Hi all,
I'm trying to solve a bit of a puzzle regarding the RS4 maf, and maybe someone here might be able to provide some insight.
TLDR: Is this sensor still in production, and where can I get a genuine bosch one? The 0 280 218 08P which supersedes this maf seems to be incompatible.
Some background:
My car uses a HFM2 sensor in stock form, and those have been unavailable for some time so I've been working on converting to HFM5.
The standard way of doing it is to hook up the sensor using a stable 5v feed (as close as possible to AREF used by the MCU) and to zero out the values below 1v as these are nagative airflow (backflow) with HFM5 (my platform does not support negative air mass values).
The rest of the table is populated with the stock table that came from a bin of a car which uses that specific maf package (sensor + housing size combo).
So far so good.
I want to use the RS4 maf for it's increased measuring range. My platform uses a 8 bit adc value in stead of 10 bit for the maf value lookup, so resolutions already suffers. By compressing the whole measuring range into 4v, it suffers even more. Therefore I created a conversion circuit which subtracts precisely 1v from the maf output signal and scales it with a factor of 5/4.
I modified the RS4 tables to suit, and tested the circuit (works perfectly).
So far so good, but unbeknownst to me the hard part is actually finding the correct maf sensor.
The part number that is usually quoted here for the RS4 maf is Bosch part no. 0 280 218 067, using the HFM-5-9.7 sensor.
This is unavailable on any of the bigger parts site in europe I have looked.
According to MS4X it has been superseded with 0 280 218 08*, where * can be P(aftermarket), X (VAG), or K (AUDI).
The only version I was able to find listed anywhere is the 0 280 218 08P, the aftermarket version, which makes use of the HFM7-R5 sensor.
The HFM7-R5 is the analog interface version of HFM7 (as opposed to the "digital" version of HFM7 which outputs a frequency signal).
Upon buying and testing this maf (0 280 218 08P), I found that its output is very different than that of the (0 280 218 067).
At very similar (steady state 100km/h) driving conditions, this maf measures 140 kg/h, while the old setup measures (86 kg/h).
I also get a DTC with air mass signal faulty. I have traced the asm for this dtc, and logged the relevant parameters, which confirmed that the maf is reading way to high.
I double checked the circuit, and also tried without it (with appropriate maf table), but the result is the same.
So the transfer function for this maf is not compatible with the RS4 maf.
The hunt is on for the correct maf, so I have scoured the web for information on this, and as usual the Bosch stuff is shrouded in mystery.
I have now ordered a maf from a 01-05 BMW 745i, Li with Bosch part no 0 280 218 077
It is listed as having the HFM-5-9.7 sensor, which I'm hoping will actually be the correct sensor.
I will be using it in the 0 280 218 08P housing, as it has the appropriate size (83mm ID same as the RS4 maf).
Does anyone have any insight if this would be the same sensor as in the RS4 maf assembly?
If this sensor does match, I intend to flow bench the others to compare the transfer functions.
Does anyone have more information on these sensors? Are all HFM5-9.7 the same?
Is there actually a better way to go regarding maf sensors?
These PMAS sensors seem interesting as well:
https://pmas-maf.com/product/mh95b-1800-95mm-housing-w-tuned-maf-1800kg-hr-black-universal/I'm not sure if they are any good though, and I would rather have something OEM.
