Mass Air Flow sensor

Pierburg Mass Air Flow sensor

The Pierburg MAF, VW number 074 906 461, has 6 pins. The wire colors and the connections mentioned to the ECU are from a 66kW AGR and a 81kW AHF engine.

Looking through the MAF

The output voltage is proportional to the air flow through the device as can be seen in the curve below.

Bosch Mass Air Flow sensor

VW uses several Bosch MAFs for its TDIs. Bosch number 0 281 002 216, VW number 028 906 461 and also Bosch number 0 280 217 121, VW number 06A 906 461. As far as I know these MAFs are equal. The drawn transfer curve drawn is from a Bosch 0 280 218 018 MAF. I'm pretty sure this curve is equal to the curve of the previous mentioned MAFs. The wire colors and the connections mentioned to the ECU are from a 66kW ALH engine.

A Bosch MAF has 5 pins:

Pin	Description	Wire color	Connected to
1	Temperature sensor output, not used in ALH engine	-	-
2	12V supply input	Yellow/black	relay 109
3	Ground	Brown	pin 4 of the ECU
4	5V reference input voltage	Red/green	pin 50 of the ECU
5	Output voltage	Brown/blue	pin 52 of the ECU

Converting mg/stroke to kg/hour

When reading data from the ECU, the mass air flow is not reported in kg per hour but in mg per stroke. Converting one value to the other is quite simple:

At 900 RPM a 4 cilinder 4 stroke engine makes 900 RPM * 2 strokes/revolution * 60 minutes in one hour = 108,000 strokes. So if the ECU reads 480 mg/stroke at idle, it equals to 108,000 * 480e-6 = 52 kg/hour air flow.

480 mg/stroke is the value you should read with a 1.9L engine during idle when the EGR is closed. One full cilinder filling is 0.48 liter. Air is about 1290 mg per liter. So one could calculate 0.48 * 1290 = 619 mg/stroke. However, the cilinder is not fully filled as it has to suck in the air in a limited amount of time. Therefore the value is a little lower. (If the turbo would be really working at idle, the air flow could be higher than 619 mg/stroke.)

The output voltage of a Pierburg MAF during idle and closed EGR is about 1.6V. A Bosch MAF reads about 2.2V in this situation. With a closed EGR you should measure about 480mg/stroke. With a 78% opened EGR you should read about 290 mg/stroke. If the values you find are much lower, something is blocking the air flow or the MAF has gone bad. The air flow can be blocked by a clogged air filter, a clogged intercooler, a (partly) closed anti shudder valve, a clogged intake manifold or bad valves. The intake manifold may clog due to soot/oil as exhaust fumes are fed back into the engine by the EGR and oil is fed into the engine by the CCV. It's also possible that the EGR is stuck open.

Below is a curve of the mass air flow of a 1.9L TDI industrial engine (AFD).

Page Last updated: 11/14/2002