Exploring the VDO Outside Temperature Display in a W124 - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 11-27-2010, 09:26 PM

This is not so much a "DIY" as a "how it works" post.

Many people have asked about repairing or replacing the digital temperature display in their W124. I took several apart and photographed their interiors. Repair-wise, there are only two things that can be done and them only by those with the proper tools and skills: (1) Replace the LCD readout and (2) Replace the light bulb. The LCD readout is not available from Mercedes but you can take one from another display. I suspect that all LCD readouts are the same but have not had access to enough for testing and they contain no part numbers. The LCD readout is made of glass and is physically fragile. It often fails by turning partially or entirely black. This is a failure of the manufacturing process that was eventually solved but early LCDs (remember the first LCD watches?) had a high failure rate.

The light bulb is available from Mercedes as a separate part (001 544 85 94). This is surprising, since the bulb is soldered to the PC board. Replacing it requires unsoldering the old bulb from the tiny PC board and soldering in the new one.

The digital outside temperature display in W124 (and W126) (and other?) Mercedes were made by VDO in various versions during the ten-year production run of the W124. The first versions (1986-early 1987) had a connector with four pins and the later version had five. The four- and five-pin versions are interchangeable if you take the plastic connector apart and swap the wires. Threads on this procedure already exist so I won't repeat them.

The temperature sensor that mounts in the front bumper is the same for both Fahrenheit and Celsius and was used in W124, W126, and W201 cars (and maybe others). It carries Mercedes part number 001 542 26 18 (latest version, there are also older versions). My point is that the sensor is the same for both Fahrenheit and Celsius; the difference is in the electronics in the display.

Other than the connector, all W124 displays are interchangeable. Note that there are Celsius and Fahrenheit versions. These carry different Mercedes part numbers, according to EPC:

Celsius "ONLY FOR CANADA"
124 542 00 27 "replaced by"
124 542 00 05 "replaced by"
000 542 71 27

Fahrenheit "NOT FOR CANADA"
124 542 02 27 "replaced by"
124 542 01 05 "replaced by"
000 542 72 27

In addition, VDO made internal changes that were designated by different VDO part numbers cast into the black plastic housing. These VDO part numbers are different from the Mercedes part numbers. Do not confuse them. Different VDO part numbers have internal differences but they work the same and AFAIK the LCD display and the little light bulb is the same in all of them.

The one W126 display I have came from a 1989 420SEL. The display is date coded "11.88" (November 1988) and carries Mercedes part number 126 542 02 27 and VDO number 631.202/020/001. EPC shows that these displays were also available in both Fahrenheit and Celsius. The outside shell is too big to fit in a W124 instrument cluster but when I took it apart, I discovered that the electronics package inside is the same as the newer (1992 300E) W124 display. This means that just the LCD readout or the entire electronics package can be swapped between some W126 and some W124 cars.

The electronic components that take the analog signal from the sensor in the front bumper (just a heat-sensitive resistor) and turn it into a digital readout are different for Fahrenheit and Celsius. They are not interchangeable and you cannot do anything to change one into the other. These components rarely fail. The sensor in the front bumper rarely fails but it can be crushed in an accident and its wires can be cut or broken.

Common failure modes:
1) LCD fails. (Symptom: display turns part or all black.) Cure: replace LCD from a donor unit or replace entire display.
2) Light bulb burns out. (Symptom: display does not illuminate with headlights.) Cure: solder in a new bulb 001 544 85 94.
3) Wire or sensor is damaged. (Symptom: display works for a few seconds, usually showing a very cold temperature, then goes blank.) Cure: find and fix damaged wire or replace sensor.
4) No power. (Symptom: display blank, may still illuminate with headlights.) Cure: check Fuse #6, look for broken wires.

To keep this post from being overly long, I have attached just a few pictures. Many more pictures are in my Photobucket account.

Jeremy