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

tbomachines · #2 11-27-2010, 09:58 PM

Good info, thanks Jeremy!

charmalu · #3 11-27-2010, 10:00 PM

This is great information Jeremy. thank for for posting this.

Deliveryvalve did a write up, installing a out side temp displays in his W123. At first he used one from a w126, then later (post# 34) he discovered the W124 was a better fit. this is a good up grade for the W123.

http://www.peachparts.com/shopforum/showthread.php?t=251755

I hope you don`t mind me tagging this on your thread.

Charlie

$turbo$ · #4 01-11-2011, 07:27 PM

Jeremy,

I have a Canadian 1987 300D Turbo (W124) with a failed LCD outdoor temp display. If I was to source a US version (displaying degrees F, and might be 4-pin or 5-pin), would I be able to just swap the crystals and still have it display degrees C? BTW I have a 4-pin module.

Thanks,
Will

aaa · #5 01-11-2011, 08:30 PM

My guess is that it would show the correct celsius temperature with an F next to it.

shertex · #6 01-11-2011, 08:42 PM

Very interesting! I suspect I will need this some day. Every now and again, the display on the 92 goes blank....at which point I employ a very delicate and technical restoration method: I POUND THE DASH WITH MY FIST....and the display, without fail, returns.

Zulfiqar · #7 01-11-2011, 09:15 PM

by what i picked up - if you disconnect the wire connector for the front bumper sensor - the display should read coldest and then go out.

In my case it reads 136F meaning short I believe then goes out after a little while - disconnecting the bumper sensor from the fuse box does not help - still the same.

Bad display?

Jeremy5848 · #8 01-11-2011, 11:43 PM

Quote:

Originally Posted by $turbo$

Jeremy,

I have a Canadian 1987 300D Turbo (W124) with a failed LCD outdoor temp display. If I was to source a US version (displaying degrees F, and might be 4-pin or 5-pin), would I be able to just swap the crystals and still have it display degrees C? BTW I have a 4-pin module.

Thanks,
Will

Although I don't know for sure, I suspect that the LCD panels are identical across the board and the difference (Fahrenheit and Celsius) is in the electronics.

Jeremy

Jeremy5848 · #9 01-11-2011, 11:47 PM

Quote:

Originally Posted by Zulfiqar

by what i picked up - if you disconnect the wire connector for the front bumper sensor - the display should read coldest and then go out.

Correct.

Quote:

Originally Posted by Zulfiqar

In my case it reads 136F meaning short I believe then goes out after a little while - disconnecting the bumper sensor from the fuse box does not help - still the same.

Bad display?

The electronics could be defective or there could be a short in the cable to the sensor or in the plug. The plug is fairly easy to take apart (make sure the pins don't pop out and get mixed up). It would take only one broken strand in a wire to short the sensor.

Jeremy5848 · #10 01-11-2011, 11:49 PM

Quote:

Originally Posted by shertex

Very interesting! I suspect I will need this some day. Every now and again, the display on the 92 goes blank....at which point I employ a very delicate and technical restoration method: I POUND THE DASH WITH MY FIST....and the display, without fail, returns.

Sounds like the LCD is not in firm contact with the PC board. If you get tired of pounding the dash, you can take the display apart and see if you can tighten up the fit of the parts.

biopete · #11 01-12-2011, 12:03 AM

I had fuse 6 keep blowing on me. My temp display didn't work. I unplugged the outside part at the fuse box -- no help. Had to cut a wire at fuse 6 to see where the short was -- either in the temp display, horn or blinkers. i think that is what shares that fuse. Not sure why but MB decided to make all the wires going into that fuse the same color. So although i wanted to just cut the temp display to eliminate it first as the problem, i couldn't tell which wire it was. Ultimately i just guessed and got lucky the first try . It was the temp display and the fuse quit blowing and everything else on that fuse worked. I'm going to try taking my cluster out some time and maybe i'll see why it was shorting.

compu_85 · #12 01-12-2011, 12:03 AM

Thanks for posting this. Interesting stuff.

Does that "extra" wire remain unused even in the late W124?

-J

babymog · #13 01-12-2011, 12:12 AM

The 5th wire goes to the speed sensor block.

compu_85 · #14 01-12-2011, 12:16 AM

Ahh, so it probably doesn't update the temp until the car is moving again?

Jeremy5848 · #15 01-12-2011, 01:06 AM

That's correct. Late "1987" diesels (IIRC, they were sold new well into 1988) and later W124s had the 5-wire display. My "early" 1987 300D Turbo (build date July 1986) had the 4-pin version; it had the speed sensor too but didn't use it for the temp display. I upgraded to the 5-pin version and connected it to the speed sensor distribution block and it works "as advertised."