I had an ECU for an air seeder blow up, so I'm rebuilding it from scratch using a Pi and other hardware. I got the shaft sensors reading fine, set up the relays and other switches and lights, but this one has stumped me. The markings give me nothing to go off of (Trison 91779847 Rev C gives me nothing useful), but I still have the borked board that I can trace out. The sensor is fed 12V and ground, and has a white wire that is marked for airspeed input on the wiring diagram of the ECU.
When I trace the white wire on this, it looks like it's conditioned by some diodes (two diodes, in opposite directions to ground) and a capacitor, with a pulldown resistor. The trace then goes to an opamp (TLC2272) inverted input via what looks like a voltage divider, and from there maybe feeds into the processor the board uses. Kind of hard to see from that point on.
I scoped it and if there's data there, it's in the 1-2mV range if I reference from ground, which makes the opamp make sense. But I don't have a very good scope where I am, I do have a Rigol at home that I could get.
I have built a very crude estimate of the airspeed just from the current draw of the hotwire using an ADC and shunt resistors, but I assume I can get a better reading from this sense wire if I can figure out how the hell it's working, via a reference circuit to help me understand. If you know this sensor or a similiar sensor that I can crib the circuit from, that would be awesome.
This is more of a personal curiosity rather than helping you.
From what I can make sense, the way this thing would work is by heating up an element. The air flow will cool the element, and the ECM measures the current needed to keep the element at a pre-set temperature.
Would that be correct?
If so, I guess it would also have temperature and humidity sensors as well? Basically measuring wind chill and working backwards.
That's my impression, but if all you have to do is measure the current draw, then you don't need the third wire. So I've built a basic ADC circuit to do that, but it's pretty crude.
In the original monitor, it was quite precise, and so I traced the circuitry as above off that other wire. I don't know enough about how all that works to replicate it and I can't seem to find much about similar sensors to learn more.
If you look up dtc for mass air flow sensors you can see from their failure modes what the ecm input is supposed to be P0102 and P0103
Sorry, typically from what I remember. The third wire is typically the signal wire. You get the 12v in then ground and the 3rd wire the ecm will read the signal via voltage drop usually.
Typically it's 5v though and not 12v.