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  #1  
Old 03-21-2009, 11:00 AM
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W124 Ambient Temperature Display Issue

Anyone have an idea? The ambient temperature display in my 1994 E320 reads 18 degrees lowall of the time. Sometimes after starting, it comes on correctly, but then "resets" itself to 18 degrees lower. It is not the bumper mounted sensor as it is new ( did it with the old one as well). Any ideas? If it is the display.. does anyne have the part #?

Thanks..

Jeffrey

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Old 03-21-2009, 01:39 PM
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I don't know enough about the LCD readout to say if the problem could be there. However, I believe that the sensor resistance goes down as the temperature goes up (a negative resistance/temperature coefficient). Since the readout does vary but is always 18 degrees too low, it may be possible that there is a poor connection between the sensor and the readout in the dash giving a higher resistance and thus an apparent lower higher temperature.

I'd suggest opening and cleaning first the connector in the engine compartment where the sensor cable connects to the wiring which goes to the dash instruments. If that doesn't fix the problem, pull the instrument cluster and remove and clean the connector to the temperature readout. This won't cost anything but time.

The readout in my '86 300E went bad and I replaced it with one off of Ebay which worked fine. I disassembled the old readout and found places where corrosion might affect sectors in the LCD pattern but couldn't learn much about the electronics.
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  #3  
Old 03-21-2009, 06:45 PM
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Thanks for the suggestion. It is pretty strange.
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  #4  
Old 03-22-2009, 08:48 PM
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Not strange at all. Mine is 20 degrees. I'm living with it...
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  #5  
Old 03-23-2009, 10:21 AM
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The problem is most likely with the sensor (about $70), not the LCD display. My 87 560SEC behaved the same and sensor replacement corrected the problem. Mark
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  #6  
Old 03-23-2009, 10:34 AM
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Our W124 wagon is off, but not a consistent (or linear) amount..... Always low, but anywhere from ~ 20*F to 50*F (was fun seeing -28*F last winter in CT).
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  #7  
Old 03-23-2009, 11:58 AM
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The display is horribly expensive, I was quoted well over $300 for a 126 display (best price I found) new. Fortunately, they are available used on ebay et al.

I'll look around, might have a spare 124 unit. Where are you located Jeffrey?
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  #8  
Old 03-24-2009, 12:55 AM
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It is most likely the thermistor on the end of the sensor, not the display. Do a search for thermistor, I believe some members have been able to fix it by soldering a new one in place. You might try searching with GSXR, I think he fixed his.

Read this thread.
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  #9  
Old 03-24-2009, 02:31 PM
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The sensor is a brand new MB unit ( from the fusebox to the front bumber ) Same result as the old one. 18 degrees off...

Babymog... I sent you a PM.

Thanks!

Jeff
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  #10  
Old 03-24-2009, 02:33 PM
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By the way.. anyone else have / ever seen a Sierra Brown W124? I think mine is the only one I have ever seen...
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  #11  
Old 03-24-2009, 04:48 PM
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Sounds like a bad resistor in the guage. I'm not experienced enough with these to know whether they are RTDs or thermistors, but it appears from the other posts/threads that they are thermistors.

Regardless, they are likely read using a wheatstone bridge. The circuit will have a constant voltage supply and three known resistors. If there were a problem with the voltage supply, the error would change as the reading changes.

Since the error is a constant number, that only leaves the wiring and the internal, known resistors. If you can read the resistance where the leads go into the guage and the value is the same as it is at the sensor, it must be one of the internal resistors. One of those in particular will give a step change in the reading.

Here's a good wiki page if you want more info:

http://en.wikipedia.org/wiki/Wheatstone_bridge

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