Repairing Mercedes-Benz Ponton 
Temperature Gauge

Overview

The temperature gauge is a mechanical unit consisting of 01) a bulb with fluid (ether) mounted in the engine head, 02) a gauge head mounted in the instrument cluster on the dash, and 03) a tube connecting the bulb to the gauge head. The gauge head is actually a pressure sensing unit.

In operation, the coolant in the engine head heats the working fluid in the sensor bulb. As the engine gets hotter, the pressure in the bulb and tubing rises. The dash head unit simply reads the pressure on a scale calibrated in degrees Celsius.

A common failure is for corrosion to lock the bulb in the engine head. The tubing is twisted off near the bulb when bulb removal is attempted. In this case, the gauge head unit is known to be working properly, the defect is in the tubing, and in the loss of fluid from the sealed system.

Repair Procedure

Caution: Ether (the working fluid in the gauge) is a highly flammable substance. Do not have any open flame near your work area. Do not use a torch for the soldering operations.

First I repaired the break at the bulb end by removing the old bit left in the bulb. It was only soldered in. DO NOT use a torch! A soldering iron is just fine. Then I cleaned the other end of the capillary tube and pre-soldered it, and put it back in the bulb and soldered it closed.

Note that there might be a small amount of ether left in the bulb — you can smell it. Soldering will evaporate the remains, but a blow torch WILL give a small, but nice explosion.

The system is now closed, but won't make your gauge needle move because once you cut the capillary tube, too much of the ether will have dripped out or evaporated. So it has to be refilled, which is easier than many people think.

I decided to refill from the middle of the capillary tube, so cut it there with a Dremel tool, and cleaned the edges of the tube (as well as an inch of both ends) with fine sandpaper — preparing to pre-solder. Try to pre-solder in such a way that you leave 3 mm of the capillary ends free from solder. You don't want to block the capillary tube when we solder it all back together.

I made up a 3 cm long sleeve from 2 mm outside diameter (O.D.) brass tubing (sourced from a model shop). The capillary tube of the temperature sensor is 1.2 mm O.D. I drilled both ends of the 2 mm brass tube out with a 1.2 mm bit, and made sure the capillary tube fit snug in both ends of the 2 mm brass tube.

Filling with Ether

I found no need for an "ice bath." Purchased 200 ml of ether from eBay (£15). Whereas 5 ml would have been sufficient, I could not find a smaller quantity to buy. With a 5 ml syringe filled with 2 ml of ether and a 25G x 5/8 inch needle, which fit very nicely in our capillary tube, I first sucked as much air out of the tube as possible and then squeezed the ether in, then pulled the plunger again to suck more air out, and pressed the plunger again to let some ether in. Then pulled again to suck, etc. It takes five or six pull-push plunges to fill the dial mechanism end, and the same for the bulb side. You can tell when both sides are full by removing the needle and covering the bulb with your hand — some ether will come out of the capillary tube. Do this for both ends. Then put the needle back in and do one last pull-push with the syringe. Do this for both sides. Then leave the needle and syringe in on both ends of the capillary tube. No ether will escape. Get the soldering iron and sleeve ready, and quickly remove needles and push both ends in the sleeve. Now solder both ends closed. DO NOT use too much solder. Make sure the joints look properly closed. DONE.

Testing and Adjustments

Immerse bulb in kettle and start to boil, and you will see the dial slowly going up to 100° Celcius. While observing the dial mechanism, you might want to cycle the sensor bulb between cold and boiling water a few times to verify that nothing is binding the movement. The gauge should now be restored to operation.

If the gauge does not read 100° C in boiling water you have two options: 01) Note the error and live with it. 02) Attempt to adjust the gauge head unit. The gauge head is a Bourdon tube connected to the indicator by a linkage. The Bourdon tube is simply a flattened tube rolled into a coil. As pressure is applied, the tube unwinds slightly. When the pressure is removed, the coiled tube returns to its original position.

Adjustment is made by bringing the sensing bulb to a known temperature by placing it in boiling water (212°F) then bending the linkage that connects the Bourdon tube to the indicator. Do not bend the Bourdon tube itself. [Chrysler 1953 page 70] states that it is possible to adjust the gauge if its reading is less than 30° different than the actual temperature.

If you have any doubts about the adjustment operation, do not attempt it. You can buy replacement sensing bulbs and tubes at any auto supply store. Getting an original gauge dash head is a lot harder.