Bypass Strategies: Relay vs. Switch

[The Rigger]

When I get home next, the first order of business (after switching out the headlamps - Hi Boris!) is putting in the new sound system (Pioneer AVH-P4100DVD, Polk 6-3/4" speakers). I plan on bypassing the parking brake interlock, but I won't be kludging up the dash with a switch; I'll use a SPDT fog-lamp relay to do the dirty work, using the blue control wire off the head unit to control the relay, switching the brake-interlock wire to ground.

Anyone else here done this? Did it work for you?

The only thing that gives me pause is the line in the AVH's instructions saying to get the video functions ot work you have to put the brake on twice, which doesn't make much sense to me... Might have to work up some kind of logic circuit to "pulse" the relay signal, if the head unit wants to see discrete ground signals.

[John_H]

I'm guessing you need to put the brake on twice so the system know you're not grounding the brake wire.

If you have a push-button interrupt of the relay the brake will look like it's going off and back on again. But at that point you might as well go with the switch in the first place.

[The Rigger]

I was afraid of that...

Oh well... Clock chips are cheap, and I haven't had an elecronics project in a while.

[John_H]

But do you know you'll have the appropriate on/off/on times for the system from start-up?

If all you have to do is power-cycle the head unit it might not be so bad but I'd hate to have to shut the car down to get the pulsing to kick back on.

You can go for time-tested hobby-style 555 timers or go for fancy schmancy and use a cute little PIC processor.