Dielectric Grease
A non-conductive silicone grease applied to electrical connectors to prevent corrosion and seal out moisture.
Dielectric grease is a non-conductive, waterproof silicone-based grease used to protect automotive electrical connections from moisture, dirt, and corrosion. Because it is an insulator (dielectric), it does not allow electrical current to flow through it. It is applied to the rubber boots of spark plug wires, light bulb sockets, trailer plugs, and electrical harness connectors.
When applied to spark plug boots, dielectric grease prevents the rubber from fusing to the ceramic spark plug insulator due to high engine heat, making future removal easier. It also seals the boot to prevent high-voltage electrical current from leaking out (spark arcing), which would cause an engine misfire. On low-voltage connectors, it seals out moisture and salt that cause terminal oxidation.
Because it is non-conductive, dielectric grease should be applied primarily to the rubber or plastic housings and mating surfaces, not directly inside the metal terminal pins (though the pressure of connection will usually push the grease away to allow metal-to-metal contact). Using the correct amount of grease prevents electrical glitches and extends harness life.