In search of an ignition wire

**witten** · 01-14-2004, 03:18 AM

I'm looking at using an ITPS for startup/shutdown control (if not for voltage regulation). As pointed out in other threads, the ITPS needs to know when the car turns on and off via a connection to an ignition wire. Some people mention that they're getting an ignition wire from their head unit. I've got a factory Honda head unit, but I'll have an adapter (SNHOND3) plugged into its AUX-in. Is there somewhere else on the head unit I can tap for the ignition signal?

Also, some people talk about getting an ignition wire from the fuse box. I have noted that there's an ACC plug in the fuse box, which would presumably be hot only when the car is on, which is just what I'm looking for -- but it's got a fuse in it! What dire consequences await me if I remove the fuse and plug a wire into the ACC location, and connect the wire to the ITPS? Are there other plugs in the fuse box that I can use instead of the ACC one? Several plugs are not occupied by fuses.

So my basic question is this: What are my general options for getting an ignition wire?

**[H]Bugster** · 01-14-2004, 03:41 AM

great question! I was going to ask the same sometime soon cause im having trouble locating my ignition wire on my 02 Ford Ranger

**craigyb** · 01-14-2004, 04:17 AM

I have had the same problem on my Merc CLK, I was trying to change the stock HU for a new panasonic one with AUX inputs, both connect with standard ISO connectors, but guess what, only a live feed for the 12v, no ACC/IGN wire in the Merc ISO plug.

It was suggested that I took a feed from the cigarette lighter +12V, I tested this and sure enough it's an ACC/IGN +12v feed. I'm running my HU, hands free kit and 7" screen from this line.

As I never let anyone smoke in my car, I should never use the cigarette lighter, so the current draw should be fine. Also I'm removing the cigarette lighter/ashtray section soon to make way for a DVD drive, Keyboard IR receiver and IRman receiver and maybe some USB ports.

Craig

**witten** · 01-14-2004, 04:53 AM

Craig, I didn't realize that a cigarette lighter could be an ACC feed. As far as I know, there are two metal parts on a lighter, one is +12V and the other is ground. Are you saying that the +12V from the lighter is only on when the car is on? Do you know if this is typical for most cars?

**craigyb** · 01-14-2004, 05:04 AM

The +12v feed for cig lighter is quite powerful as it's driving a heating element, the cig lighter itself.

On my Merc is is a switched accessory, you can test it with meter, check the feed with no key and then turn to first position see if you get +12v.

I think most cars work like that, but I can't be sure. I seem to remember my dad lighting cigs when parked up, but I was so young, I didnt know anything about ignition systems at the age of 7.

Craig

**witten** · 01-14-2004, 05:06 AM

Okay, so assuming that the cigarette lighter is only on when the car is on, wouldn't that be somewhat of a problem for an ITPS, or any shutdown controller? As I understand it, the way the ITPS is designed is to take both an unswitched constant +12V line and an ignition line. That way when the ignition line cuts off, it can send a soft power down signal to the computer, while still providing it with the +12V during the computer's shutdown sequence. Then after a few seconds the ITPS can cut the +12V to prevent the computer from draining the car battery.

Now if the cigarette lighter only has power when the car is on, then the ITPS wouldn't be able to give the computer any juice during the shutdown sequence. It would just suddenly lose power when the car turns off. This sort of defeats the purpose of a shutdown controller. So what am I missing? Are you getting your constant +12V power from something other than the cig lighter? Or are you not using a shutdown controller?

**craigyb** · 01-14-2004, 05:30 AM

Doesn't the ITPS require a switched ACC/IGN feed and a constant +12V feed. As I understand it the original question was, where can I get an ACC/IGN source from. I answered that question.

If I understand the ITPS properly (I don't have one and only saw one for the first time 2 days ago) the +12v constant feed comes from the battery via a fuse. So the +12V ACC/IGN feed is used for starting up and shutting down the PC.

Craig

**craigyb** · 01-14-2004, 05:43 AM

Hmm, looked at the ITPS again, not sure without a wiring diagram what it does.

Anyway, the question was where can I get an ACC/IGN feed. The cigarette lighter feed is not powerful enough to drive a DC-DC converter or a high power inverter > 100W.

If you need a switched +12V of greater power > 20A then it really needs to come from the fuse box or ignition switch.

Craig

**neuroplague** · 01-14-2004, 06:55 AM

The accessory line that a shutdown controller requires doesnt need to be powerful, it only needs to be a 12 volt source that is switched from the ignition... you can get this from any source that doesnt work when the key is switched off. On most of the car stereos I have installed the yellow wire is usually the switched line. As for the constant 12 volts I suggest you run a heavy guage wire either 12g or preferably 10g directly from the cars battery (dont forget to put an inline fuse on that) and ground to the cars body .. remember to keep the ground wire as short as possible to reduce interferance.

I hope this helps