Patching into power in the engine bay

[Nepharius]

As part of my meth install I need to tap into a 12V source that is only on when the ignition is on. I figured I could tap into a fuse in the engine bay but I have no idea which of them is ignition only. Is there anything in the engine bay or will I have to go through the firewall to something like the cigarette lighter fuse in the passenger footwell?

[Doug_MPS6]

Stay away from using the lighter as a tap-in point. Everyone does it but a) it's a pain to access, and b) it's a lightweight connection and repeated tap-ins can overload it and the attendant wiring and c) cigarette lighter plugs are crappy devices and often short circuit - even if you don't use one, say, as a non-smoker, if the plug is in place, some wally will depress it one day and if it is dodgy it can short. I keep mine out altogther.

Set up a new insulated and fused multiple point power-board block under the bonnet or on the interior firewall (much less desirable due to restricted access and considerations of bringing heavy current cabling inside cabin), that you can then use for multiple hot points. Pull power off the main fusebox next to the battery. A test light will define the take-off point for you that is only on when the ignition is on. If in doubt about your skills get an auto-sparky to set it up.

Make a grommeted access port through the firewall to bring power inside the cabin. Take precautions against chafe and fuse everything - not to protect your new component but to stop a short from frying the entire wiring system to which it is connected. Best side to do this is the LHS, in front of the glovebox, approx 2" (50mm) to LHS of the two heater pipes that pass through the firewall which you can see from both inside the cabin behind the glovebox, and from under the bonnet behind the battery box, and use as a reference point. Drill at that 2" distance from and at the level of the half way point between the two pipes. There is some latitude to this but not a lot, say +/- 1cm for comfortable access. The interior fibreglass and rubber-backed insulation is thick there and will wind up around your drill bit - it's too thick to use a chassis punch on. The rubber is sticky stuff when it gets warm on the bit. Start with a small 1/8" hole and progressively enlarge it to about 5/8" or 7/8" for which you can get standard grommets. It helps to remove the glove box to enable access and identify your drill site, and remove the battery so you can hold a (compact) drill straight.

Hope this helps.

[Nepharius]

I could have sworn I replied to this...... I'm not really after this 12V ACC for power for the meth kit, but to activate a relay. If I understand the way it works the principle is to prevent the pump activating by some freak accident while the car isn't even on and filling the intake with water. What I really want is a fuse in the engine bay or something like that I can run a wire off. Something a big like this Wire Tap Blade Fuse 10A - Jaycar Electronics ensuring the tap is on the opposite side of the fuse to the power source. If I can find where I put my multimeter I might do a fuse hunt.

I learnt my lesson about the importance of fuses in cars a while ago, I directly shorted the battery in the old celica, the metal heatshield that did the shorting basically disintegrated and shot sparks everywhere, fun fun.

[Ranton]

you will find there is a empty fuse holder somewhere in the fuse box which you can tap switched 12Volt power off either that or separate the fuse box and find the one that has switch 12 Volt (probably one of the wires going to one of the relays) and tap the switched power off that.. after all you would only be activating two relays then!)

but check the empty fuse bays first some have pre exisiting contacts in them

[rob82]

Why wouldn't you use a fuel pump feed? That way the device can only turn on when the fuel pump runs.

[Ranton]

id be more inclined to use the injector feed

so it only runs when the injectors are..

Fuel pump primes just when you put your key in...

[rob82]

The MPS's use high voltage current controlled drivers to operate the direct injectors. DO NOT USE THE INJECTORS TO FEED ANY DEVICE as you will induce extra current loading into the injector drive circuit and most likely blow up any relay or device as the flyback voltages will be huge - like 100-200 Volts or so.

You will also find that most cars injector feeds are ignition fed anyway so the fuel pump relay would be my recommendation in most circumstances.

[Ranton]

The MPS's use high voltage current controlled drivers to operate the direct injectors. DO NOT USE THE INJECTORS TO FEED ANY DEVICE as you will induce extra current loading into the injector drive circuit and most likely blow up any relay or device as the flyback voltages will be huge - like 100-200 Volts or so.

You will also find that most cars injector feeds are ignition fed anyway so the fuel pump relay would be my recommendation in most circumstances.

If you open the fuse box rob there is a relay called "injectors" im pretty sure that same 12Volt Switched relay doesnt carry 200 volts...

Im talking about the switched 12V line being a means to switch another relay with its own deidicated power feed from the constant 12V of the battery

[rob82]

If you open the fuse box rob there is a relay called "injectors" im pretty sure that same 12Volt Switched relay doesnt carry 200 volts...

Im talking about the switched 12V line being a means to switch another relay with its own deidicated power feed from the constant 12V of the battery

Really - thats interesting. I'm 99% sure that both wires that go to the injectors go directly back to the ECU. I wonder if the relay is a direct feed for to the ecu for injector control? The voltage spikes I was refering to was the "flyback voltage"(ie after the injector has fired). The injectors from memory operate at about 70 volts.

I think you will find that the injector relay will be an igniton feed anyway - so I would still use the fuel pump feed.

[Doc]

Maybe ive misread this..

why not run it straight off the battery with a relay tapped off the fuse box behind the glove box.. that is what i have done

Doc