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Why doesn't my TEMPERATURE GAUGE work? - DT-101

safety gauge with no temperature reading This one is at the top of the list of most often asked questions about MGA dash gauges. When I tell you how the analog temperature gauge works, you might already figure out why it doesn't.

Internally the water temperature gauge is actually a pressure gauge, nearly identical to the oil pressure gauge. This is why these two gauges are quite often assembled in a single instrument housing, as they are easy to manufacture on a single assembly line. This is commonly called a "safety gauge", as it monitors safe operation of your engine. Otherwise it may be called a combination gauge, or simply oil/water gauge.

Thermal sensor bulb
There is a small fluid bulb, about the size of two joints of your pinky finger, screwed into the side of the cylinder head just below the thermostat housing. This is connected to the back of the temperature gauge with a long stretch of small bore tubing. There is a steel wire coiled around the small tube for physical protection. The bulb and line are filled with a volatile fluid, most commonly ether, which boils and produces vapor pressure when heated. The vapor pressure is roughly proportional to the temperature change, and this pressure drives the temperature gauge.
Connection on back of gauge
Internally the oil pressure and water temperature gauges are very reliable. Treated kindly, the temperature gauge may continue to work indefinitely. In fact many of the original instruments from the 1950's are still working well in the next century. The most common cause of malfunction of either gauge is the loss of a pressure signal. For the temperature gauge, all that is required to kill it is a small crack in the fluid line that would allow loss of the working fluid. Unfortunately, many people find out the hard way that the small fluid line is easy to abuse and fracture. Some people may even cut the line thinking it's a wire that might be easily reconnected. Ouch! This little mistake can be expensive.
ViceGrips on flare nutFlare Nut Wrench on flare nut

Use no ViceGrip, as this will surely damage the thin wall hollow nut.
Use only a proper flare nut wrench on the temperature sensor.
Save the nut, save the fluid line, and save yourself some money.

The repair for the broken fluid line is generally not a home garage project (unless you happen to be handy with laboratory equipment). The small tube is first soldered to the fluid bulb. Then the flare nut is installed, because it cannot be removed or installed with the line connected to the gauge. The bulb and line are placed in a vacuum chamber to evacuate air from the assembly. The open end of the line may be closed with a temporary seal, then the vacuum can be relieved to retrieve the part. The end of the tube is then submerged in a vessel of liquid ether (generally kept on ice to avoid boiling), and the seal is removed. This causes atmospheric pressure to force ether into the tube to fill the bulb. The bulb can be placed in ice water to keep the ether in liquid form. The open end of the tube is then soldered to the back of the gauge to affect a permanent hermetic seal that will contain the working fluid indefinitely.

Most people do not have the required equipment in hand, so the expedient move is to send it out to a pro shop to be repaired. You might expect this repair to cost about $90, and add about $5 for shipping each way. I have known a few pro shops to charge considerably more than others (I hate to say gouging), so it can pay to shop around a bit. Here are a couple of sources for this service:

Mo Ma Manufacturing - instrument repair
1321 Second Street. NW
Albuquerque, NM 87102
Ph (505) 766-6661 - Fax (505) 766-5419 - Email momanm@qwest.net
Say hello to Margaret Lucas

West Valley Instrument Specialists - instrument repair
Now a division of West Valley Auto Electronics
19314 Vanowen St.
Reseda, CA 91335
Ph (818) 758-9500 - Fax (818) 758-9504

Addendum 1/22/07:
There is an alternate repair you might do at home without the fancy lab equipment. The idea is to buy a cheap analog temperature gauge from a local auto parts store, put the thermal bulb on ice, then cut and graft the signal tube with the ether in it to your old gauge. See off site instructions here: http://www.ply33.com/Repair/tempgauge.

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