W124-126-201 Outside Temperature Gauge Calibration
 

Here's some data that may help people trying to figure out whether their outside temperature gauge is working correctly or not. This should apply to all W124, W126, and W201 cars and maybe to others -- I don't know if this design was used in other models.

Mechanically, the displays do not interchange but electronically they appear to be the same. The displays read from -36°F to +136°F or the Celsius equivalent. If I can find a Celsius display, I'll post numbers for it. The actual LCD panel is the same for all displays, it's only the electronics (degrees F or C) and the physical package that changes. The temperature sensors should all be alike.

The only other change is the "fifth wire" added after 1987 to connect the display to the speedometer so the display won't update when the car is standing still. This keeps the display from reading super-high temps when the car is stopped in traffic on a hot summer day. This fifth wire was not added to W126 cars.

If the temperature sensor fails or otherwise shorts to ground, the display will show +136°F for a few seconds and then goes blank. If the sensor fails or a wire breaks so that an open circuit (disconnect) is created, the display reads -36°F for a few seconds and then goes blank. Either way, the display stays blank until the power is turned off and back on. Once the fault is repaired the display operates normally.

Outside temperature gauge calibration (1991 300SEL)
°F Ohms
136 (Short to ground - display blanks)
136 680
123 1000
103 1500
80 2200
69 3300
55 4700
39 6800
27 10000
15 15000
3 22000
-11 33000
-19 47000
-26 68000
-36 100000
-36 Open circuit (display blanks)

I do not claim a lot of accuracy for the above data. It is based on my testing one display from a 1991 300SEL on my work bench with a 12 volt power supply and an inexpensive Heathkit resistance substitution box. However, it should be enough to give you a rough idea of whether your unit is working properly or not.

Jeremy

This is good information. My display read 55.5 C today and it was 9deg.f :eek:

The only other change is the "fifth wire" added after 1987 to connect the display to the speedometer so the display won't update when the car is standing still. This keeps the display from reading super-high temps when the car is stopped in traffic on a hot summer day. This fifth wire was not added to W126 cars.

That explains a lot! Thought the display was pretty useless.