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kirkt
10-01-2007, 09:19 PM
Hi folks. I recently discovered that the water and oil temp gauges in my SIII lightweight are mechanical, not electrical. For some reason, the water temp gauge's capillary tube was cut at the probe, so no more ether in the system and the gauge does not work. As the gauge internals look almost spotless, I am going to attempt a repair, as outlined here:

http://www.ply33.com/Repair/tempgauge

Anyone ever repaired one of these gauges before? Any words of wisdom?

Thanks,

kirk

http://www.mgaguru.com/mgtech/dash/dt101.htm

has a brief description of the mechanical temperature gauge.

kirkt
10-05-2007, 09:29 PM
Here is a rough draft of a write up for the repair of a mechanical water temp gauge. Comments?

Thanks.

Kirk

kirkt
10-05-2007, 09:29 PM
Introduction

I am in the process of taking care of a few (!) issues involving a 1974 right-hand-drive Land Rover Series III ex-military FFR 1/2 ton (aka Lightweight) named "Pongo". The original vehicle was a FFR (Fitted For Radio) 24 Volt vehicle, quasi-converted to 12 volts by a previous owner. The conversion gets the job done for the basic services (alternator, some electrics). In my quest to compile a To Do list, I started digging through the electrical system. I found a beefy wire from the back of the water temperature gauge dangling loose - the opposite end was a cut stub coming out of the back of the temperature sender threaded into the block adjacent to the thermostat - or so I thought.

I ordered a new sender - electrical sender. Upon closer inspection of the gauge and the sender, I started to wonder where the +12 volts was supposed to go into the gauge. Turns out, the system was not electrical, but mechanical. I had no idea....

The web quickly answered my many questions. The mechanical temperature gauge is a marvel of thermodynamics and is not a relic, but still in use - thankfully. it turns out that the solid "wire" that was cut is actually a capillary tube - a hollow tube. The sender is a probe, a bulb that sits in the coolant stream. The system is sealed and filled with ether. When the system heats up, the ether expands and the vapor pressure with the ether-charged system increases. The increased pressure causes an element within the gauge head (a Bourdon tube) to deflect the indicator needle to the appropriate temperature reading on the gauge face. Very freaking cool and elegant. Also voltage independent, so useful in a 12 volt or 24 volt vehicle, and not dependent upon a voltage stabilizer like the fuel gauge or electrical temperature gauge. This explains why I could not find the oil and water temperature gauges on the 12 volt wiring diagram (http://kirkt.smugmug.com/gallery/2242674#123953305-L-LB) (or 24 volt diagram) for the Lightweight.

I do not know why someone previously cut the old system, but I found that the gauge could be "restored" commercially for a LOT of money - about $100 or more. I entertained the option of replacing the gauge with the Land Rover genuine electrical equivalent (I had already purchased an electrical sender [:o] but that was just as expensive (about $100)). I also thought about an inexpensive, round, aftermarket mechanical or electrical gauge - but that would just leave me with a dysfunctional stock water temperature gauge occupying a third of the stock military gauge cluster, and would not be tidy (read that as "cheesy").

In true thrifty Rover owner spirit, I decided to try my hand at fixing the damn thing. What the hell, Rover owners can fix anything, or at least burn cash and bloody our knuckles trying, right?

Acknowledgments

The inspiration and guidance for this fix goes to Tod Fitch at his website:

www.ply33.com (http://www.ply33.com/Repair/tempgauge)

and his terrific, detailed step-by-step account of his attempt to fix a mechanical temperature gauge on his classic car.

Props to Tod. :)

WARNING

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


Materials

You will need a basic knowledge of sweating/soldering plumbing joints, or at least a basic knowledge of soldering.

- soldering iron, solder, flux
- emery cloth
- 1/8" copper tube (available from the local hardware store)

- ice water bath = ice, rock/table salt, styrofoam cup - the official styrofoam cup is from a value size Chik-Fil-A #1 meal - mmmmmm, Chik-Fil-A.

- Inexpensive mechanical water (or oil) temperature gauge (I used a Sunpro from the local auto parts store). This will be the donor for the repair. Make sure the gauge you choose has the correct NPT (or metric) threaded connector for your block and has a metallic (copper, preferably) capillary tube. Do not get one with a nylon capillary tube or you are on your own.

- Something to cut the capillary tubes on the original gauge and the donor gauge as well as the 1/8" copper tube. I used a small set of end cutting pliers, as the tube/pipe cutter I had for household plumbing only cut 1/8" pipe on the small end of the range. the idea is to use something that will cut a pipe without crimping it shut. I tried a Dremel with a diamond cutting wheel and it still closed off the lumen of the capillary tube.

- Something to boil water and a candy thermometer. I used a backpacking stove. I used this at the end of the process to check that the repaired gauge worked and indicated the correct temperature.

If I have forgotten something, forgive me.

WARNING

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

kirkt
10-05-2007, 09:30 PM
Methods

Please consider reading through the entire methods section before attempting this repair. Please. Before commencing, put the probe of the donor gauge in the ice water and salt bath. It needs to chill to decrease the vapor pressure of the ether within the donor capillary tube so that when the capillary is cut, a minimal amount of ether leaks out. Let it chill while you do all the other prep work.

1) Remove and prepare the original gauge. Here's mine (Figure 1).

http://kirkt.smugmug.com/photos/204163362-M.jpg

2) Here's a side view (Figure 2). Note the black, plastic insulation, covering a fibrous cloth insulation. You will need to cut the capillary tube a few inches from the back of the gauge, leaving enough room to play with, in case the first attempt does not go well. I left about 5 inches or so.

http://kirkt.smugmug.com/photos/204163386-M.jpg

3) Strip away about 2 to 3 inches of the insulation (Figure 3). Make sure you give yourself plenty of room between the solder joint and any of the plastic insulation remaining, as you do not want the plastic to melt and contaminate the solder joint. Note that you want to tin about an inch of the exposed capillary tube but do not tin the area within about 0.5" or so of the tip of the capillary tube, just to be safe and prevent any solder from running into the open lumen of the tube and clogging it.

http://kirkt.smugmug.com/photos/204163412-M.jpg

4) Now comes the delicate part that requires a little patience. Use the end cutting pliers and gently apply pressure to the tube. Turn the tube within the jaws of the pliers to slowly cut your way through the tube without crimping it closed. Take your time. If you are successful, you will see the open lumen in the tube (Figure 4) at the tip of your cut. Use emery cloth and deburr the cut you just made, making sure that the tip of the capillary tube is finished and the same diameter as the tube itself.

http://kirkt.smugmug.com/photos/204163456-M.jpg

kirkt
10-05-2007, 09:31 PM
5) Figure 5 is a detailed view of the tinned and deburred capillary tube, ready to be test fitted to the copper sleeve. Note the area adjacent to the tip that is not tinned and the area between the tinning and the insulation, leaving plenty of room to preventing melting of the insulation during soldering.

http://kirkt.smugmug.com/photos/204163488-M.jpg

6) Figure 6 shows the prepped capillary tube and the 1/8" copper tube sleeve ready to make the first solder joint. When prepping the two components, make sure to use emery cloth on the capillary tube and then apply flux to the tinned portion of the capillary tube. Slide the copper sleeve over the capillary tube so there is about an inch of capillary tube inside the sleeve. Carefully ream the sleeve with a Dremel and small (1/16") drill bit if the inner diameter of the sleeve is too small to admit the capillary tube.

http://kirkt.smugmug.com/photos/204163518-M.jpg

7) Taadaaa! Figure 7 depicts the soldered joint between the original gauge capillary tube and the copper sleeve. Apply flux to the end of the sleeve prior to heating the sleeve and capillary tube. When heating the joint prior to soldering, be patient and bring the tip of the iron to the underside of the copper sleeve, bringing the critical portion of the joint to temperature. Tin the soldering iron and start feeding a steady bead of wire into the joint. Check the joint for voids and apply more solder carefully if necessary. In Figure 7 you can see the "buffer" of stripped capillary tube behind the joint so that the plastic insulation would not melt and contaminate the joint. We're halfway home....

http://kirkt.smugmug.com/photos/204163557-M.jpg

8) Now comes the preparation of the donor gauge capillary tube and probe for transplant onto the original gauge head. Figure 8 depicts our donor gauge capillary tube. This particular gauge has the capillary tube nicely protected from crushing and kinking with a spiral wire wrap. Carefully cut the wrap near the back of the donor gauge head to expose the underlying capillary tube. We are now going to tin an inch of the capillary tube and mark a spot about 0.5" in front of the tinned area where will will eventually cut the tube. DO NOT CUT THE TUBE YET!!

http://kirkt.smugmug.com/photos/204163581-M.jpg

kirkt
10-05-2007, 09:33 PM
9) Here comes the moment of truth. You want to cut the donor capillary tube without letting out the ether in the system. I decided to use a pair of wire cutters to cut the capillary tube first - this will cut the tube and crimp it so that it will not leak. Then you can do all of the prep work like test fitting the tube into the copper sleeve, etc. without losing ether. When everything is ready to go, use the end cutting pliers and carefully cut the capillary tube - be patient so that you do not accidentally crimp the tube shut. Once you cut through the tube ether will begin to leak out. You can momentarily seal the system just by putting your finger over the opening. Apply flux to the tinned area of the tube and heat the copper sleeve with your soldering iron before you take your finger off of the tube opening. Slide the tube into the sleeve and work quickly to apply the solder to the joint as before. You may see the molten solder bubble a little as the ether leaks out while the solder cools. Again Check the joint for voids and carefully applying more solder. Do this sparingly, as reheating and melting the solder may cause more ether to escape. Figure 9 shows how you too can achieve great success.

http://kirkt.smugmug.com/photos/204163598-M.jpg

10) If all goes well you now have your original gauge attached to a new probe and capillary tube filled with ether. It is time to test your handywork. With the probe still in the ice water bath, check the gauge for its zero point. Figure 10 shows my gauge with a slight offset above the 30 degree C mark (low temp). I used some needle nose pliers to make a small adjustment to the needle so that it rested on the 30 degree mark at this temperature.

http://kirkt.smugmug.com/photos/204163637-M.jpg

11) Fire up your heat and boil some water. I used a small backpacking stove and a candy thermometer to check for boiling. Bring the water to a rolling boil, check the temperature and cross your fingers.... If all went well, the needle should shoot up to the red line, indicating boiling. I was amazed that the needle even moved, let alone settled right on the red line on the first shot. Figure 11 - victory.

http://kirkt.smugmug.com/photos/204163678-M.jpg

Give yourself a big pat on the back and grab a cold one. Nice work.

Again, a huge thanks to Tod Fitch and his write up on this topic at his website:

http://www.ply33.com

Kirk Thibault
October, 2007