Retro-fitting an Ambient Temperature Gauge to a W124 .


My wife had mentioned for years that she had missed not having an outside temperature gauge (OTG) on our 1994 E200 Wagon.

One time recently when I was changing instrument illumination bulbs it seemed obvious that there was a suitable space for this display, and that a retrofit should be possible. In addition, archive posts from this group had described where most of the components were located so I looked around for a kit.

Mercedes-Benz UK wanted a huge amount of money for the kit, and so I decided to obtain one used. After all, my car’s 9 years old, why should I need NEW parts?

A wrecker yard near Manchester said they could find me one, and at a fair price too.
The parts duly arrived and comprised the display unit, a loom with several terminations, and a separate sensor unit with associated wiring. As a bonus the supplier had labelled all connections, but I had checked these out with a wiring diagram anyway.

These connections were:
On the OTG upper loom, a black/red stripe wire, for ignition controlled 12v.
a brown wire, the ground (earth) connection
a grey/blue stripe wire, for dimmed illumination
a multi-connector to fit into the back of the OTG
a two-pole connector, comprising a brown wire and a green wire, to link into the corresponding connector from the sensor
And a black wire that was labelled as fitting into the rear of the speedo, more on this below.

Before I began the process of dismantling the vehicle, I checked to see if the components supplied actually worked, by connecting everything together, using a spare car battery, and using only the black/red stripe and ground wires. As soon as power was connected the unit displayed 22 degrees C. The heat from my hand on the sensor made the display move up, so all was well.

Firstly I disconnected the battery.
I then removed the front number plate, and located the sensor from behind the bumper into it’s position.
The wire was routed around the inner wing – the fender for you who drive on the “other side” - and into the base of the fuse box, via the cable-ties on the existing loom and a rubber grommet on the outside of the box, doing my best to ensure a snug fit with the sealing grommet. It proved impossible to push the two-wire connector through the tiny grommet, so I took the connector apart and pushed the wires through individually. The fuse tray was unscrewed and lifted up to gain access to both the passenger compartment and to the underside of the fuses. This male connector was left in the base of the fuse box.
From inside of the car, I removed the instruments and the plastic flap from above the driver’s feet in order to gain access to the underside of the fuse box.
I could obviously see where the temperature display would fit, but it took a while to work out how to remove the blanking plate that would let the display show through to the instruments’ front face. With the instruments facing you, peek around the left hand side of the assembly, and at the side, directly behind the warning lights for high beam, battery etc. you will see the end of the multi-coloured plastic strip on which the warning symbols are printed. You can pull this out with pliers, and cut off the last couple of inches. Check before you cut! When slid back into position, it exposes the OTG display.
With this done I put the upper part of the loom into position behind the dash, and managed (head under the dash resting on the pedals, feet either side of the headrest!) to get the two-wire connector (brown and green wires) and the switched 12v supply lead (black/red stripe) into the base of the fuse box.
I used fuse B for the supply, as this controls the electric mirrors and also had an existing black/red stripe wire attached. I joined the end of the sensor wire with it’s male connector to the female connector I had routed through from the passenger compartment, and closed up the fuse box.

The upper OTG loom has a grey/blue stripe wire, which was marked as fitting to a lighting block located behind the radio, under the heating controls area. My car doesn’t have air-con, so yours may differ.
I removed the radio, and routed the grey/blue stripe wire across the dash avoiding any heater or movable controls, trying as best I could to follow existing cable runs, and using ties to keep things tidy.
Behind the radio, about 12” in, lay the connector block with several other grey/blue stripe wires attached. However, it looked very difficult to access this without removing the heater controls and also getting access from one side, so I decided, as this was only dimmable instrument illumination to make a connection using a 3M in-line connector, and so I took this supply from the rear of one of the switches above the heater block, as they all had the grey/blue stripe wire running to them.

The brown wire from the OTG loom was connected to the main ground point behind the instruments, which left only a single wire, which came from the OTG connector, and ran to an existing sensor which is fitted into the back of the speedometer. I have no idea what this does, as the OTG seems to work perfectly without this connected, and this sensor (I presume it is some sort of magnetic pickup by it’s design) has two connectors on its rear, a small double-pole one with a polarity marking, and the single pin, which goes to the OTG display connector. Originally, both of these connectors were not used at all in my car.

I rebuilt the dash, replaced the instruments, reconnected the battery, and turned they key. My OTG now displayed 24 degrees C.

In all, this took me about 3 hours, but of course, like most things, next time it would probably take half that!!

I hope this is of use. After I posted a few questions to the group regarding the possibility of retrofitting an OTG I had several members contact me asking for details if I successfully managed to do this.
Please note that the information above is relevant to my right-hand drive European spec model, and is merely an account of the way I fitted this to my car, not necessarily the correct way.


Barry

1994 E200 Wagon 111,000 miles.

Very Good! With regards to the speedometer connection I have seen mention that new models use a vehicle speed cutout to disable the OTG from updating it's display when the vehicle is not moving to avoid reading that would not be accurate because of engine heat affecting the sensor when the vehicle is at a standstill.

Thanks for your suggestion re the unused sensor.
You could well be right.
It's made by VDO, and while I can't locate the exact part number, on a number of sites which have part numbers listed, it's very close to several existing numbers, marked as "hall effect " switches.

Cheers for your support

Barry

E200 Wagon 1994 111,000 miles.