Bosch CIS-E Airflow Sensor Potentiometer
Replacement and Adjustment
Replacing an Airflow Sensor Potentiometer is not a very difficult job. You'll need some basic hand tools and a digital multimeter for the electrical adjustment. This sensor applies to every CIS-E fuel injected Mercedes-Benz model, ranging from the mid 80's to the early 90's. Depending on model, it can take as little as half an hour, to as much as 4 hours. Most are very easy (M102 8v 4 cyl, M103 6 cyl, M116 V-8, M117 V-8, M119 V-8) but the 190E 2.3 16v can be a challenge, usually needing to remove the entire airflow sensor housing to gain access to the mounting screws. This part is not available from the dealer, they will only sell you the complete Airflow Housing assembly, usually between $450-$1000 depending on the model, which makes this a very nice repair to save you money. The current Bosch part number is #3 437 224 035, there is no MB part number. It has been updated over the years and replaced the previous version part number of #3 437 224 015. Unfortunately this sensor is no longer in production by Bosch as of 01/01/07 and is getting very hard to find. If you can't find one you'll be forced to buy the whole airflow housing.
Here's a good look at the replacement Airflow Sensor Potentiometer, front and back along with seal. You can see on the front side that there are 3 pins for the connector, the 4 elongated mounting holes allowing for adjustment, and the fine adjustment screw just to the right of the 3 pins. You can see on the backside the potentiometer sweep contacts. The new sensor does not come with a new seal so be careful on removal to not damage it.
For this demonstration, the vehicle is a 1989 300SE with M103 inline-6 cylinder engine, but the procedures are the same for most models. We'll start with removing the air cleaner by removing the three 10mm nuts (2 on the left side of the air cleaner at 7 and 11 o'clock, and one on the right lower side of the air cleaner). Pull off breather hose from the top of the air cleaner and pull off air cleaner.
With the air cleaner removed you can see the black plastic potentiometer. There are 2 black plastic covers over the mounting screws, you'll need to pry them off with a small pick or screwdriver. You'll also need to remove the fuel line (17mm) from the fuel distributor that goes to the damper and unbolt the damper from the airflow housing (10mm bolt/clamp) so you can move the damper over to access the screws. Next pull off 3-pin connector from potentiometer by squeezing the top and bottom clips. You can then unscrew the 4 mounting screws (#15 torx) and remove the potentiometer. Be careful to not damage the seal during removal as it does not come with the new sensor.
Here you can see the airflow housing with the sensor removed. Be careful to not damage/bend the contact arms. Transfer the seal from the old sensor to the new, and install the new sensor. Tighten the screws until they make contact, but not quite finger tight. This allows the sensor to still move slightly for adjustment. You can now feel how the sensor pivots on the housing due to the elongated mounting holes. Just try to center the sensor for now, adjustment will come later. Reinstall the fuel line and clamp for the fuel damper so the engine can be started.
Here you can see the electrical tools I use for back-probing the connector for adjustment. These are made by Hirschmann and can be purchased on-line or at many electrical outlet stores like Radio Shack or Fry's. You could also make your own, just be careful so that the 2 contacts aren't touching and shorting during the adjustment process. You'll need to back-probe pins 1 and 2 (the top 2 pins) and then reinstall the electrical connector.
With a digital multimeter hooked up to read DC volts, start the car and let it warm up to a steady warm idle. With the engine at idle adjust the potentiometer body by pivoting it slightly until 0.70v (plus or minus 0.10v) is obtained. This is a very fine adjustment and it doesn't take much movement to make a big difference, so be sensitive and move it slowly and gently. Then gently tighten the 4 screws in a cross pattern, being careful to not change the adjustment while tightening. With the sensor tight and adjustment within spec, shut the car off and install the black plastic screw covers on the sensor and reinstall the air cleaner. The fine adjustment screw next to the 3-pin connector should not be used for adjustment as it is only for very minor adjustments and should only be used in conjunction with a flow bench tester.
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