OBD2 & MSnS-E playing nice together
 

Hi guys-

I'm plotting and planning my MSnS-E / LC1 install on my 96M next spring (spent all this year's money on the car itself!).

Is there someplace where I can read more about the factory ECU and what all it does in OBD2?

Here's what I'm thinkin:

Leave factory ECU in place, disconnect injectors and spark triggers. Splice in coolant temp, TPS (I have a variable TPS, right?), and the cam/crank sensors. Question #1, will having the factory ECU connected to the temp sensor skew the readings, or is the internal resistance small enough not to be a factor?

Run a new wire or two for the air temp sensor; while I'm at it, run wiring for boost control and water injection.

So far so good, the factory ECU doesn't know that it's not calling the shots anymore; but it still can control idle, ASSUMING that the idle control is not done via fuel injectors, but rather an air valve. Question #2, is that right?

Then install the LC1 - will the OEM bung be okay in an NA app? I know it's not ideal, but I don't much like the idea of spending a lot of time on the NA exhaust.

Okay, all of that I could figure out on my own... here's the biggie: What about the AFM? I want that thing outta there, but I don't want to throw a code. Any ideas? I was halfway thinking of somehow convincing MS to output a pretend MAF signal based on MAP. I don't know how easy that is to do in practice, though.

Are you trying to keep the factory ecu for emmisions inspection reasons? If so having anything disconnected will throw a fault code.

OBD2 generally does a lot of stuff, like the charcoal canister, fuel vapor leak testing, blah blah blah. I'd like to preserve as much of that functionality (as well as OEM idle control) as possible.

*IF* disconnecting the AFM throws a code and a CEL, I can live with that (prob wire in a switch for the CEL) but I don't want it to go into some kind of oddball limp mode where it fails to drive the fuel pump or idle control or tach or other oddness.... hence wondering where I can find out what the OEM ECU actually does.

Similarly, if I can trick the ECU into thinking the AFM is still there - AND getting reasonable airflow data (which is why I can't just plug the brain of the AFM into the plug and leave it there), then I'm good to go, no codes, and could probably pass smog without doing any reversion to stock.

Leave the AFM wired in, but not connected to your intake. I'm in the same boat as you, I'm almost done with my complete plug n play MSnS for my 97 OBDII. I'm just worried disconnecting the idle valve from the stock ECU with throw a CEL. I want to get all of this sorted out because IRTBs are going on shortly after, and I don't like seeing the CEL on.

Thanks Weasel. What about throwing a code for:
http://www.miata.net/garage/obdii.html

The more that I've been thinking about it, I probably need to just get over the OBD2 codes... I'm going standalone, so why worry about it? Only big deal would be smog; disconnect the wastegate to prevent boost, switch back to OEM ECU, stick the AFM on the front of the turbo, and drive it to complete the drive cycles.

your idea bout having the MS output the fake MAF signal is dead on. I did that with my Tec3 and it worked great. Your best bet is to hook up the MS to your car and just log stock sensors with the stock ECU controlling them. You'll essentially build an "AFM map" that you can use. It's not SUPER critical to get it right but the closer you are the less likely you are to get code.

The Tec3 used its own air, water, and crank sensors. It's not hard to wire in another water temp sender really. You can just drill and tap the thermostat housing.

The stock ECU doesn't use the injectors to control idle.

Matt

Thanks Matt. Do you know the voltage range?

If you live somewhere that doesn't scan for codes during inspection if the CEL is off you can wire the CE light to the battery light so it turns on and proves out like it would if attatched to a fully functioning ECU.

You have a flapper right? Mine was the 01 style. I assume 0-5V derived from a PWM duty cycle table. I have a rough table in my old tec3 archive if you need it.

It's easy enough to drop a voltmeter on the signal wires to the ECU and push the door open.

It's a hot wire.