Im hooking up my tranny temp gauge and it says I have to use the third prong as an engine ground. What does that mean? If i have to ground it to the engine, what is the purpose of that instead of a regular ground?

it is an autometer electrical gauge. I have everything else already hooked up.

 

they didn't say chassis ground? All of my gauges (boost, electric fuel pressure, and oil temp) are grounded at the nearest convenient chassis ground with no problems whatsoever.

A ground is a ground and whether they be engine or chassis grounds they end up back at the battery anyhow.

 

Everything is grounded to the engine chassis ground bolt, thats where the battery connects to. The biggest chunk of metal in the car is the engine block, thats the common ground. (it's located below the TB by the coil)

They want you to use that because thats the most direct cleanest grounding location with the least chance of stray voltages and bad connections. Also EMI could be a factor.

For example, when installing a aftermarket ignition your always supposed to use the engine ground bolt, not just scrape some paint off the chassis and attach it there..................