A Noid light is a little device that allows you to quickly check your fuel injectors are getting an electrical signal. Available in sets to suit most common injector wiring harnesses, they’re a useful and inexpensive tool for testing the electrical side of your fuel injection system. To know how to use one it’s helpful to understand a little about how injectors work.
A fuel injector is a valve that’s opened and closed by a solenoid. A solenoid comprises a magnet inside an electric coil. Energizing the coil moves the magnet one way, de-energizing lets it move back.
Behind the injector fuel is held at high pressure. Opening the valve lets this spray through a very small hole, or sometimes several holes. On engines with port injection the injectors spray into the inlet tract just before the valve. Newer engines using direct injection spray straight into the cylinder.
The fuel pressure regulator holds the gas at a constant pressure differential, so when the injector opens it sprays the same volume per second every time. This lets the Engine Control Unit (ECU) regulate the quantity of gas injected on each cycle by varying the time length of time the injector is open. (A longer open duration means more gasoline injected and more power produced.)
If an engine is running poorly, perhaps missing on one cylinder, one possible cause is that it’s not getting a signal from the ECU. This is where the Noid light comes in.
Working one cylinder at a time, unplug the wiring harness from the injector. (On some engines this may involve removing or undoing a metal clip or retainer.) Plug the Noid light into the harness connector, ensuring the terminals on the light match those on the harness. Most fuel injection systems have only two wires at the connector, although there are a few with four.
Have someone crank the engine while you watch the light. (It helps to not be in bright sunlight when doing this.) If the ECU is sending a pulse the Noid light will pulse on and off. If no light is seen, or if the light remains on all the time, that indicates an electrical fault.
On a poorly-running four cylinder engine sometimes unplugging each connector in turn is enough to determine which cylinder has the problem. (Disconnecting a functioning injector will make it run even worse; disconnecting a malfunctioning injector won’t make any difference.) However, this is not an especially reliable method and on six and eight cylinder engines it can be difficult to discern a difference in the way they idle.
Purchasing Noid lights
As manufacturers have different connector designs Noid lights are usually sold as a set. Most have lights to suit connectors on common vehicles such as Chevy and Ford as well as Bosch injection systems. As with all tools, for anyone who does a little work on their vehicle, these can be a worthwhile investment.