Outside temperature gauge replaced - still blind
 

my 1990 300CE has no outside temperature reading. I replaced the display unit but it remains the same. There is no digit visible, just background light.

What is broken? Thanks!

Could just be the sensor. It is either attached behind the front plate or the bumper...depends on year so I am not sure about yours. You could try probing the connection when it is on maybe?

You most likely have no power to the module's temp electronics because if there was power the display segments would be visible even with no sensor connected. Without a sensor a powered module will read 136+. There appears to be a ground connection because the illumination works. If you have the illumination bulb lit all the time with the ignition turned on, it may very well be that you have the connections to the illumination bulb and the module power switched. Double check those connections and that might get things correct! Good Luck!

Yes, the illumination is permanently on with the ignition tunred on. It is only one connector to the display module. I am going to check if the connector can be inserted other way, else I don't know how the connections can be switched. Thank you for your diagnosis.

I see the sensor near the license plate, how is the it connected? not directly to the display module I guess. I am looking for the connector in-between, where is its location?

The 5 pin inline connector is designed to be connected only one way! Perhaps at some point in the past someone had opened the connector and failed to position the wiring correctly when reassembly?

If you do a search there is a listing of the correct wiring positions for conversion from the 4 pin square type to the 5 pin inline configuration. The key is that the illumination is constant with the ignition in the on position. Another diagnostic step that could reveal more is to with the ignition on, turn the lights on and turn the interior illumination knob up all the way, if you then get something happening with the display it will pretty much confirm the wiring issue. Good Luck!

thanks Billy, I am going to check if the pins are correctly placed into the connector this evening after work. I think your diagnosis is very close.

Off the top of my head I'm not certain with a 124 chassis, there is a two pole male connector either alongside near the fuse box or inside the fuse box which plugs into a mating female connector. The wiring from the sensor is sheathed in black with one brown and one green wire inside. The wiring runs with the other wiring along the driver's side wheel well, you need to loosen the black plastic wiring tye's along the way to remove it.

Doesn't matter, it's not the temp sensor, see post #3 above. It's power, ground must be okay or it wouldn't light, Billybob has your answer. Check fuses yet?

Billybob and you are right, the display has no power, the illumination comes from that of the cluster backlight and that can be seen when adjust cluster backlight.

I am still checking the fuse and will report back. Thanks for all feedbacks. I appreciate it.

Sensor (If it becomes the problem later)
 

Substantially,less expensive than the MB part #
[Allegedly more rugged, Too]

The "PigTails have to be soldered onto the end of your existing cable.

Nice tip. MB should have designed that way that the actual sensor, just the tip, be connectable and thus replaceable and not the whole 5 ft cable with connector placed in location difficult to reach.

FYI, my temp gauge was not displaying anything and the background light worked okay. I ruled out the sensor by testing with a 3.3K Ohm resistor and still nothing on the LED. I purchased a used LED display on e-bay for about $30 (found it under "124 display"). Swapped it out in about 2 minutes. Now it works fine. Must have been the LED display.

I did change the LED display prior to posting the question. It must be bad luck should the replacement be also defective. I checked the fuse and it is OK, it is the same fuse for turn lights.

Now I am back to square one. Next is checking power supply and ground connection to the display.

I tested the sensor using an ohmeter at the display connector and ice water/warm water applied to the sensor. Since I got a change in resistance with change in temperature I figured the display was bad. A good used display from e-bay fixed it.