Mass Air Flow converters
On a rainy Monday, when you're late for work, suddenly your TDI looses power. I turns out that your MAF has gone bad. You can get another MAF for a small amount of money, but the MAF you need is a Pierburg and the MAF you can get is a Bosch. And when you needed a Bosch, you could only get a Pierburg for a fair price. But you remembered a post on the TDICLUB web page or your search engine found the web pages of TECH4TDI. So here you are.
Below you'll find the schematics of two converters, together with data from the MAF manufacturer and measurement data of the converter. But first some global information:
No IC's, large temperature range
The first thing you may notice looking at the circuits is that it seems quite simple. Just a few resistors, transistors and one or two capacitors. No IC's. You could make the same circuit with IC's, but it would be more expensive, less robust and no better performance. But most of all: the temperature range of normal IC's is too small. The circuits below are fully temperature compensated and can handle a temperature range of -50deg.C. to +125deg.C. without any problem. IC's for this range would be expensive as they have to have military specifications.
Fault tolerant
The circuits are designed to handle all kind of fault conditions. Shorting the inputs or outputs to any voltage between 0 and 12V will not damage the circuits. The supply voltage is 5V, but if you would apply 12V, nothing will go bad in these circuits. Although the circuits here are fault tolerant, I do not know if the MAFs or ECUs are. The ECU can handle voltages between 0 and 5V for sure, but I'm not sure it will not be damaged by voltages above that.
Accurate
The conversion is quite accurate as you can see in the measurements below. It's best to use 2% resistors. You can use 5% resistors, but you may want to check the actual resistance value. Normally 5% resistors are within a few percent, so good enough.
Low cost components
The components used are all very low cost. 0.25W resistors and standard small signal transistors. You can find these at any electronics shop at a cost of something like one dollar in total for the parts. I used BC547/BC557 transistors, but many small signal transistor will do. (2N2222/2N2907 is okay too) Don't waste your time making a beautiful printed circuit board. Just use an experimental PCB. That really is good enough. The circuits are not critical on PCB layout at all.
Casing
For the casing it is important that it is water tight. If not, the circuit will corrode and will go bad in no time. The circuits are so small however that it will fit in a 35mm film container. Just punch small holes in it for the wires and seal it with silicon. The container can be placed next to the MAF.
Intellectual property
You are free to use the information presented here. I do not ask any money for it, but also do not accept any liability. You are not allowed to sell the schematics or printed circuit boards with these converters with a profit. If you find a circuit valuable and are going to use it, I would be happy to hear your experience with it.
Page content:
Using a Pierburg Mass Air Flow sensor
with an ECU programmed for a Bosch MAF
The schematic is shown here. It basically is an attenuator that also does some level shifting and curve shaping. In the interesting air flow range from idle to full power (50kg/hour to 400kg/hour) the conversion is very accurate. This circuit has been tested for quite some time in a 81kW TDI. It work beautifully. No error codes in the ECU (of course not, the ECU doesn't even see the difference between a Bosch and a Pierburg MAF. So why should it complain when it does get the right signals.)
Using a Bosch Mass Air Flow sensor
with an ECU programmed for a Pierburg MAF
The schematic is shown here.
It basically is a low gain amplifier that also does some level shifting. In the
interesting air flow range from idle to full power (50kg/hour to 400kg/hour) the
conversion is very accurate. This circuit was tested, but only on the lab table.
As the conversion is very good too, there's no reason to suspect that this
circuit will not work.
Note added Jan 08, 2006:
Somebody who tried the circuit, reported good results. Only the top speed was
limited from over 200km/hr to 187km/hr (81kW AFN engine). This is due to the
fact that the circuit starts to deviate at very high airflows. To compensate for
this you can increase the value of R3 (3.9 kohm) a little. This will however
result in a bit less accurate transfer at low airflows. This might result in a
little more smoke and a little higher fuel consumption. (and maybe a little more
power at low RPMs)
Page Last updated: 01/08/2006
