A few pre-installation Qs:



The factory ECU gets its power from a 7.5 amp normal fuse in the fuse box, which I do not think supplies any other device with power (I will go through the stock wiring diagrams again to confirm but I'm pretty sure that's the case). If I replace this with the 3 amp fuse, can I use the power lead available on the front harness or should I deliberately avoid doing so and go with the haltech fuse box?



Likewise, the injectors and sensors get their power from the main relay which then sends +12V through the front harness to the ECU area (FEM-02). Can I draw my power for these items from this location on the front harness or is that also not recommended?



Doing these power lines this way will allow a complete integration into the stock system without having to add a fuse box and run completely new power lines ACROSS the engine bay. I really don't want to have to take out the dash again to run power neatly unless I absolutely have to, and I won't run power lines across the engine bay unless I can't get them into the fuse box area conveniently to run them behind the dash.



Final Q: is there a problem with using the stock wiring to the ignitors or do you strongly recommend I run my own wires there? I've figured out exactly where the stock wires terminate and its (obviously) right at the ECU location on the "front" (dash-side) harness which doesn't get replaced (connector F-31, along with convenient ECU power and switched power). I might as well use these unless there's a good reason not to.



I'd appreciate some comments because I don't really want to have to run more wire than necessary.. and the front harness isn't usually brittle, in fact mine seems perfectly fine, so if it would all work as the diagrams seem to indicate... I'd like to try it. Comments?

 

thats basically how we do it. we use the connector under the dash for water temp (dont forget it!), injector power (black w/ yellow), emissions or bac valve or any other switched 12v stuff, ecu power (switched), and something else? ah fuel pump, i'm actually using the haltech harness for that on mine



the 3 ignition outputs we go into the ecu plug that goes to the dash/front harness



and road speed goes way over to cruise control.



i actually just did an s5 t2 where we pulled out the stock ecu and harness and we made the haltech harness plug into the car

 

I have an aftermarket water temp sensor and my stock one is busted anyway (hence the aftermarket) so that's irrelevant for me, but worth noting for others for sure.



Its good to know that others have thought of this and done it successfully. It seems like an "elegant" integration to me, although I was cautioned by one person to run power separately simply for the robustness of the new and more significantly sized power lines. I don't necessarily disagree.



Is road speed important at all? I yanked my cruise box entirely... what does it allow the Haltech to do that it wouldn't without such an input?



I rewired my walbro fuel pump so I figure I'll probably just cut out the resistor pack I so carefully installed in my trunk, and just use the factory wiring as the switch to the relay I've already installed. That won't make the car run pig rich at idle with the haltech controlling it will it? I know I *could* use the "alternate fuel pump" output to switch that resistor pack easily, but nobody seems to have done it so I dunno if its pointless.



BTW if its useful to anybody, the AMP part number for that ECU Plug (Number 1, which is Front Harness connector F-31) is 174515-7 (or the functionally equivalent 917989-7). Note that this part number doesn't include the pins.

 

yes in the fc, its silly to add the extra fuse box when the cars already got all that stuff wired up! plus this way the fuses still do what they say they do. i'm doing a 510 soon and since that doesnt have any efi wiring/relays i'll use the full harness.



the road speed just shows up in the datalogs, ive never actually used it myself. it doesnt need the cruise control box to be in to work, its one of the inputs to said box.



you dont need the resistor, it wont run rich, thats why you are buying an ecu



water temp sensors up to you, id be tempted to run the wire anyways, you may change your mind, i have.