Mechanical temp gauge repair

Rat Rods Rule

Help Support Rat Rods Rule:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

offroadrolls

Well-known member
Joined
Mar 20, 2012
Messages
1,333
Location
North side of Deer Mountain
Not really an electrical question but kinda.
The mechanical temp gauge in my old Volvo does not work, you know, the kind with a capillary tube. So I did some investigating to find out how they work. I learned the bulb on the end of the capillary tube that’s screwed into the engine is filled with a gas, ether to be specific. When it gets warm it expands thereby actuating the mechanical gauge on the dash at the other end of the tube. It’s a sealed unit. If the tube gets crimped it will not work. Or worse, if it gets a hole in it the gas escapes rendering the gauge inoperable. So continuing my studies I found that they can be repaired by grafting a good tube with its gas from a working gauge to the tube on your gauge. The trick is you have to keep the bulb from the good gauge in ice water lower than the rest of the tube while you cut the tube and solder a copper splint you make to graft the 2 tubes together. Keeping the bulb from the good gauge in ice water keeps the ether liquefied in the bulb so it won’t escape the tube when you cut it. Sounds like a good way to waste a Saturday afternoon. I’m gonna give it a shot. Has anyone ever tried this? Or does anyone know a better way to repair a mechanical temperature gauge?
 
Might work. I always thought if you wanted one repaired you had to send it out to a shop. Worst case you ruin a good gauge and waste a day doing it.
 
How did this work out?

I assume using the good gauge was not an option.

The only other question that comes to mind is, Is the capacity, and therefore the message it sends, the same as the one being replaced? This could have an effect on the accuracy of the gauge after the repair.
 
I would think they are made in a vacuum and introducing air to the system would definitely effect accuracy.
 
I haven't gotten around to fixing it yet. Just too busy keeping shop trucks and DD's on the road on top of my regular work. I have thought of all things mentioned above though, volume, vacuum etc. I'll post something as soon as I get it figured out.
 
Dang, Offr, I must have read the same article and I really want to try fixing one of those gauges,------- second, right after you try it and tell us all about it. I have a '34 Dodge brothers gauge in a near perfect dash that should be rehabilitated, instead of slapping in a Princess Auto special. Good Luck.
 
OK so I had some time today to get to the repair of this old Volvo gauge. First, I'd bought a cheapo mechanical temp gauge at Advanced Auto parts to use as the donor for its capillary bulb and tube......$17.00

attachment.php

attachment.php


Next I made a few coupler tubes out of some brass round stock we had laying around. The tube on the old Volvo gauge was .058" and the new tube is .054".
I decided not to bother trying to match each end of the coupler to the size of its prospective tube and drilled it .059". The guy on the post on the hamb said to go to the hardware store and get some copper tubing to use for the coupler. I didn't try that because I figured the closest I'd get is .062"

attachment.php


I made extra couplers because I wanted to get plenty of practice before I performed the actual operation. I tried using a normal plumbers type propane torch and found it to work so good that as the solder sweat into the joint it would go too deep and plug the hole in the capillary tube. I also tried using a soldering iron like the guy on the hamb but I couldn't get it hot enough because of the thickness of the coupler material. Last I tried a jewelers torch and found you could still get it too hot like the plumbers torch but it was much easier to control. Here's one of 4 or 5 practice sessions using pieces of the old Volvo gauge tubing and the extra couplers.

attachment.php

attachment.php


So here's soldering the coupler to the gauge. After 4 or 5 test runs I found the best way was to pre-tin the tube right where it entered the coupler about 3/4" from its end. Then using the jewelers torch get it just hot enough to lay the solder on top of the joint but not so hot it sweats into the joint. Every time I got it too hot it would go too far and plug the hole in the tube.

attachment.php


The next part I forgot to photograph so I'll just explain; I put the bulb of the new donor gauge in a big cup of crushed ice, salt and snow down on the floor about 38" below my benches work surface. The inferred thermometer said it was about 24 degrees. After about 10 minutes I cut the donor tube with good scissors. (note: scissors work better than wire cutters because they shear the tube instead of it pinching it closed.) I pre-tinned the tube in the area where it would enter the tube about 3/4" from the end where the hole was. Then I stuck it in the tube and soldered it using the jewelers torch while being careful not to get it too hot and just lay the solder on top where the tube enters the coupler. Here's the finished product.

attachment.php


Next it was time for the big test! I used the microwave and dropped the new bulb into 185 degree cup of water...........

attachment.php


........IT'S ALIVE!! [cl

attachment.php


So in conclusion, the gauge reads high but not so high I can't live with it. Also, 180 is half or straight up normal and 190 is at the top end of normal.
I'm guessing the donor gauge had more ether in it than the old Volvo gauge was designed for. I suppose I could freeze the bulb, de-solder one of the coupler joints and empty some of the ether but that would be.........nutz!
 

Attachments

  • IMG_2254.jpg
    IMG_2254.jpg
    116.5 KB · Views: 44
  • IMG_2255.jpg
    IMG_2255.jpg
    110.5 KB · Views: 44
  • IMG_2256.jpg
    IMG_2256.jpg
    99 KB · Views: 42
  • IMG_2263.jpg
    IMG_2263.jpg
    148 KB · Views: 5
  • IMG_2264.jpg
    IMG_2264.jpg
    174.1 KB · Views: 41
  • IMG_2261.jpg
    IMG_2261.jpg
    57.6 KB · Views: 39
  • IMG_2262.jpg
    IMG_2262.jpg
    115.8 KB · Views: 39
Thank you Oroad, you've given me confidence, ------- and some trepidation. You've blazed a trail for us.

LOL! To be honest it was not all that difficult with some patience. One does needs to remember the first rule of sweat soldering copper pipe...everything needs to be clean.
I forgot to give Skip cred for calling the results of volume in an earlier post.
I was able to bend a small piece of the linkage in the back of the gauge helping it to read a little less to the hot side. Today I might try enlarging the 2 face plate screw mount holes so I can try rotating the face plate a bit. A coworker suggested I put a 160 degree thermostat in the engine, ha ha.
 
Nice going.
icon14.gif


Team work.

I'd say as long as you know where "good" it, who cares where the needle points. You could put a smiley at optimum temperature. :D
 

Latest posts

Back
Top