Hi. I have a 1996 Mercedes C280. The outside ambient air sensor has gone bad and now reads about 30 degrees F high. The estimate to fix this is nearly $400, which is a lot. There are two types of sensors for this model and year. One is a plug-in sensor that looks like a small pointy cone. The other is a long cable with a metal cylinder at the end that looks like an old-style stereo headphones jack (the thick kind). I have this second type.
I would like to know whether, instead of replacing the whole wire and sensor combination, which involves taking out the instrument cluster and rerunning the wire, I can simply cut off the wire at the sensor end and solder in a new sensor (cut off from a new part).
The new replacement part costs well over $100, as opposed to just over $20 for the pointy cone type. I am wondering whether it would be possible to just cut the existing cable near the sensor end and then solder in one of the pointy cones. There is a youtube video that shows replacement of a similar type of pointy cone sensor on a BMW, and it looks like the assembly that accepts the sensor is the same, so if I couldn’t find a Mercedes plug-in receiver assembly for my old car, I could maybe use the BMW one and then plug in the Mercedes sensor. That would let me do another plug-in later without resoldering.
Since this sensor has gone bad once before, I have to think that a replacement won’t last more than five years or so. It would be good if I could find a way to get it fixed for less than $400 a shot.
I am not sure whether the outside temperature has an effect on other aspects of the car, such as injection mix. I presume it has to go through the on-board computer, because it must be the signal for the little snowflake that appears on the dash when the outside temperature drops to freezing at the road level. Does anyone know whether leaving the sensor alone and just living with a high reading will cause problems? Right now when the outside temp is around 70 degrees F the sensor reads about 105 degrees F.
Any help would be appreciated.
Just drive on. I’m not an expert on MB - but I doubt this sensor feeds info into the engine management system.
This sensor is for the cluster only
The outside air temp sensor is not a part of your engine’s demand signals to the ECU. The ECU only cares what temperature the engine is, not what temperature it is outside the car. Induction air temperature may be measured, but it’s unrelated to the outside air temperature sensing. In short, you can safely ignore this problem if you choose.
What you have is probably a “thermocouple” sensor. It contains two different metal bonded together at the end that generate a very small DC voltage (in millivolts) due to the slightly different way each metal reacts to temperature. This signal is then converted to its temperature reading by a chip. The wires leading to the thermocouple may be a part of the total signal used in the sensor’s calibration. The wires add resistance to the circuit, and when you’re dealing in millivolts that can matter. This means the wires cannot be clipped and a new end soldered on. Thermocouple sensors are probably the most common temp sensors because they’re dirt cheap to make and do not require constant current flow, as do thermistors.
Thermistors are pretty much universally use for ambient temperatures. Thermocouple systems have issues with reference voltages. All the dirt-cheap home digital thermometers use thermistors.
Both are extremely cheap and extremely common. I would argue that thermocouples are probably in more common use, but it really doesn’t matter. The OP’s Merc could be using either. I simply flipped a coin based upon the OP’s physical description. Either way, the important points are that (1) it won’t affect engine operation, and (2) the OP cannot simply clip the old one off and solder on a new one.
All the ambient temperature sensor does is displays the ambient temperature on the dash. Nothing more.
So if you want, you can live with it.
Tester
I can’t add anything to what you’ve been told other than if the display can be changed maybe you could switch it to Celsius and not have to stare at triple digit heat… 
Personally, I’d leave well enough alone and just mentally subtract 30 degrees from what it says on the display.
One idea however, if you are a gambling type, buy the $20 version and clip the old one off and solder the new one on. Sure, doing this “experiment” could cause some unanticipated and very expensive to fix problem. But you are the gambling type right? So it also result in the correct reading showing up on your dash display.