1991 BMW 850i

page 9

  1. MAF testing
  2. SMOG results

The BMW 850 uses 2 x Hot-wire air-mass meters (MAF) to measure the air mass entering each bank of 6 cylinders. They are manufactured by Bosch and can significantly affect engine performance.

They work by suspending a heated wire in the intake air flow (see pic below taken with the protective wire gauze filters removed). The wire’s resistance changes with temperature, so to maintain the temperature of the wire as the air flow increases, more current is fed into the wire. The output signal from the MAF is essentially an indication of the current flowing into the hot wire. The MAF is compensated for temperature by a thermistor also mounted in the airflow (sticking up in front of the heated wire in pic below) but the MAF itself does not provide an indication of intake air temperature (a separate air temperature sensor mounted in the rear of each intake manifold does that).

To clean a MAF, remove the gauze filters and spray the suspended wire with MAF cleaner. Expect to see the engine side of the wire clean and shiny as below but the intake side may be brown and dull. Blow out any dust particles on the inside faces of the MAF.

Because the hot wire may become contaminated over time with particles of dirt or oil that might affect its accuracy, these are removed in a burn-off cycle that occurs each time the engine is shut down. A logic high signal applied to the MAF (Pin 1) for approximately 1 second causes the wire to heat up to 1000deg.C and burn-off any contaminants.

Over time MAF accuracy can degrade as a result of component ageing and hot-wire erosion. This could cause engine performance to gradually degrade. I have read that after about 80K miles, there is a good chance a Hot-wire MAF has drifted out of tolerance but this was by someone that offers a rebuild service so I'll need further convincing....

Below is the pin-out for the Bosch MAF used in the BMW 850i (Bosch part# 0280212010 or 0280212025). The pin numbers are clearly labeled in the connector.





Burn-off signal (open or low = normal. +5V = burn-off)


+12V supply (up to ~ 1.7A)


Mixture (is connected to a potentiometer in the MAF) is not used by the BMW 850i. Some versions of the MAF have this, others do not.


Main Ground (12V return)


Signal ground for output (internally connected to main ground)


Signal output (voltage proportional to air flow)


So how to check if your MAF is working correctly?

Here is the setup I used to evaluate MAF performance (I later replaced the alligator clip leads with female connectors that provided a much better contact).


A leaf blower powered from a variac is my source of air flow…. by controlling the voltage applied to the leaf blower I can change the air flow rate. Since this is not a calibrated setup it is used to compare the performance of 2 MAF’s connected back to back (a good reference and an unknown unit). The 2 MAF’s are taped together at the end of the leaf blower’s output tube. I used a bench power supply to apply +12V to each MAF (up to about 1.7A each at max airflow) and a dual channel digital oscilloscope to monitor the output signal from each unit.

I purchased 2 inexpensive used MAF’s on Ebay for $25 incl shipping!



These are both slightly different... #1 has a plug for the idle screw adjust and #2 does not, #1 has a sticker with part number, #2 has part number etched into lid. Both have part number 0280212010.

They came from a car with ~ 168K miles. I doubt they are original but they are both genuine Bosch.

I also purchased this much newer Bosch factory rebuilt unit from a parts breaker (unknown miles but labeling and physical condition suggest it is a much newer part). New Bosch ones are $400+ each! Chinese rebuilds are available for much less but of questionable quality. I paid $45 for this one incl shipping. Its part number is 0280212025.


Here is a close up of my test setup... a reference MAF and a unit under test plumbed in series.


With no airflow across the MAF, the current draw at 12V should be very close to 0.5A and the output voltage about 1.4V. Whilst looking into the MAF and applying +5V to Pin 1 for a second or so, you should see the hot wire glow like a light bulb filament. These are the first 2 tests to determine if a MAF is good or not.


The response time of the MAF is very fast (Bosch advertises <5mS), so it is possible to capture the air flow rise as the leaf blower spins up. Below is the response of the 2 older MAF’s (#1 and #2) that I purchased on Ebay. There is a small glitch in one of the output signals half way up its response curve. This is something to do with the test setup (turbulence perhaps) as when I reverse the positions of the 2 x MAFs the glitch moves to the other MAF. It is the MAF furthest from the blower that always has this disturbance.


The picture below captures the blower turning on and then off. As you can see, the blower spins up fast and then slowly decays. The violet trace captures the difference between the 2 x MAF outputs at a magnified scale (100mV/div). The difference between the 2 MAFs is always less than about +/-40mV which with a nominal 2V output would be +/-2% so I'd say these 2 x MAFs are very closely matched and well worth the purchase price! The blower speed was set to produce about 3.9V at the MAF output (the no-airflow voltage at the start of the traces is about 1.4V).

And here is a comparison of the newer MAF vs. #2 older MAF

The difference is a little more than the difference between the 2 older units but not by much... still within 2 or 3 percent of each other!

The final test I did was run them continuously at about 4V output and tap them with the head of a screw driver to make sure there are no intermittent connections.

My conclusion..... I was either very lucky or these things are a lot more rugged than some would have you believe. I now have 3 good spares!

Here's what they look like under the cover. The circuitry is encapsulated in RTV.


Here are the MAF's out of my car...

The 2 MAF's in my car are different so I guess at least one of them has been swapped out at some point.

The one on the left does not have a mixture adjustment screw, the one on the right does. It is my understanding this is not used on my car so not necessary.

The traces below capture the fan ramp up.

One on the left matches my reference unit perfectly. But the one on the right is showing its age and is about 2.5% low (the limit of acceptable drift)


And these traces are of much longer duration, capturing the ramp down...


Looking a little close at the 2nd MAF I noticed a speck of green on the temp sensor (salt corrosion perhaps).

I cleaned this with some alcohol and a Q-tip and then cleaned the whole thing with MAF cleaner.

And here's what it looks like now... not perfect but much much closer!

Since I have a few spares, I'll still go ahead and replace this one with my reference unit.

A word of caution... If you are going to replicate this test you need to be sure that you have solid power and ground connections to the MAF's. The alligator clips I was using proved to be unreliable and affected the results - perhaps even causing the error I saw with the 2nd unit from my car. After deducing the leads were affecting my results I used some female connectors that fit the MAF pins perfectly and from then on saw very consistent results.

Feel free to contact me if you need a MAF testing.


Since 2007 this car has been registered non-operational.... stored for 6 years by the previous owner and in my garage for the past 3 years! To re-register the car a smog check is required and was performed today.

Here are the results... PASSED!!!

All that's left is to wait in line at the DMV and she'll be street legal once again!

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