Note that this thread might be totally useless, as on the "DI/APC to T5 Conversion to T5Suite" thread Mike D states that he can get the DI/APC trigger wheel to work with T5. If that's the case, that will be a much better solution.
Anyways, there is so much in common between the LH2.4.2 / DI/APC and the T5.2 system that it seems like it may be possible to do without replacing the wire harness (though some modification will be necessary). By hacking apart an APC box and an LH2.4.2 box and getting a T5 ECU plug, you can build a "mini-harness" that will allow you to use the T5.2 ECU in your car.
A good majority of the pins match up one-to-one (function-wise) between the two systems, but there are some differences that I need to have clarified. Note that my car has ETS.
- On the LH box, there is a single output for the fuel injectors. It passes through a junction that splits it to four separate leads that then connect to the individual injectors. The T5 box, on the other hand, has four separate outputs for the injectors. Does the APC box drive each injector individually with its own unique signal, or is just junctioned internally? I guess I can answer that myself if I took my 5.2 box apart, but considering how hard they are to find, I don't want to risk it unless I have to (though the fact that on the T5 all of the injector wires are the same color strongly suggests that they are part of the same output). If the T5 does use unique signals for each injector, I guess I can utilize the unused leads for the MAF.
- The purge canister on the LH box is driven by a single wire that completes the ground in the circuit (the other side connects through a junction which in turn goes through the main relay). The T5 box has two pins labeled "Signal from purge valve" and "Signal to purge valve" (21 and 27, respectively). Note that they are both yellow/red wires, and in the diagram appear to be connected at a junction before reaching the canister (connecting them electrically?). Would I be wrong in assuming I just use pin 27 matched to the equivalent pin on the LH box?
- Obviously I don't have a MAP sensor, so that will have to be added. I'm guessing the sensor on my delivery pipe is my IAT sensor, but I don't see anything on the LH or APC wiring diagrams indicating an IAT input. There is one pin on the LH box (23) labeled "EGR, temperature sensor," but I don't think that's it. I guess I'm going to have to do some testing with a multimeter to find out what's going on, but if anyone knows, I'd be happy to know.
- Are the O2 sensors compatible between the two? Parts listed online seem to indicate this is the case, but I just want to make sure. The wiring info for the LH box says it operates in a 0-1v range, but the T5 specs show 0-1.5V, leading me to think there may be a difference. Additionally, The on the LH box the O2 preheating is controlled through a relay under the dash whereas on the T5 it's controlled through the ECU itself. Probably going to have to do some splicing here.
There's some differences with the DI cassette plug, namely the addition of the wire for ionization current knock sensing. It'd probably be best to splice the plug I have on my spare T5.5 harness and use the existing lead for knock detection on the APC box.
Additionally, are there any specification changes between the years for different parts that I may have overlooked (coolant temp sensor, etc.)?
Additionally, if anyone has a spare LH2.4.2 box (or 2.4 if the plugs are the same)- good or bad, it doesn't matter, or a T5 plug with about a foot of wire coming off, I'd be happy to take them off of your hands if you want to sell them.
Thanks for your help; I'll update the progress as it comes along... which will probably be slowly.