老賓士維修保養-w126、w124、w201: 空氣流量感知器-Air Flow Sensor

怠速不穩、耗油、加速無力，目前台灣已不單獨出售新品(料號3437224035)，原廠新品連同空氣流量感知器一併出售售價約台幣30000元左右；感知器正常使用下約20萬公里電路板感應層將逐漸耗損,導致電壓值不穩進而影響電腦判讀空氣流量數據。

跟據BOSCH產品文件，料件編號:3437224035；F026T03021；3437224015指的是同一個零件；整個零件分為三個機構:

1.防水外殼及橡膠密封條
2.4支固定螺絲(其中2支為訊號調整螺絲，可以改變進氣量的大小)
3.IC電路版(電路板上有一層薄膜，透過與節氣門的指針摩擦產生電位差)

常見的故障大多集中於薄膜磨損，導致訊號無法依節氣門的指針正確的傳導；當訊號不正確ECU就無法計算正確的供油量，引擎就會怠速不穩、耗油、加速無力(前提是點火及氣密條件沒有問題)。大部分的保養廠會調整分油盤供油量來解決車主的問題；比較可行的方式應該是:

1.移動空氣流量感知器測試電壓是否會改變
2.將空氣流量感知器電壓調整至預設值(螺絲須先假固定稍微拴緊來調整)
3.調整分油盤
4.確認引擎工作溫度時怠速轉速(冷車時轉速會比較高)
5.測量CO值(老師傅檢查排氣管用聞的要不惡臭，排氣尾管最好要有水滴)

如果已將上述問題排除(換新料件或是調整至最佳值)，還是無解那就必須檢測其他系統；機械噴射系統的供油及供氣系統不像新型的電腦噴射引擎，全部交由程式來調整，它必須藉由維修人員依手冊及車況來微調，這也是老車好玩的地方。

Testing this component is carried out with ignition on and engine off. The potentiometer is connected by a three pin plug, with locking tags at top and bottom. Ease these out and slightly withdraw the plug, so that the socket pins can be accessed by test leads from your multimeter. The top and bottom pins should have nominally 5 volts across them, as supplied by the computer. The centre pin feeds back to the computer a voltage less than 5 volts for each position of the air flow meter. Connect the multimeter between the centre and the top pin and watch the voltage reading as you very gradually deflect the air flow meter plate. The voltage should increase continually, never dropping out to zero. If it does, you need a new potentiometer. Mercedes only sells these already mounted on a replacement air flow meter and you may think that its cost is comparable to the value of your vehicle. There is information on the web about obtaining a replacement potentiometer at a much lower cost. If you go this way, you'll have to remove the old pot and replace it with a new one, then calibrate it, but that's a simple matter really. One reason for which Mercedes provides an all new solution may be that the sliders that run over the resistance tracks remain in the housing and they may also be worn, so installing a new pot may not produce any improvement. Removing the old potentiometer involves using a very fine screwdriver to insert in the gap between the two roughly circular plastic covers on the front at either end of the potentiometer housing and easing them out, exposing four Torx screws that mount the potentiometer to the air flow sensor body. At this stage it's a really good idea to put a pencil mark around the pot on the air flow sensor body, as an aid in getting the replacement one roughly in the right position. Undo the screws and gently withdraw the pot from its shaft. Look at the tracks and marvel at how the car operated at all! You'll note that the four mounting screws go through slotted holes in the pot housing that allow it to be rotated relative to its operating shaft and this is how the pot is calibrated. Once you have replaced the pot with the screws just loose enough to allow its rotation around the shaft, you reconnect the three pin plug, again with the socket pins exposed for measurement, then measure the voltage at the centre pin. Turn the new pot around the operating shaft until you read about 1 volt. At the 1 volt level the engine will be running rich, but if you try a lower initial setting, you may not be able to start the engine. Now start the engine and get it up to operating temperature, then adjust the pot position until you read 0.7 volts between ground and the centre pin. The pot is now calibrated."