MS3x Wiring Diagram Confusion
 

Hello all,
I am planning on building up a MS3x as one of the first steps to turbo goodness, but I have run into a bit of confusion. After reviewing the wiring diagrams on trubokitty for the 94-95 NA, it seems as if the tableswitching and data logging inputs will be used for VSS in (X28) and A/C in (X30) respectively. These are both ground based inputs which I will want to use for their intended functions, so its a bit confusing as to why the MS3x would need VSS and A/C to be routed through them. My general plan is to build a small switch panel to control tableswitching, ebc, data logging, and launch control. As a bit of background I am a second year biomedical (bioelectronics specifically) engineering major with an electrical engineering minor so I have a solid grasp of whats needed to do a simple harness job like this. Any help would be greatly appreciated.

The A/C compressor clutch is turned on and off by the ECU. The A/C input wire tells the ECU the A/C button has been pressed, the ecu can then increase idle before engaging the A/C clutch.

Someone else will have to enlighten us on what is lost without VSS. All I can think of is stuff like traction control and boost by gear.

I totally get the need for the A/C input to the ECU, both for idle up and engaging the AC fan. Looking through the hardware manual it seems JS7 on the mainboard works as a spare ground-switching input. It would just need a pullup resistor and appropriate connections made.

As far as I know the speedo and odometer are both driven off the mechanical cable from the trans. It seems odd there would be a need for a VSS sensor in a non-abs NA8. Can anybody confirm or deny the presence of said sensor?

Looks like it must be used for trans control on automatic vehicles and cruise control. If Aidan is to be believed in the first post in this thread, your 1.8 will have vss at the ecu even if it didn't come with cruise or an auto.

If you have to ask why you need vss then you don't need it and you can repurpose that input for whatever you need. It can be used as a permissive for some idle stuff, launch control, traction control and a few other odds and ends. Not a necessity but nice. Do you not have any other spare inputs you can use for logging or tableswitch? I use the nitrous input to trigger logging for example.

Well maybe I will have to do a bit more digging then, maybe look at some more oe shematics and study the stock ecu pinout a little more closely.

I don't need to ask. Would be very useful for boost by gear. How did you get the 12v nitrous input to trigger logging? Realistically I don't need table switching at the moment as getting flex fuel and everything else required for corn is far, far in the future. So I suppose the best use of the inputs would be DL on the Nitrous input, VSS, AC, and Launch control on the three primary ground switching inputs.

So this might end up as a double post, because I replied days ago but a mod has not approved it for some reason. Boost by gear would be great down the line, and the VSS would be perfect for that, so I definitely want to utilize one of the inputs for it. Does the nitrous input simply need to see 12v, as opposed to ground, in order for it to be activated? If so that will be pretty easy to rig up off of switched power and a toggle switch.

Tableswitching will not be used for a few years, so I would be willing to sacrifice it for now. It seems from the MS3 hardware manual that the JS7 pins on the mainboard can be used as a ground switching input, so I could always fuss with that later on down the line when its time for corn. Building a protective circuit with some diodes in the proto area would be pretty simple, so maybe its best just to do it when I build it the first time?

Another thing I am considering is adding the improved knock sensing module right off the bat, and throwing in a Bosch sensor. From all my research the included MS3 knock sensing circuit is pretty sub-par.