TPS Functionality

[Gavin]

After more than a year on the shelf I’m resurrecting my E30 2.7i ITB project.

All the mechanical work in terms of the ITB’s has been completed, now I’m beginning the painful process of installing engine management and tuning.

I have a basic question regarding the functionality of the 3-wire TPS sensor my car is equipped with – and I’m hoping this group can help me out:

I understand that there is no variable 0 – 5v signal output from the factory 3-wire TPS. Just an on signal at the closed throttle position, no signal through the variable throttle position, and an on signal at the WOT position. My question is what does the factory ECU key on to determine fuel addition in each of these conditions?

Closed – ?
Variable – ?
Wide-Open – ?

Without any engine management at this point I can see that with a closed throttle (but tricking the ECU into thinking the throttle is in the variable position) my AFR is good. Closed throttle + closed throttle TPS signal and the car runs lean. Closed throttle + WOT TPS signal and the car bogs-down rich. This makes me think the ECU is triggering on different inputs, or at least triggering differently, at each of these three conditions.

[nando]

closed - it idles 2D idle fuel and timing maps
"open" - changes to 3D part throttle fuel/timing maps
WOT - changes to 2D full throttle fuel/timing maps

[Gavin]

So if I’m interpreting you correctly- At closed and wide-open throttle the ECU references RPM only to meter fuel addition. And at variable throttle positions it’s formulating MAF vs. RPM to meter fuel?? All with modifiers applied for air temp/O2.

[nando]

yep

it runs closed loop with a real-time, short term O2 correction for idle and mid-range. Based on how much correction it has to do to stay on target, it will begin to adjust a long-term fuel trim that applys to all operating conditions.

[Gavin]

Thanks nando, that helps