How to scale the HPFP pressure sensor

[djuosnteisn]

Okay, so there's no way to just solder in a diode or resistor to drop the reported hpfp pressure sensor voltage, because there is a pull up resistor on the ecu side, and the sensor is effectively "sinking" the current. All a diode will do is stop all current flow, and make the ecu see 5V and lean the hell out of the fueling. And if you reverse the direction of the diode, it'll just do the exact opposite of what your trying to achieve.

Resistors don't work cause all they really do is alter how much current the sensor is sinking.
.
.
.
Long story short, you need to supply a scaled voltage to the ecu, and for this, we use an op-amp.

I noticed that there was more than just a pull up on the ecu side, but also a significant resistance to ground as well, basically a voltage divider. I figured that this could be some sort of fail safe for a failed sensor. It's about 4.6k to the supply and 6.8k to gnd, which would result in a voltage of 2.92V reported to the ecu if the sensor were "disconnected". I figure this was put there for a reason, so i mimiced it in my circuit.



There's not much to the circuit, for those DIY types, just a non-inverting op-amp with a adjustable voltage divider on the input.


The circuit limits the amount of adjustment you have. So you can turn the scaling all the way off, basically running normal. Or you can turn it all the way up, but it won't let you turn it into the danger zone, lol. That's what the "limiting resistor" does.


Here's a plot of the voltage output with the pot set to minimum (no scaling):


You can see that Vout clips at just over 4V. This isn't an issue cause the ecu will never command more than 4V.

Here's a plot of the voltage output with the pot set to maximum (full scaling):


The full scaling is roughly 22% increase across the board.

At maximum scaling Normal Volt Scaled Volt Normal PSI Scaled PSI % increase 1 1.22 450 549 122.00% 1.25 1.52 562.5 684 121.60% 1.5 1.8 675 810 120.00% 1.75 2.14 787.5 963 122.29% 2 2.45 900 1102.5 122.50% 2.25 2.75 1012.5 1237.5 122.22% 2.5 3.06 1125 1377 122.40% 2.75 3.36 1237.5 1512 122.18% 3 3.66 1350 1647 122.00% 3.25 3.98 1462.5 1791 122.46% 3.5 4.27 1575 1921.5 122.00% 3.75 4.58 1687.5 2061 122.13% 4 4.89 1800 2200.5 122.25%

Here's a vid of the circuit working on a bench, with a power supply acting as the sensor input:

showinghowcircuitworksgoodcircuit.flv video by djuosnteisn - Photobucket

showinghowcircuitworksgoodcircuit.flv video by djuosnteisn - Photobucket

This is when i was about to put it on the car:


And it on the car:

mintomidpotwithSTFT.flv video by djuosnteisn - Photobucket

mintomidpotwithSTFT.flv video by djuosnteisn - Photobucket

Here's a vid with minimum setting and an idle injection pulse width:

minwithinjonscope.flv video by djuosnteisn - Photobucket

minwithinjonscope.flv video by djuosnteisn - Photobucket

Here's a vid with maximum setting and an idle injection pulse width, you can see that the injection duration is indeed brought in. I'll do much more testing when i get my upgraded internals in and can actually drive the car without worry .

maxwithinjonscope.flv video by djuosnteisn - Photobucket

maxwithinjonscope.flv video by djuosnteisn - Photobucket

I mentioned in the other thread, if it proves to be safe on my car, and enough people want to do it, but don't want to build their own, i can order some pcb's and parts and stuff.


*disclaimer*: Don't do this to your car unless your willing to assume all responsibility of damage incurred from this "mod". This is thread is not intended to be an instructional "how-to". But... if you do choose to do it, GL and let us know how it works