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You have 2 cars with 2 different things. The one that beeps when you get too close and says driver intervention required is ACC. The things in the front bumper are parking sensors. My XJR does not have the parking sensors but does have ACC.
If you have an XJR then you have Adaptive Cruise Control.
So, it works in 2 ways. One, if you have ACC enabled....meaning you have cruise control 'On', then it beeps and slows down as you approach a vehicle from your front.
ALSO, on the headlight switch panel, there is a button for sonar braking which when you are driving (without ACC enabled) this will beep and automatically apply the brakes to help avoid a collision.from the front. You will know if this is enabled by a small red light on the button itself.
I'm typing this from memory so check the owners manual.
You can see this button here in Red:
I just never bothered to figure out the ACC. I'm assuming its the sonar that's bothered me and why I generally shut it off. And as much as I used the cruise control I've always had it shut off while driving with cruise on. I am actually going to play with this. Cant believe I've never tried it all these years. Seems both cars have ACC.
Just as an aside (from the Owners Handbook), the ACC applies brakes when active, but it does NOT apply brakes when inactive (where it is called "Forward Alert Function" "The brakes will not be applied.".)
It does not appear to be related to the more modern "Emergency Brake Assist" (EBA)
Unfortunately I bought a bum car with a blank switch. But I can live through you guys!