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Temperature Gauge

This component is part of Dashboard Instruments.

Definition


Celcius Temperature Gauge

The Temperature Gauge (German: Fernthermometer) indicates the temperature of the engine.

Two versions of the Temperature Gauge are available, one in degrees Celcius (40-120ºC), and one in Degrees Fahrenheit (100-250 ºF).

Normal operating termperature ranges are listed here.

Function

The engine temperature device in Mercedes-Benzes until 1972 are of a capillary tube design, a thin line running between a sensor on the engine and a Bourdon tube movement inside the dial on the dashboard. The systems capillary tube is filled with diethyl ether.

Operation of the Gauge: The gauge dial is activated by expansion or contraction of the filling in the approximately 3 foot (1 meter) long cable which terminates with a sealed thermo unit. The cable is filled with a substance that expands when heated to operate the gauge. Once the cable is broken (pin hole, cut, etc.) the filling in the cable escapes and the unit will no longer operate. Note that if the outer cable covering is torn or cut but the inner filled cable “tube” is intact, the gauge can continue to operate.

Maintenance

Testing the Gauge

The gauge can be tested by removing the cable from the engine. In a safe place hold a small flame or other heat source to the brass end; can test by dipping the end into hot water. If the dial does not move, the gauge must be repaired.

  • Repair Options: The entire cable must be replaced and a new cable soldered to the gauge. Merely patching or taping an open part of the inner cable will not allow the gauge to work unless the cable filler material is restored.
  • A method to transfer the filler material from one gauge to another while working in a freezer is as follows:
There is a way to repair it believe it or not, if the gas has escaped. Take a similiar bulb from a cheaper unit. Either place the unit in the freezer or more preferably in dry ice. The gas inside will condense. Cut the capillary tube on the gauge close to unit. Make a splice tube that will fit over both capillary tubes and solder it to the old gauges' tube. After you have cooled the new cheap gauge and sender pull it out and cut the tube to the right length and quickly solder the bulb side of the tube to the splice. If you can keep the bulb in the dry ice it will keep more of the gas in the bulb. After you get it together try the water test again with a thermometer and see how close it is.
  • As the “freezer transfer” might be impractical for most of us, a better option might be to have the unit repaired. In January 2008, in the USA there are two repair shops in California that repair these gauges with a one year guarantee (takes 2 or 3 weeks, about $150). These service shops are listed in the Suppliers List. There may be other shops that do similar repair.
  • Another option is to order a new temperature gauge from Mercedes Benz. A new Celsius dial gauge is available for about $350 (January 2008). Not sure if the Fahrenheit dial gauge is available.
  • SLS in Germany (see our Suppliers List) offer both Celcius and Fahrenheit gauges with complete units.

Figure 1

Removing the Temperature Gauge from the car

  • Remove the cable end from the engine port. This port should be plugged with a same size dummy fitting to prevent water leaking if the engine is started with the cable removed. (When the Center Instrument cluster is removed the dash end of the oil sensing cable must be plugged with another oil gauge to prevent an oil spill in the car if the engine is started.)
  • Remove the Center Instrument cluster from the dash (see Dashboard Removal).

Disassembly of the Temperature Gauge from Instrument Unit

  • After the Center Instrument Unit is removed from the car, the Temperature gauge can be removed to allow repair. These steps also apply to the other Center cluster gauges which are removed in a similar manner.
  • The example Center Unit is from a 1969 280 SL (VIN ending near 007000). The Gauge is a Celsius dial unit. A Fahrenheit dial gauge unit is removed the same way.

Figure 2
  • At the back of the Cluster unit take off the two knurled nuts on the unit-holding bracket if they have not been removed. Remove the four flathead screws holding the light brown Bakelite back to the unit. Note the one long screw at the top edge of the back.
  • Evenly and carefully pull the back away and off the unit. Careful of the long pin that connects the rheostat to the light dimmer dial.
  • Each of the four gauges has its own aluminum cover. These covers are held to the back of the Central Unit by two slotted nuts. Only one nut must be loosened for the two gauges on each side of the unit.
  • Loosen the slotted nut holding the Temperature gauge back panel. The nut does not have to be removed, just loosened. The gauge can now be twisted up and out of the Center Unit.



Figure 3

Note that the two flat head screws on each side of the temperature gauge cable are NOT loosened or removed. These two screws hold the aluminum back plate to the temperature gauge works. This plate is not removed when sending the gauge for repair. The entire gauge with back plate, inner works, cable, and engine port fitting is sent to the repair shop.

  • Suggest changing the dried out grey seals surrounding the Central Unit, the Speedometer, and the Tachometer (rev counter) while access is easy. Also may be time to replace the 2 watt bayonet base bulbs in all the units. (Can use 4 watt bulbs for a brighter dash.)



Figure 4

Installation

Installation is the reverse with a few cautions:

  • When re-installing the back of the unit, make sure the long pin is correctly mated with the rheostat panel light control knob. Before pushing the pin all the way in, you may have to turn the knob back and forth a few times while gently pressing down on the cluster back to orient the flat part of the pin end with the knob. When turning the knob moves the rheostat slide back and forth, the pin is in the correct position.
  • The temperatue gauge cable and electric cable are crowded very tightly next to each other. The gauge cable might have to be moved to make room for the electric cable. Use a very slight and gentle push when moving the temp cable. Rough handling of this cable can cause a break and require repair of the temp unit. The photo above of the assembled cluster shows the tight fit of these two cables.


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