Always connect auxiliary power to a point on the vehicle that is electrically active all the time. Here's why.
The safest and most-reliable point may be the test connector under the left side of the dashboard. Recently, several readers have asked me why I didn't recommend connecting auxiliary power to a cigarette lighter socket or similar power port. Let me recap a previous discussion.
I had recommended applying external, backup power to a vehicle before disconnecting its battery for any reason.
This topic appeared in the March 18 issue on page 7 ("Auxiliary power saves vital computer memory").
Linking auxiliary power to the electrical system maintains computer memory that might otherwise be canceled when a technician disconnects a battery during a repair.
One common backup power source is a "jump box" or "booster box" that techs usually use to jump-start vehicles. Another is a properly charged automotive battery.
Note that the negative terminal on a backup power source connects to a comparable negative terminal on the vehicle.
A negative terminal, which has an electrical pressure of approximately zero volts, is often called "ground."
The auxiliary power source's other terminal is electrically positive with an electrical pressure of approximately 12 volts (battery voltage).
Traditionally, techs have called this terminal "hot" or "live."