Why must car start in P or N and not D.
 

Why are cars designed so they can't start in drive? Is it just a safety thing so you don't start moving right away, or is there a more mechanical reason?

What I'd like to implement it an off button for use while at red lights. Something that cuts the engine when pressed, and starts it back up when released. Building such a device would be easy, I just want to make sure it isn't going to do any damage.

So one doesn't accidentally run over someone or some thing. Generally though, you don't want to start the car in drive since your starter will be both turning over the engine, TC and trans. Your starter might have enough grunt to do it for the rest of its life or you could kill it off quick.

For the record I don't think this is a wise mod and you shouldn't do it. But all you'd have to do is ground out the NSS usually. That way the trans computer thinks its always in neutral or park.

I'm guessing it's because in drive there would be an extra load on the starter of trying to move the Torque Converter. In P or N that load isn't there. At least I don't think it is.

That makes sense. The alternative would be to place the button on the shifter, so I can push it into N, hold the button to keep the engine off. When the light changes I let go of the button, and pull it into drive.

well, you'd have to crank the starter over to get moving again though. So the button would be a kill switch, then you'd have to turn the key to start it again

so yeah. this is what I do anyways. drop in N, kill it. Light goes green, fire it up, drop in D and go. Only problems I'm having are the key/brake/shifter links don't like this much, because it won't give me the key back sometimes. I believe its just a solenoid problem, and starting it again, put it in gear, then park, all while holding the brake, key off, release brake, the solenoid kicks off, key removed. Annoying? yes. A deal breaker? no. Worth building a secondary circuit for on-off work? Not even close.

I've bypassed the neutral safety switch in a 1976 Chevy van for over 20 years and not had a starter failure. My ignition switch went bad, so rather than replace it, I just wired in a relay and a push button switch (I was poor at the time) and its worked fine ever since (my dad still owns the van). And I've often started it in gear. It is a little weird to start moving as soon as the engine starts though. At cranking speed there is very little resistance from the torque converter, so it doesn't really add much to the starter's load. The reason manufacturers don't let you start in gear is strictly liability, remember Audi's "unintended acceleration"? That wasn't the same issue, probably a pedal placement problem, but the point is the same. Imagine what the average driver with a cup of coffee and a cell phone would do if the car started moving as soon as the key was turned and you see the problem. If I was going to wire something like this up, I would probably use a three position toggle switch mounted within easy reach with the center position normal, one direction kills the ignition / injectors, and the other direction to the starter. It would also allow easier EOC if you're so inclined, which saves a lot more gas, according to the people who do such things.

In my application, I'd be worried about transmission wear.

Reason being: I would likely be tempted to kill the engine while in gear and just key-on to restart (providing the speeds allow the TC to remain locked). Otherwise, I'm trying to see the advantage...

How would this react without rev-matching from Neutral?

RH77

This is intended for use at stops only, I don't do EOC in my car. As soon as I let up of the break, the vacuum assist is gone.

The two way toggle is a great idea. I'd prolly mount it in the center console near the gear indicator. Now all I have to do is wrap my head around the poor "schematics" in my haynes manual. I may go to the library and look at the Mitchel1 book, since those schematics are properly drawn.

We have an 89 dodge spirit (same as your car but a 2.5 turbo) and someone hardwired the neutral start switch. You can start it in any gear, but if you do it in drive it doesnt really move while cranking, but after it starts it starts rolling forward a bit.