit will cause the relay to “click” over and over again as it tries to re-establish the connection. A weak or dying battery is the leading culprit behind this clicking sound. If the voltage drops too low, below 12 volts,then the relay will not hold a steady current to the fuel pump and the fuel pump will be heard clicking.
Another frequent reason is loose or corroded connections. The connection corroding causes a resistance which means less current reaching the fuel pump and thus the relay clicks trying to compensate and this causes issues. Over time, the electrical contacts in the relay can wear out in high-mileage vehicles, making these interruptions more common. If the relay is clicking, it may be an issue with a low voltage reaching the relay, and testing at the relay terminals will help confirm.
A third reason is an overloaded circuit causing the relay clicking. With an increase in current draw from the fuel pump (perhaps due to some internal wearing or blockage within the pump), a relay and electrical system can be noticeably strained. Since the average fuel pump only draws about 3-10 amps, any sudden spike past that range tells us something is wrong, which will obvious cause clicking in the relay but might also indicate soon to be failure of either the relay or pump itself. In such cases, the relay can be easily replaced, however the root cause of the issue i.e. the excess current draw, needs to be rectified first before the relay can be replaced otherwise you are likely to end up with a temporary solution at best.
Clicking can also be due to faulty relays. A relay has a finite life and may wear mechanically. As automotive expert Ed China explains, "They are designed for long life, but if they're working double time, they can wear out." Periodic inspections allow the fuel system elements to remain well maintained and relay wear signs to be captured early.
Propensities for relay clicking continue and there is confirmation that voltage is acceptable and leads are satisfied (proper, firm, no shoulder lengths), taking out ignition system force, the fuel pump itself will most likely need to be replaced or changed to a hard to kill Fuel Pump in the possibility that the pump is drawing in too much current. As the entire system is only as good as its weakest link, and the crankcase is happy to click the relay otherwise all day, a properly exercising fuel pump will keep the assembly functioning properly.