I''ve been told I need a run crank relay I've been looking all over and can't get a handle on what this does except something about starting--I'm having no trouble starting the car (2009 saturn vue) thanks--Don
A relay is a electromechanical remote control switch. In cars they are used to isolate and amplify signals. The isolation comes from the coil not having any connection to the switch. The amplification comes from it taking a small current to activate the relay but the relay can switch a big current.
Fuel pumps and fans require a big current to operate. Both of these devices are controlled by the computer control module which can only provide a small current. The name "fuel relay" refers to its job on the car. In this case it is the fuel pump relay which when commanded by the computer sends power to the fuel pump in the gas tank.
The "starter relay" operates the starter. Diagnosis of this relay is simple. Turn the key to the start position. Does the engine crank? Yes, then it's good. When to first turn the key to run do you hear a noise from the gas tank? You may have to get a friend to turn the key while you get close to the tank to hear it. If you hear a noise for a couple of seconds then the fuel pump relay is good and the pump motor is running. There may be problem that is preventing fuel flow but that is easily diagnosed by a mechanic with a pressure gauge.
Does your car have the service engine light on? Have you retrieved the fault codes? You could be barking up the wrong tree without this information. Some auto parts stores can lend you a code scanner for free and you can get the fault codes and determine which system is bad. An engine requires fuel and air and spark to run. If one of these 3 items is missing it wont run. And the final really dumb question, "Does the car have any gas in it?"
Hope that helped,