The early Bosch way to meter air was with a mechanical flap that moves with air flow. On the top side of the air flap is a circuit board with a wiper arm using resistance value to meter the fuel. Primarily you will see this on Digfant 1 and 2 systems on Volkswagens. At first it was used on Eurovans where you didn’t really care about acceleration or performance but did end up in late model Mk2 cars 88-92. Thankfully VW went away from Digifant as it proved to be no too laggy for a car and quickly switched to a newer version of Motronic for the Mk3 generation. The same AFM was used with early Motronic systems in Porsche 944’s, but eventually Bosch upgraded it to a hot wire Air Flow Sensor since it works better and costs less. The AFM wasn’t the main cause of “laggy” acceleration in cars but it didn’t make it any better either. Most of that can be blamed on the archaic Digifant 2 system that all though worked well, wasn’t designed for highly responsive driving. If you drive a car with a Digifant 2 system consider trying to find the bits and pieces to retrofit a Digifant 1 system from a G60 Corrado/Passat, or California version of the same car. You can adapt the Digi 2 harness easily while adding in wiring harness for the TPS and new wiring harness for the injectors as Digi 1 is still batch fire but wired differantly. Even if you have no other modifications to the engine you will have more fun with you car. Anywhoo……
Over time with age the carbon track the wiper arm swipes across can wear. Particularly at the lower end of the range. This causes a “flat spot” in the acceleration, or a “lull” before it accelerates again. These units are not made anymore. Used ones are normally as worn or poorly adjusted, and the odd chance you find a new one some wear it will cost you an arm, leg and both ears. Another very common fail is the ambient air temp sensor witch sits just after the air flap. They can be found still but require a lot of work to swap out. Normally with a failed air temp sens the car feels repetitively gutless until its actually up to temp and on cold days can cause a higher loap / bouncy idle. To change it the entire inside of the AFM has to be removed, re installed and then checked that its working properly. Its easier to buy a good used one from a part out.
Normally you would do most of this while its still on the car as there is no need to remove it. This one I’ve shown is a dud that I have on the shelf keeping papers from flying around.
First picture: The ugly little things! Carefully pry the cover off and try not to break the guts inside. Note that the hole on the top left part of the shot should have a tamper proof plug pressed in.
Second and third pictures: Hidden treasure, The workings. Note the curved carbon track the wiper arms above it ride on. Also note on the big gear wheel a paint dot next to the “030”. Ill explain later.
Now there are a variety of ways to electrically test the AFM, but it largely depends on its serial number to be honest. At this age the highly accurate test isn’t worth it as its OLD AS HELL! You can measure resistance with it disconected to rule out if it works at all or not. Test between terminals 3 and 4 should be between 500 and 1000ohms, and between 2 and 3 should vary with movement of the arm. If either are out get a good used one and start over. To more accurately test you’re getting into testing output voltage but that varies between models and years. IE Porsche 944 late models came with a very linear read out where as the older ones came with a more progressive one. Without a degree wheel to measure it spot on you just wont know. The resistance way isn’t as accurate but it will tell you before and after if its working.
Fourth picture: Clean up! Grab yourself a bit of computer paper and slide it under neath the wiper arm tip a few times. This will clean off any deposit from the carbon track nicly without hurting anything else. You can see in this picture the extent of how worn the carbon track is on this particular AFM. Particularly bad spots you’ll see the resistance change one way and then the other as you pass over it with the wiper arm. This is what causes the problems.
Before you call clean up done remove the electrical connector out of the side of the AFM. Nothing to break here. You will see how the electrical connectors just rest on the circuit board. Give them a good wipe with your computer paper too. Any extra resistance yields bad metering.
Fifth picture: If the carbon track looks as bad as this one use a good sharp screw driver and crack loose the screws holding the board onto the base. These are loctited at the factory. You will need to push down hard and mind not stripping them. Once loose you will have a little wiggle room. Re position the whole board so the wiper tips are on fresh carbon. This will only get you so much moment. Late model ones you have almost no wiggle room so use method #2. Another way to do it for more moment and less risk of breaking anything is to loosen the clamp bolt on the whole wipe assembly and lift it up very slightly. Re clamp it and then add a little bend to the wiper arms themselves to give them a good positive springy connection. You can see the clamp bolt in the first two pictures. If the carbon track isn’t so bad and the resistance reads like it should skip these steps. Give the carbon track a quick wipe with a Q-tip with some alcohol on it, seal it back up with RTV and go test it on the car.
If you did re position the board or re position the wiper you will need an accurate multi-meter with alligator clips, or in my case I have an old bit of harness with connectors that connect to my multi’s pins. Re check at that point that now you have a smooth change in resistance and not any dead spots. You will never get it perfect. Bosch laser calibrated these at the factory. If you still have some dead spots go find a good used unit.
Now the tamper proof plug – in the first two pictures – Under this is the Co2 adjustment. You do not want to touch this unless you have an exhaust gas analyzer handy, know the procedure for adjusting, and the specs. This is how the base fueling for the car is set, it varies with what altitude you’re at as the ECM can only auto correct itself so much. Lots of older German car specialists still have these, and you can get away with using a very good 4 gas analyzer similar to what a natural gas tech would use too if the test wand is long enough and the right diameter to slide into the exhaust test port on the manifild. Again the adjustment varies on altitude. If you’re installing a used one because of internal wear or a faulty Air temp sens consider paying to have the Co2 calibrated. You’re car will feel so much better for it.
Now the paint dot on the gear wheel. If you’re buying a used one and see something like this do not buy it! For a little while people thought one neat mod was to de tension the spring holding the air flap by 3 teeth. The idea was that it gets a little richer quicker, and boosts your acceleration. In reality it does nothing as the car still reads the O2 sensor and leans it out again. If you see the paint dot on one you own or are working on just leave it where it is as you don’t know how far out of adjustment it is and it can be made worse because of this. There is a lot of math and science in that setting and its not worth messing with.