Though the running of a fuel pump without a relay is possible, it is not recommended as relays control when and how most efficiently the fuel pump operates. The fuel pump relay is essentially an electronic switch that allows low current voltage to pass through it, giving the ON command to the electric fuel pump. Ensures that power is delivered to the pump... usually 12 volts when the engine is running and switches off when the engine stops, If you were to direct wire the pump to a power source without an relay, this can cause:
However, one big problem with bypassing the relay is risking electrical overload. That's because a fuel pump is a high-current-draw item, and wiring and switches (and we need not diagram it) cannot handle the kind of current required by the pump. It may cause the fixture to overheat, short out or pose a fire hazard. Relay failures and other electrical problems, for example, contributed to around 8 percent of fuel system-related failures according to the National Institute for Automotive Service Excellence (ASE) in a report from earlier this year so adequate electric management should also be addressed.
On top of this, when doing so under high loads with no relay, expect the fuel pump to get hot and quite possibly burn out. Normally, the relay also controls when a pump operates—meaning the pump would run even when it isn't needed (and they can get hot to begin with). This continuous operation will eventually wear the pump out prematurely as typical longevity is about 100,000 to 150,000 miles under nornal conditions. When you bypass this relay, the odds of a burnout increases and may shorten up to 20% of the pump life.
While a bypass of some sort could start the car if the fuel pump relay has quit on you, it will not cure the long-term problem. Fuel pump relay replacements typically range from $20 to $50, which is a lot more economic that spending around 6 times the amount fuel pumps cost ($400-$600).
As automotive legend Carroll Shelby once eloquently said, "Power is nothing without control," and the relay effectively manages that flow of power to help ensure your fuel pump has longevity. If the relay stops getting a proper control on, almost none of the system could function.
So to sum up, you can operate a pump without a relay but the potential consequences are great for both your vehicles electrical system AND its expensive fuel pump. For further details about fuel pump maintenance as well a relay replacements Visit Fuel Pump