Exploring the VDO Outside Temperature Display in a W124
 

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.

http://i323.photobucket.com/albums/n...9/VDO_4549.jpg

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.

http://i323.photobucket.com/albums/n...2/VDO_4538.jpg

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.

http://i323.photobucket.com/albums/n...9/VDO_4548.jpg

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.

http://i323.photobucket.com/albums/n...2/VDO_4558.jpg

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.

http://i323.photobucket.com/albums/n...9/VDO_4562.jpg

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

Good info, thanks Jeremy!

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/diesel-discussion/251755-w126-outside-temperature-display-installation-w123-300d.html

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

Charlie

LCD with Degrees C
 

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

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

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.

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?

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

Correct.

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.

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.

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.

Thanks for posting this. Interesting stuff.

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

-J

The 5th wire goes to the speed sensor block.

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

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."

Odd that it isn't hooked up on the W126, all W126s have electronic speedometers so a vehicle speed signal is always available.

-J

It will update when not moving, but it does it more slowly.

Well Done
 

Nice write-up Jeremy!!!

My display was fried. I bought a complete instrument cluster that came from a 93 or a 94 (for the gas gauge) and the LCD has the five pins on it, my plug has 4 sockets. I pulled the sockets out of the plug and connected the corresponding sockets to the newer display unit and got nothing. I did not know the 5th wire had to be connected to the speed sensor block.
Can someone identify the connection to the speed sensor block for me?

X53/5
 

X53/5 in the following picture is the speed sensor distribution block. The temperature display (green wire, 3rd pin of the 5-pin plug) can be connected to any one of the terminals of block X53/5. The green wire coming out of the front of the block goes to the speed sensor on the speedometer housing.

Jeremy


http://i323.photobucket.com/albums/n...ing-Module.jpg

Thanks Jeremy.

Very interesting and thanks for posting. Luckily for me my temperature display is working fine for now.

Based on your trouble shooting for my '91 300D(display illuminates, but it's blank)I should begin by checking fuse #6 and then search for broken wires?

Trouble-shooting
 

Trouble-shooting always begins by looking at what is working and what is not. In your case, the temperature display illuminates so the illumination wire (gray/blue) and ground (brown) are good. Your display is otherwise blank, usually a sign that the sensor is defective or has come disconnected from the display. Can you put a date on when the display stopped working or did the car come to you in that condition? If you can date the problem, did anything else happen at that time (like running into a brick wall at 50 MPH)?

Then there's the matter of what usually breaks and what usually doesn't, and what is easiest to get at. Fuses are easy to get to but too easy to look at and say "looks good to me." You need to actually remove the fuse, check it for hairline cracks, polish the ends, etc. The electronics inside the temperature display usually don't fail but it is not impossible, just unlikely.

The sensor, buried in the front bumper, is usually safe from harm unless it is smacked hard (brick wall, etc.). If you are comfortable removing the instrument cluster from the dash, you can check the power wire (black/red or black/violet) for 12 Volts when the ignition is on.

If you have a digital voltmeter, you can unplug the connector from the temperature display and check the continuity (Ohms) between the pin with the green wire (goes to the sensor) and ground. You should get a reading of a few tens or hundreds of ohms or maybe a thousand or so. A reading of zero means a short-circuit and a reading of 9999 (infinity) means a broken wire.

Unfortunately, the wire is buried in a maze of cables; fortunately, it's fairly safe. There is a connector somewhere in between the display and the sensor but my schematics [Electronic Troubleshooting Manual] don't say where it is located -- probably somewhere in the left wheel well near the ABS pump would be my guess.

The speed sensor wire does not have to be connected for the display to work -- I have seen W126 displays (still use the old 4-pin square plug) in which the 5th wire is simply cut inside the display.

Jeremy

Well I can't put an exact date on it, but it stopped working when I installed a new battery and alternator. Everything else works just fine except my cruise control. I'll pull my dash this weekend, follow some of your testing procedures and see what I can find out. Thanks for he quick response.

Jeremy, as stated in post #5 by aaa I think the LCD crystals might be different. I think the "F" and the "C" are fixed characters in the crystal and not segmented like the numbers. I'll soon find out... I have an "F" unit on the way.

If I want to bench these could you provide the wire colour codes for the wires going to the pins of the unit. With the wire colours I can test either 4 or 5 pin models. I am assuming that the orange wire is for the speed input since it is cut in one of your photos.

Thanks,
Will

Awesome thread!!!

I removed my cluster from my 300TE a few weeks ago since the odo stopped...I'm very aware of the fragility (?) of the LCD displays and I couldn't have been more careful. I put it on carpet, correct orientation, removal was slow and steady, yet it STILL bled out!! :mad: Its like a black plague slowly making its way from right to left....

I have tried swapping just the LCD before on my other W124 since it had the same issue, but the LCD when turned on was screwed up and showed an error code I guess?

You say the LCD's are interchangeable though? Can you think of any reason why it didn't work for me before?

Error code?

Did you swap to a 5-wire temp display? Sounds like there could be a mis-wire.

Well I changed out my sensor today and now everything works like a charm. By the way, the actual sensor and wire have a 2 pin connector before it actually reaches the cluster.

I successfully fixed the display on my 400e yesterday and plan to tackle the wagon next. I first tested other displays and then found that if I unplugged the sensor (below the fuse panel) and jumped a resistor across the leads, I had a working (but incorrect display). My sensor read 2-3 Ohms. I went to he local yard and found a couple of sensors and found ~3.2k resistance. Once I installed a new sensor (cut the wire at the bumper and soldered the "new" one on) I had a working display. The moral of my story would be to start with the fuse box connector and go from there. It's simpler than pulling the cluster.

I havent taken apart my early W124 temp display (it looks like it has the integrated board)

But I have to later 5 pin temp displays. With the integrated board can I somehow swap to a 5 pin?

Displays
 

I did some experimenting with the various displays a year or so ago. It did not go well -- I could not swap the little LCD readout panel among different electronic boards and get them to work. I think the pinout changes from version to version. I didn't put enough time into the process to really be sure and there is no available documentation unless you have a friend at VDO and even then the design is 20-30 years old so nothing may have survived. To be safe I would advise against swapping parts inside the black plastic case.

The complete display can be swapped with no problem as they all work the same. You can swap Fahrenheit for Celsius and even 4-pin for 5-pin if you rearrange the pins in the plugs.

Jeremy

Guess I should have grabbed the plugs from the junkyard instead of the display. The displays I have came out of E320's and do not have any signs of black smudges. So they might work?

Ill go back and get the plug and then switch plugs. Even if I do not tap into the speed sensor block it should work, correct?

I have a '85 W201 190D. The odometer recently stopped so I'm going to be pulling the cluster to address that problem. While I'm in there I'd like to also change the current sporatic Celsius unit with a Fahrenheit one. The sporatic systems are the unit usually works when the car is first started (expecially when the temporature is cold) and will read accurately for a few minutes then goes blank.

Based on your info above, I wouldn't have a VDO produced unit? Is so, what donors should I pull a replacement unit from? Since my current display sporatically provides an accurate reading, is it likely my sensor is functioning and won't need to be addressed?

Thanks!

If my LCD was blacking out, I'd just be tempted to buy something like this off ebay for sub-$10, and separate the LED from the electronics and stuff it all in the spaces within the original temp gauge. If you look harder you can get ones that are "auto" dimmable (input from headlight switch kinda thing).

0 36"Blue LED Digital Car Thermometer Temperature Meter 55 125°C DS18B20 Sensor | eBay

They're different
 

The W201 got a physically different temperature display; the W124 (and W126, for that matter) versions won't interchange. I can't speak to the insides -- they may interchange but I've never taken a W201 unit apart. To be safe you'd have to get a replacement from another W201.

If your unit works occasionally it may just be a dirty contact in a plug or a wire that has most of the strands broken at a connector and is just about to pull loose. That is something easily fixable once you track down the problem. The pins in the connectors may be silver-plated and shouldn't be scrubbed hard with steel wool or sandpaper -- the plating will come off. Pretty much any metal polish will remove the corrosion -- I use Mother's Mag Wheel polish and a Q-tip for such things.

Jeremy

Substitute
 

I had such a thing in my W123, which didn't come with an outside temperature gauge. Your eBay suggestion looks good and the price is certainly right. Do you know whether the temperature sender in that product is permanently attached to the display? That would make it more challenging to install in the instrument cluster if the sender couldn't be detached. Also, I notice that the item is in China.

Jeremy

Bringing up this older thread as I need assistance. My ambient temp display is the 4 wire version and was working fine. I pulled it out to replace the bulb with an LED (including resistor). After replacing, bulb lights up fine when I connect battery directly the gray (pos) and yellow (neg). When I put the unit back in the car, I am not getting any light or temp read out. I have a spare unit that I tried and getting the same result with both. Is there a way I can try to get a temp readout connecting it directly to a battery? I'm thinking the LCD is not getting power, however am not sure how it is suppose to as there does not appear to be any physical contact between the PC board and LCD...how does this get power? Any ideas to test or get working would be appreciated. Thanks.

The green wire is not negative ground. The wiring for the 4 wire version is: pin 1 +12 volt power - black/red, pin 2 ground - brown, pin 3 temperature sensor - green and pin 4 is illumination - gray/blue.

Thanks- my wire colors are grey (which I believe is illumination)-pin 2, yellow-pin 4, green-pin 3 and white-pin 1. When I put 12v to grey and ground to yellow, the bulb lights. I can't get the LCD display to come on regardless of which wires I apply power/ground. I'm starting to worry that when I soldered the LED and Resistor on the board, I messed up the board and killed it.

Another Pic

The colours that I gave you are for the mating harness although the pin numbers and functions should be accurate.

Awesome. I'll take a look. Thanks.

The LCD contacts are along the edges. There are two small "rubber" strips that act like brushes. There is someone on one of the lists who rebuilds these, new lcd and everything, maybe on 190revolution?

The five and four lead versions should interchange. The 5th lead is a speed sense, which corrects for roadspeed airflow. If it's missing, the gauge works without correction.

So I should get voltmeter reading of 12v on pin 1 of the harness? Does ignition need to be on?

I do not have any rubber strips on either side of my LCD. The white lamp diffuser is the only thing between the PC board and the LCD/reflector.

Found the rubber conductors that had fallen off. Everything working fine!

W201 temperature display
 

Hi! I am new here.
I own an W201, year 1985, with 2000cc gas engine, automatic gearbox.

The instrument cluster was without odometer (engine rpm) and without outside temperature display.
I found an odometer from other car (gas engine - it have the "economy" display, just like my car), and an outside temperature display. I did build a new instrument cluster with them.
Now, I have to make them work!
I did make the wiring for odometer "+" to "contact" position of ignition key, "TD" to alternator, but it don't have the "W" connector.
Also, the outside temperature display is working, but is enter in sleep mode because I don't know where/how to connect the "4" pin to the engine speed sensor or somewhere...
Can anyone help me, please?
...sorry for my rusty English...