Here are some pictures of my Subaru Liberty's (first series) AFM (air flow meter) which I removed from the car in order to troubleshoot what i perceive as a minor loss in performance at low rpms.
After removing the rubber and breaking small bits of the case (nothing major) a nice copper screen is found.
After the screen is gone the somewhat basic circuitry is not much to look at. It looks like the green transistor functions as a heater. The ceramic (?) main pcb has a couple of diodes which might be used to keep the temperature constant. The fifth pin, not connected internally is 5V on the harness.
After desoldering the top board I found that this is about as much as this afm is going to be taken apart. The board is glued to the backing heatsink. Anyway, there is not much else to look at. The 2 screws hold the inner thermistor assambly. It could be removed if needed.
I have not found out much during this exercise. The AFM uses unregulated 12V , but there is a regulator in the IC so that does not matter. The meter works, but presumably the printed film resistors or the thermistors are aging. I measured the thermistors as 530 Ohms for the small one and 20 for the larger, rectangular one. They are both PTC. (room temp 22C). With a hair drier they changed to 586 and 22.5.
My array of tests were mostly for fun. The AFM could be used to measure wind speed, but sadly I cannot think of a use for it right now (or ever). It's response time is good moving to higher amounts, and fairly bad from high to low (1 second). Output ranges from 0.2 approx at zero to 3V at my hairdryer's output (probably higher). The hairdryer was surprisingly repeatable with 1% error if placed in exactly same position.
Some error seems to be introduced by air temperature, but it was very small. No error is introduced by power supply voltage.
Since the grounds are connected together there might be ground loops present. I will look at the car wiring diagram in hope ground can be improved.
In conclusion if the afm calibration is off due to aging no used afm will be immune to it. Even new "old stock" units might have this problem. But without any reference there is no way to measure this item's calibration.
