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Old 09-12-2017, 06:04 PM   #1
BillLee/Chandler, TX
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Default How does water temperature gauge work?

I have one of the after-market water temperature gauges on my A. It uses a stiff cable-like connector between the water outlet neck, through the firewall at the junction box, and then to the gauge which is mounted under the dash.

Today I was driving and the water temperature was indicating just fine. I looked away for a minute or two and the gauge was suddenly reading WAY over on the too hot range. Yet the engine was running normally. In the past when i have over heated the engine, it was quite noticeable in the way it ran. When I got back to the garage I used an infrared thermometer and the radiator showed 190 at the top and 165 or there-about at the bottom. The water outlet was reading about 175 right under where the temperature gauge sensor is plugged in.

Under the hood I noticed fluid of some sort on the head at the back, under the junction box and coil. It looked oily. The ignition cable was wet with it as well as some on the head.

I am looking for knowledge!

Bill Lee
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Old 09-12-2017, 06:15 PM   #2
BILL WILLIAMSON
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Default Re: How does water temperature gauge work?

"Maybe" the temp gauge bulb blew up or the wire wound tube covering shorted against something around the junction box or in the dash tunnel????
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Old 09-12-2017, 07:24 PM   #3
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Default Re: How does water temperature gauge work?

Bill, is it possible that the coil is leaking? They do have oil in them. Jeff
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Old 09-12-2017, 07:31 PM   #4
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Default Re: How does water temperature gauge work?

Usually when a mechanical temp gauge fails it will not read any temp.

I do not think I have ever seen a mechanical temp gauge fail and read too high.

It may be trying to tell you something.

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Old 09-12-2017, 07:32 PM   #5
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Default Re: How does water temperature gauge work?

Torque your head bolts. One is leaking.
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Old 09-12-2017, 07:37 PM   #6
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Default Re: How does water temperature gauge work?

I agree with the shorting passibility. If it did, and heated up the sensing fluid in the tube, that would drive up the gage. Look around for a burned spot on the cable/tube.
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Old 09-12-2017, 08:04 PM   #7
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Default Re: How does water temperature gauge work?

Bill,
Had 3 of the gages (REX-A-CO)purchased from Model A Vendors.
Same symptoms as yours. Pointer pegs Hot, engine temp. normal.
Installed Stewart Warner gages and problems went away
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Old 09-13-2017, 06:25 AM   #8
BillLee/Chandler, TX
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Default Re: How does water temperature gauge work?

I'm still unclear on how the temperature gauge works.

The gauge is not electrically operated since there is no electrical connection.

Is the sensor/tube/gauge full of fluid?

If that is the case, I suspect I had a coil boil and drop hot oil onto the sensor tube right at the back of the engine. (This being the source of the "fluid" on the cylinder head and ignition cable.) The gauge saw that very hot oil on the tube and it showed up on the temperature reading.

I'll check more closely today. This is part of a larger problem that I am working on.
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Old 09-13-2017, 07:11 AM   #9
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Default Re: How does water temperature gauge work?

The gauge has ether in the bulb at the end which expands at a known linear rate as the temperature increases. The gauge itself is a pressure gauge with the dial calibrated in degrees.

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Old 09-13-2017, 07:24 AM   #10
BILL WILLIAMSON
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Default Re: How does water temperature gauge work?

Chuck bought 4 mechanical gauges @ NAPA, before he found one that passed the 212 Degree "BOILING" test ! !
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Old 09-13-2017, 07:56 AM   #11
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Default Re: How does water temperature gauge work?

Is the sensor/tube/gauge full of fluid?


Yes, it's full of fluid
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Old 09-13-2017, 10:11 AM   #12
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Default Re: How does water temperature gauge work?

Quote:
Originally Posted by Pinesdune View Post
Is the sensor/tube/gauge full of fluid?


Yes, it's full of fluid
Well.... it was...
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Old 09-13-2017, 11:49 AM   #13
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Default Re: How does water temperature gauge work?

This information is from the Second Chance Garage.


There are basically two types of temperature gauges, electric and mechanical. Today's cars use electrical gauges but not too many years ago many used mechanical gauges that operated with a Bourdon Tube.
The Bourdon Tube

A Bourdon Tube is a thin metal — usually brass or copper — tube that is filled with an easily vaporized fluid, typically alcohol. It is sealed at both ends. At the gauge end it is formed into a circle or spiral with its end attached to the indicating needle by some form of linkage. The other end is fitted to a water-tight connector that is in direct contact with the coolant in the engine.
As the coolant warms up the alcohol in the Bourdon tube expands. The expansion transfers its force to the coiled end of the tube inside the gauge. As the coil or spiral unwinds it pulls the linkage on the needle, which in turn shows a temperature reading on the gauge face. The gauges are calibrated during the manufacturing stage and are not adjustable afterward.
Since the Bourdon Tube design is purely mechanical the gauge will continue to read some temperature level even after the engine is shut off. As the engine cools the gauge's needle will return to its rest position.
Bourdon Tube gauges aren't used anymore because of cost and convenience factors. The tubes are delicate and must be carefully routed from the dash to the appropriate fitting on the engine. The gauges themselves are far more expensive than electric or electronic gauges and if the tube is kinked or split the entire gauge assembly must be replaced.
Electric Temperature Gauges

Basically, an electric temperature gauge is a voltmeter. The scale on the gauge face is reading temperature but the instrument itself is reading voltage. The gauge itself is comprised of a bimetallic (two different metals fastened together) "hairpin" assembly. This assembly is attached to the needle.
The gauge requires an electric circuit and a sending unit in order to read temperature. The sending unit is a temperature-sensitive material that is part of a variable resistance, water-sealed unit that sits in the coolant stream in the engine. As the engine warms up the resistance in the sending unit is lowered gradually until the system reaches maximum heat. The sending unit is the "ground" portion of the circuit.
In the completed circuit the battery voltage passes from one side of the gauge, through the bimetallic spring and onward to the sending unit, which is grounded to the engine. When the engine is cold the resistance is high, so little current passes through the gauge. This small current doesn't heat up the bimetallic spring, so the gauge reads a low temperature. As the engine warms and the sending unit's resistance lowers more current passes through the gauge and the needle reads higher and higher because the bimetallic spring expands further.


Electric gauges can fail to read accurately because the sending units fatigue or rust over, or simply lose their connection to ground. The bimetallic spring can also fatigue over time, rendering the gauge inaccurate or inoperable.
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Old 09-13-2017, 05:07 PM   #14
BillLee/Chandler, TX
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Default Re: How does water temperature gauge work?

Good information, Bob!

Would you say that the "typical" temperature gauge we add to our cars is Bourdon tube gauge? There is no electrical component to the one I have.
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