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DIY repair of a mechanical temperature gauge
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TOPIC: DIY repair of a mechanical temperature gauge

DIY repair of a mechanical temperature gauge 3 years, 3 months ago #2245

  • YLG80
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  • Posts: 664
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Vintage Temperature Gauge repair.

Two years ago, I've removed the temperature gauge from my WWII Jeep Ford GPW in restoration.
The capillary tube was broken at the sensor bulb level screwed in the engine head.
This is an original painted can SW temperature gauge.


The bourdon tube (detailed picture)


I've decided to try to repair that gauge myself, but the first trials were unsucessful.
At that time (during the winter when nothing else can be donne on the jeep), I've replaced the capillary tube
and resoldered the tube to the gauge and the sensor.

Easy task!

But refilling the sensor with ether was another challenging problem.
Despite of several different techniques, I was simply unable to inject ether into the circuit.(with a syringe)
Today, I'm still wondering how they were making that operation in mass production.
If someone has that information, please share it with us.
So I've temporarly left that project aside while finishing my GPW restoration, with a repro gauge, but still trying to find the solution to that tricky problem.

Today, I've eventually found a surprisingly easy way to refill the gauge.

It uses what I would call "thermal pumping" or "thermosiphon".

This is an interesting method (and exciting for me) because this the application of a well known physical rule expressed in an equation : P * V = nR * T (Boyle & Mariotte)
(see wikipedia with a nice animated explanation : http://en.wikipedia.org/wiki/Boyle's_law

The gauge refill "thermal pumping" method is explained a pdf file.


Summary :

1- Ether is difficult to handle because it has a boiling temperature point of 35°C.
If you simply take a syringe filled up with ether in your hands (37°C), you will immediately see an ether jet coming out of the syringe.
The syringe will be empty in a matter of a few seconds.
2- Ether is a very good organic solvant, which does not help when in contact with plastic parts (syringe and piston gasket)
3- Ether is difficult to buy. Here in Belgium, you have to give your ID card and fill up papers if you want to buy it in a pharmacy.
Likely just to be sure that you don't buy it for sniffing !
4- Fortunately it's very easy to find a substitute for ether : a Start Pilot (or equivalent of other brand) spray used to ease the engine
start during the wither. It contains ether.

Thermal pumping

As explained above, the ether does really not want to be injected in the gauge circuit. The first obvious reason is the presence of air in the circuit.
In the document, I will explain a workaround to that issue using another spray commonly used by electronicians (as I am) to debug breakdowns in electronic circuits.
This is called the "Super Cold" spray, or -51°C/52°C spray.
With that spray, the technician can quickly cool down a chip or a part of the circuit to track down thermal or intermittant breakdowns. That's a really efficient debugging method.
We will use that spray to quickly cool down the gauge bulb and the bourdon tube to pump the ether into the bulb, the capillary tube and the bourdon unit.
It takes less than a minute to complete the operation.

What you will need

- The gauge and its sensor. If the sensor is missing you could build up a new one with an brass tube. I give the original sensor dimensions.


- The gauge mechanism + bourdon unit should not be broken or modified. This would make it almost impossible the recalibrate.
- A capillary tube if you cannot afford to use the remaining length on your broken gauge. External diameter of a .078'' to .08 ‘’
A capillary tube can be found on eBay or in an old gas refrigerator, or in a old gas water boiler.
This the tube connecting the flame sensor to the safety valve that switches off the gas supply.
- Medical ether or a Start Pilot spray
- A Super Cold Spray -51°C spray used to debug electronic circuits. Such a spray can be found on Amazon for about 9.90 U.S. $


- A 100W-150W soldering iron to seal the circuit after refill (no flame for safety)
- A gas blow torch to resolder the capillary tube and sensor parts if necessary. You could also use your soldering iron.

For the complete method, please read the pdf attached.

File Attachment:

File Name: RepairTemperatureGauge-rev1-022014.pdf
File Size: 418 KB

File Attachment:

File Name: Reparation-ancienne-jauge-temperature-rev2-022014-FR.pdf
File Size: 746 KB

Contrary to what I’ve read elsewhere in this forum, you can repeat the process several times as long as you don’t modify the bourdon spring mechanism.
If you miss the refill, just remove and replace the sensor capillary termination used to refill the circuit and retry.

I hope that this method will help you with the repair of you valuable original gauge.
Please share you experience with us.

Last Edit: 1 year, 11 months ago by YLG80. Reason: add file in French

Re: DIY repair of a mechanical temperature gauge 3 years, 3 months ago #2248

  • YLG80
  • Moderator
  • Posts: 664
  • Karma: 26
To complete the information on my repair method, I've checked the calibration of the repaired gauge against a °C thermometer and by calculation in °F.
Here are the results :


The accuracy is not good for low temperatures, but who cares providing that the accuracy is good in the useful range, between 60°C and 100°C.
(Take also into account the reading error on both instruments, one with a needle and the other with a red alcool index.)

I could not reach 100°C, but I believe that the reading would have been close to 212°F.

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