Understanding amperage draw on existing circuit

[Vettett15]

I am trying to understand how this older (~late 80's) vehicle sensor works in more detail. 



It consists of a Air Flow Meter shown above... The copper arm rotates on a shaft connected to a flap, as the vehicle pulls in more air the arm rotates and moves along the voltage divider network.  All of the wires from this sensor go to the car's ECU.  What's interesting about this is the resistance across pins 7 and 26 aren't linear as you rotate but the voltage output is.


I was also interested in the current draw on the +5V source and output voltage wires.  So i utilized a current measuring tool and took measurements on two cars:




What I thought was interesting here is I expected the output signal (pin 7) to have very little current on it as I assumed it was going straight to the ECU's ADC.  The other odd thing is the large measurement difference here, used the same gauge, cars have the same ECU and AFM part #'s. 

I decided to dig in and see what i could find on the ECU, picture below is the car's wiring diagram (left side) vs the Motronic 1.3 schematic.  Note that I believe on the schematic they have the two names flipped... Reason I believe this to be correct are two fold.  1.  The pin #'s are correct otherwise and 2.  I assume since the temp sensor (pin 44) is a NTC thermistor it would have a voltage divider as you see on the pin 44.



I'm not seeing anything on here that sticks out on why i'm seeing so much current on this voltage output (pin 7).  Any ideas?

[coromonadalix]

have you measured the ohms value between pins 3 and 4 ?   surely if you say its not logarithmic

say 30 ohms value

5v divided by 30 ohms should give around 166 ma total

the car 1 seems fine

have you measured the ntc value at air temp ?  ex : 23degree = x value ?
have you heated it or frozen it to see if its a positive or negative ntc ?


pine 44 and 26  what is the supply voltage  ... the lowest you measured and the highest ??  to says its around 40 ma?

[Vettett15]

Sorry I meant to put some different pictures in.

The resistance measurements don't seem to back up the higher current I'm seeing on the cars.

To be honest I haven't played around much with taking measurements on the thermistor, don't really care about that one.

This sensor takes in 5v from the ecu, between pins 12 and 26.  Depending on where the wiper is it outputs between .25v and 4.7v between pins 7 and 26.

I took some measurements in my lab /office.  Note that this was done with a different multimeter than the one for the real cars. 

Here i just used a 5v wall plug in converter and interface that to the sensor.  I need to redo the current measurements below, think my multimeter is fooling me for those.

[Vettett15]

Pictures I mention above

[floobydust]

I think your current measurements are out to lunch, I hope you are not using the multimeter on millamps with (-) lead to ground. Measuring current needs to be done with the wire connection broken and multimeter in series. Such a hassle it's not preferred, as voltage can tell you most of what you need to know to troubleshoot. Your multimeter might not be working.

Moving-vane MAF sensors are really nonlinear (airflow verses angle), I faintly recall exponential to 4th power so the potentiometer is made non-linear to do correction, and giving the A/D more usable span.
Nippon Denso (in Toyota) in the 80's made a custom analog IC to do the calculation for the ECU's 8-bit processor.

[Vettett15]

Yes I had the meter in series during those measurements.  You're right about the non linearity, this device is linear with respect to angle vs output voltage, I've verified that on stock units. 

I just don't understand with the resistance  measurementsì I've made I don't understand the high current measurements.  Note that for the on car measurements in the first post I actually used a clamp on style milli amp reader so there was no need for the meter to be in series.  Perhaps I need to back those measurements up with another meter to verify.

[floobydust]

Clamp on DC ammeters are vulnerable to stray magnetic fields, including the earth's. You have to zero the meter.
I had one that read silly all the time and it was due to a magnetized jaw. I had to degauss it and then that fixed it.

Your multimeter, maybe try measure current on some small known loads and see if it works ok.
If a reading does not make sense, it can lead you on a goose chase so I back up and make sure the multimeter is working.

The vane MAF sensor should have a calibration screw on the body, I've ahem turned those to get better performance 

[Vettett15]

Agreed I'll go and do those measurements.

On another note, what is the purpose of these resistors and capacitors on the front end of the adc?  Filtering?  Unfortunately the datasheet doesn't give values so I'll need to take one of the ecus apart and measure the component values.

[Vettett15]

Ok finally got a chance to do some investigating.

1.  Used an inline meter to make amperage measurements.  Current is 1mA or less.  Previously was designing under a bad assumption.

2.  Took measurements of the components on the ecu side and made some high level schematics of the oem sensor and my design. 

So with the much lower output current requirement and the details of the capacitive load I'm trying to determine the best path forward.   I believe I need an op amp that can drive that 38nf cap, am I going down the wrong road?