Fixing e-brake signal

I have a head unit that uses an e-brake signal pattern-- needs to see 12V (ebrake off) after powerup followed by 0V (ebrake on) as a trigger to allow access to setup functions, video, etc. Obviously it uses other triggers in combination with knowing whether the ebrake is on or off, but it uses the +12V to 0V drop to keep folks from bypassing it by wiring to straight ground. All good by me. Trouble is, the factory wiring in the car doesn't provide that +12V to the ebrake wire unless the engine is running. So say I want to go into the garage and tinker with the head unit setup. I have to start the car and carbon-monoxide myself to make the head unit happy, and then I can cut the engine. My idea to fix this and still keep the intended safety behavior is to supply the ebrake wire with +12V switched for the key ON+engine stopped condition, and +12V from the existing source if the engine is running. At first I was thinking of using a relay but was having trouble figuring out what (12V) to tap as a signal that the engine was running. Then I caught a digram using 2 input sources, diode-blocked, to a single action point. Makes sense. That way I can have both 12V sources connected to the ebrake wire that is tested by the head unit. But... one Q. Doesn't that leave me with 24V instead of 12V on that ebrake wire, due to +12V on the switched wire, and +12V from the factory/running wire? How do I step it back down to 12V?

Some more detail... First, no more 4am posts because if I can get 12V to become 24V just by a wiring trick, I need to patent the idea and beat the riches of cold fusion to the punch. But I still don't know quite how to wire this thing! Vehicle is a 1995 Subaru Legacy LSi wagon, LHD with factory-installed security system (no, not the Code Alarm rebadge, the actual they-designed-it kind that you don't see as often now). The e-brake switch (closed when e-brake applied) provides a path to ground. The +12V wire to that switch (which is what the head unit taps into) comes off of the ground side of the brake indicator lamp in the combination meter. The wire is always hot when the key is in the On position. If the engine is running, it measures at +12V. If it is not running, it measures at 1.5V. This is why I have a problem. The head unit, which is an Alpine IVA-D310, needs to see the voltage drop from +12V (or something greater than 1.5V, at least) to zero to know that the e-brake has been applied (applying the e-brake grounds the wire). I need to supply +12V to the wire with the key in the On/IGN position but *without* having to crank the engine. That way I don't have to start the car for the head unit to see that I've applied the e-brake. There are two other paths to ground for the brake indicator lamp. One is a switch in the brake fluid reservoir, so the lamp is dual-purpose "e-brake is on" and "you need more fluid." I want to preserve this functionality. The other path is to the alternator, and it is diode-blocked based on the factory wiring diagrams. I don't fully understand this connection-- I don't get how my alternator works. But when the engine is off and the key is in the on position, this is where most of the +12V goes. If you disconnect the alternator and turn the key to the On position (don't crank the engine), +12V instead of 1.5V magically shows up on the e-brake wire. It seems like the way to make the Alpine work as intended is to supply the e-brake wire & switch with IGN-switched +12V when the engine isn't running, and the factory-designed +12V when it is running. So what's the *right* way to pull it off?

I designed a circuit to fix the problem. Works great, and keeps the safety feature intact-- if the e-brake is not engaged, the DVD picture and setup functions are inaccessible. But I don't have to run the engine to get into them, the ACC position will do the trick. Cheers!

this is alpine you over thought this way to much get your self a pac tr-7 it will bypass that stupid block everytime you turn the radio on. or do this Hook the possitive input wire or yellow black to the possitive when pressed brake wire of your car. then get a small toggle switch connect one side of toggle to ground and the other side to the yellow blue of radio. now when you turn the radio on. car off or on does not matter. press and hold brake flip switch on-off-on-off-on. let go of brake, all blocks are disabled untill the next time you shut the radio off and turn it back on. but if you use the pac tr-7 it will bypass the block everytime you turn the radio on by itself. Jon

Installer/Help Technician

---coral springs florida---

mecp certification is not always needed. I have it and it has not helped me out at all. my experience out shines it.

I avoided the toggle switch approach purposely; I wanted to keep the block (annoying as it is) for some very specific reasons, but not have to crank the engine. But being a noob, wish I had heard about that TR-7 earlier. Neat product. May come in handy for some other work I have in mind. Thanks for the tip!