Outside temperature display question - PeachParts Mercedes-Benz Forum

psaboic · #1 01-03-2014, 11:25 PM

Anyone who has put an outside temperature display into their 300D please feel free to chime in.

I have an outside temperature display from a W126 in my 300D. It had worked flawlessly for the last two and a half years....until today. Started the car today and the display worked normally, registered the correct temperature and the backlighting came on with the outside lights.

After about 15 minutes, the display started counting up to +127 F, and then back down to 0 F all by itself. It did this several times and then bottomed out at -36 F and stayed there. I stopped the car for fuel, and when I turned it back on, the display was blank but the backlight was on. Got to work and shut the car off. Fast forward 10 hours and I start the car. Backlight on the temperature display works fine, but the display read +19 F all the way home (outside temp was about +35 F)

What do you think, bad ground, bad display, bad temperature probe?

Thanks,

Glenn

charmalu · #2 01-03-2014, 11:48 PM

Here is a thread Deliveryvalve did a while back.

W126 Outside temperature Display installation in a W123 (300D)

Jeremy5848 did some trouble shooting repair.

Exploring the VDO Outside Temperature Display in a W124

Charlie

psaboic · #3 01-04-2014, 09:23 AM

That is the DIY I used to install the unit a couple years ago :-)

I was just wondering if, based on the symptoms I described, anyone has a scientific wild ass guess as to which part (or wiring) is bad, or if someone has experienced something similar

Glenn

jay_bob · #4 01-04-2014, 09:24 PM

Open circuit sensor = -36F
Shorted sensor = 127 F

I say you have a bad sensor.

To prove it, try substituting a couple different fixed resistors for the sensor and see what you get.

10 k is about zero c or 32F
3.5 k is about 20 c or 70 F
1 k is about 50 c or 120 F

psaboic · #5 01-06-2014, 11:45 PM

Well, for the last two days, the display has now been totally blank. Backlighting still works ok. I'm now leaning toward a bad display. Or should I just cut my losses and get a new unit and sender? I really like having a functioning temp display, as it makes the long drives on the backroads at night a little better knowing if it is below freezing or not.

psaboic · #6 01-09-2014, 11:33 PM

Does anyone have a working temperature sensor probe/wiring harness or a working outside temperature display (Farenheit) from a W126 that they would be willing to part with at a reasonable price? I'm looking at fixing my bad display. The back of my display unit takes the 4-prong connector so I would need either a sensor/wiring harness with the 4-prong connector or a display unit with 4-prongs on the back.

jay_bob · #7 01-10-2014, 02:39 PM

This data is from my 124 but I suspect the 126 is similar.

Remove the screws around the perimeter of the fuses and lift up the fuse tray. Be careful so you don't short anything out.

Check in the spaghetti pot under the fuses. There should be a 2 pin connector that joins the sensor harness to the body harness. Green and brown wires.

Separate the connector and get an ohmmeter. Put the ohmmeter on the side of the connector leading towards the front of the car.

You should be able to read resistance according to the temperature as listed above. If you get proper resistance the sensor is good. Extra credit, dismount the sensor, stick in ice water for 0C and put in warm tap water for the high range.

The 4-pin and 5-pin sensors are interchangeable. You will need the connector head from the 5-pin donor car. You just have to open the connector heads and swap the pins around.

On the donor car, find the 5-pin OAT display connector behind the cluster. Carefully open the connector and remove the shell. Carefully slide the sockets out of the connector body, no need to cut wires. You will notice one of the wires is green and yellow. Carefully remove this wire from the wire pack, it goes to X53/5 behind the cluster. Unplug the connector from X53/5 and save the entire wire with the ends intact.

On your car, open up the 4 pin connector shell and remove the contacts with the wires still attached.. Transfer the 4 wires from the old shell to the new shell as follows:
Old pin 1 - blk-red - power from F6 - to new pin 5
Old pin 2 - brown - ground from W1 - to new pin 1
Old pin 3 - green - sensor - to new pin 3
Old pin 4 - gry-blu - dash lights from X6/1 - to new pin 2

Now take your salvaged green/yellow wire from the donor and insert it into pin 4. Put the 5-pin connector body together, you may have to cut back the jacket slightly on the wire pack so everything dresses into the connector neatly. Bundle your green/yellow wire onto the harness and lead it to X53/5 and plug it in.

Connect the plug to the back of your new OAT display, turn the key to pos. II and see if it works. Turn the lights on and see if it lights up along with the dash lights. It will light up even if the car is off, but will not read anything.

jay_bob · #8 01-10-2014, 03:13 PM

Just to add, the X53/5 wire is the vehicle speed sensor. I believe that if it is not hooked up the display will still work.

The purpose of the speed sensor is that it helps the display to compensate for the effects of air flow on the sensor to help improve accuracy.