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8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc

410 water temperature gauge

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Old 01-01-15, 02:00 PM
Join Date: Mar 2011
Location: Walton on the Naze, Essex, UK
Posts: 42
Default 410 water temperature gauge

The Water Temperature Gauge on my 410 does not work. I can't find anything about it in the handbook or the workshop manual so am stumped.

The fact that it does not show on the wiring diagram leads me to think that it is not electrical and there is a coiled uninsulated wire coming from behind the dash to a plug on the inlet manifold just behind the thermostat housing that I assume is the sensor mechanism. Can anyone confirm this?

Is there a way of testing whether it is the actual gauge or the temperature sensor that is broken or is it possible to get the whole assembly? I have seen water temperature sensors for sale on ebay but these all appear to be electronic types. Also it is possible to but a complete gauge and sensor but these would not look right.

Any advice or experience would be gratefully received.
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Old 01-01-15, 06:30 PM
Join Date: Sep 2009
Location: Bologna, Italy
Posts: 95


yes, there is a water temperature gauge and the bulb in my 409 (same engine as the 410) is located in front of the carburetor, I hope that this picture is clear's a purely mechanical/thermal gauge and yes it looks exactly like what I had in my old MGs but with a longer coiled wire and goes to a single-needle dial.

Have a nice time

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File Type: jpg 409 engine water temp bulb.jpg (359.8 KB, 17 views)
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Old 01-01-15, 06:38 PM
Join Date: Mar 2011
Location: Walton on the Naze, Essex, UK
Posts: 42

Thanks Stefano

Yes, that is what I thought it was. I have been searching the internet for information and it seems that if the unit has broken the only solutuion is to replace it. The capillkiary tube is a sealed unit and can't be repaird it seems.

Happy New Year.

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Old 01-01-15, 07:46 PM
Join Date: Apr 2009
Location: York
Posts: 808

Maybe of use !? From an old USA magazine article.... Also, Speedy cables in the UK offer a repair service for original units and Bristol Cars offer replacements.

The temperature gauge is a “mechanical” unit consisting of a bulb with fluid (ether) mounted in engine head, a gauge head mounted in the instrument cluster on the dash, and a tube connecting the bulb to the gauge head. The gauge head is actually a pressure sensing unit.

In operation, the coolant in the engine head heats the working fluid in the sensor bulb. As the engine gets hotter the pressure in the bulb and tubing rises. The dash head unit simply reads the pressure on a scale calibrated in degrees Fahrenheit.

A common failure mode is for corrosion to lock the bulb in the engine head. The tubing is twisted off near the bulb when bulb removal is attempted. In this case, the gauge head unit is known to be working properly, the defect is in the tubing and in the loss of fluid from the sealed system.

For a more detailed description, a number of articles about the operation of this type of temperature gauge have appeared in Skinned Knuckles over the years. The earliest article I have is from the March 1984 issue.
Replacement parts

Mechanical temperature gauges are still being manufactured and available in most auto parts stores. The goal of this procedure is to graft a new bulb (with fluid) and tube onto the old gauge head.

Find a gauge that has a spiral wound protective cover over the tube. There are some that use a plastic coating. While these can be made to work, they will not look correct. As of October 2004, it is possible to find a suitable donor gauge in California auto supply stores for about $15.

The Procedure

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.

Sleeves made from thin walled copper tubingMake up a sleeve from some tubing. In this case the capillary on the donor gauge had a slip fit into 1/16" inside diameter copper tubing found at the local hardware store. The capillary on the original gauge was slightly larger, so the 3/4" long sleeve was drilled out halfway through to make a slip fit for the old capillary.
Salt and ice to make a cold bathTo keep the ether from escaping we use some crushed ice and salt to create a “cold bath” for the sensing bulb.
Sensing bulb being chilledAnd we might as well be chilling the bulb while we do all the other preparation work.
Trimming the capillary tubing on the original temperature
gaugeTrim the capillary tubing on the old gauge a couple of inches from the gauge head.
Checking that the tube is not been closed offMake sure that the tubing has not been closed off when trimming to length.
Tinning the original capillary tubingWe might as well do as much as we can before cutting open the donor gauge. So the next step is to “tin” the capillary on the old gauge near the cut. You don’t want to tin all the way to the cut as you don’t want to encourage solder to fill the capillary opening.
Attaching the copper sleeve to the original capillary
tubeNow we can attach the sleeve to the tinned capillary. In this case we seated the capillary tubing to the limit that we opened up the diameter by drilling.
Checking that the capillary tube is still openAgain we want to make sure we have not closed off the capillary. A piece of stiff wire small diameter can be used to verify that the capillary is not clogged with solder.
Cutting the protective spiral spring off of the donor gaugeCut the protective spring on the donor gauge capillary near the point marked for the length you need (indicated by the tape).
Tinning the capillary tube on the donor gaugeIn the spirit of doing as much work as possible before breaking the seal on the donor unit, we now tin the part of the capillary that will be inserted in the sleeve.
Soldering the new capillary tube into the copper sleeveAlmost done! Verify that the sensor bulb is still in the ice solution. Cut the capillary tubing, assure that the center is open and now solder it into the sleeve.
Inspecting the joint prior to removing the sensor bulb from the cold bathBefore removing the sensor bulb from the ice solution, double, triple and quadruple check that you have a good solder joint with no voids or other possible leaks on both sides of the sleeve. When you are happy with your work it is time to check the function.
Testing the gauge with boiling waterImmerse the sensor bulb in boiling water while observing the needle of the gauge. It should move up to the 212 F mark. While observing the dial mechanism, you might want to cycle the sensor bulb between the ice solution and boiling water a few times to verify that nothing is binding the movement. The gauge should now be restored to operation.

If the gauge does not read 212 F in boiling water you have two options:

Note the error and live with it.
Attempt to adjust the gauge head unit.
The gauge head is a Bourdon tube connected to the indicator by a linkage. The Bourdon tube is simply a flattened tube rolled into a coil. As pressure is applied the tube slightly unwinds. When the pressure is removed, the coiled tube returns to its original position.

Adjustment is made by bringing the sensing bulb to a known temperature by placing it in boiling water (212F) then bending the linkage that connects the Bourdon tube to the indicator. Do not bend the Bourdon tube itself. [Chrysler 1953 page 70] States that it is possible to adjust the gauge if its reading is less than 30 different than the actual temperature.

If you have any doubts about the adjustment operation, then don't do it. You can buy replacement sensing bulbs and tubes at any auto supply store. Getting an original gauge dash head is a lot harder.


Avoid sharp bends or kinking the tubing when installing the repaired gauge.

Last edited by GREG; 01-01-15 at 07:52 PM.
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