Temperature Gauge Notes 
(Applicable to 544/122 only.  Early [carbureted] 1800s had a system using similar technology, but with a different Coolant/Oil temp combination gauge on the dashboard.) 

5/2002  R.Kwas  Revisions on-going

Twisting Off Temp Sending Bulb
Overheating vehicle...are you sure?
Checking Calibration

Best Practice for Installing Temperature Sensing Bulb


The Temperature Indication on vintage 544/122 Volvos is by way of a "Filled Thermal System", really a thermo-mechanical system consisting of a sensing bulb, located at the back of cylinder head, connected by a capillary tube (easily identified by its' spiral or braided wire protective wrapping), to a Bourdon Tube  gauge (actually a low pressure indicator) having a non-specific scale in view normally, (but actually calibrated in Degrees Celsius if observed from a low viewing angle).  It is therefore a Temperature Indicating System (TIS), and that is what it will be referred to as in these notes.  The TIS is filled with ether so takes advantage of that material's thermal property of being liquid at ambient temperatures when contained, and gaseous at elevated temperatures.  Within the TIS this state change brings with it a pressure change, and this is what is actually displayed on the Dashboard.  The TIS is self-contained and other than the electrical power required to light it up so we can read the gauge at night, needs no electrical power to actually function.  Reader will have noticed that the gauge indication is not affected or bothered by the Ignition Switch position!   

ANY compromise of the capillary tube large enough to allow loss of the tube content, (so this doesn't have to be very big, and can occur anywhere along its' run, from cylinder head to instrument cluster) will cause the loss of the precious ether from the system, and render the TIS functionless.  This might be through years and miles of vibration (non-working gauge, and we didn't specifically damage it), or more direct mechanical insult (tube was twisted off as we tried to remove it, because sensing bulb was gummed into over-nut.  Note 1).

Note 1.  Twisting Off Temp Sending Bulb (TSB):  This is the most common reason for compromise and failure of the Temperature Indicating System of vintage Volvos...use penetrating lube and work EXTRA-CAREFULLY and slowly to avoid making this costly mistakeDo NOT allow capillary tube to twist more than a few degrees as over-nut of Sensing Bulb is loosened!  Loosen nut slightly and apply penetrating lube or solvent like Carb Cleaner to slowly and carefully free nut from Sensing Bulb!  These actions must be undertaken carefully and meticulously to have any chance of avoiding turning the TIS into scrap!   See:  Best Practice for Installing Temperature Sensing Bulb


Temperature Gauge of a 122 (544) removed from Instrument Cluster. 
Visible are the Calibration marks.


Often, the question arises if the operating temperature of an engine "is normal"...this is simple question, but with a complicated answer!  A bit about the Cooling System here (...much more here!).  First of all, the engine operating temp is set by the Thermostat (or T-Stat), as it regulates the coolant flow to the Radiator at a rated temp, and thereby determines rate at which Cooling System sheds the extra heat generated by the engine.  Once engine has warmed-up, and a stable thermal condition has been reached, the exact temperature we see indicated is therefore a function of the actual temp, plus any error in the indicating system...so it would naturally make sense, that before questioning the engine temperature, we confirm the accuracy of the TIS. 

Overheating vehicle...are you sure?  If we add a cal mark for 90C to the above picture, add to that a few percent calibration error for a 50+ year old instrument, we notice the normal indication for an 92C would put the indication quite near the upper white area, maybe alarming someone who is used to seeing the Temp Needle on their modern vehicle, in the lower third of the indicating range! 

Temperature Gauge with a cal mark for 90C,
 and some reference temps for common T-Stats added!

Given the absolutely normal reading one can expect is already on the high side of what one might be used to from a modern car, if one is still concerned with the Temp Indication, and that their engine is (or just might be) running hot, we might want to check the calibration of the gauge system next, so we know we can trust what it is showing us! 

Checking Calibration:

The temperature sensor bulb, connecting tube and gauge are a sealed, single assembly, and if the gauge needle is indicating at all, can be considered to be working...the exact accuracy of the needle is another story...but this may be simply checked...by removing the Temperature Sending Bulb from the rear of the top of cylinder head (carefully, without compromising the capillary tube!), placing it into a Styrofoam cup, filling the cup with (truly) boiling water, and noting the position of the gauge pointer. There are calibration marks on the upper edge of the gauge face (use a flashlight and look from below to see them), the high temp cal mark (100 deg. C) is pointed to when pointer is in middle of upper white region. If that's not were the needle winds up, it's just age of the gauge making itself known.  Simply make a mental note of where pointer IS, and keep that in mind later, when driving, and you look at that Temp Gauge (better yet, make a little mark or even a pointer of a sliver of Red tape on the glass of the gauge - if you decide on this method, assure you apply it while in the normal driver's position to avoid Parallax Error.  Parallax Error explained:  https://en.wikipedia.org/wiki/Parallax#Parallax_error_in_measurement_instruments  ). 

Adjustment of the Bourdon tube (actually a pressure gauge) needle is possible, but I don't recommend it for the inexperienced or those not comfortable with micromechanics. 

This TIS design is very good in that needs NO electrical power, and has a virtually instant response time (certainly much faster than the ngine could ever change temp!)...so it will even show the temp drop as you raise the revs and coolant circulation improves...or it will show temp climbing past the normally indicated area that one is used to, only to suddenly fall as the Tstat opens the first time...this is perfectly normal!  It is recommended that cooling system can hold pressure, which makes it more efficient.  Yes, it IS a bit disconcerting to see the temp needle take off toward and into the red (white) after shutting OFF the engine, but it's also perfectly normal due to a bit of post shut-down heatsoak which occurs, because as a filled thermal system not needing electrical power, it is still indicating after the engine and therefore coolant circulation is shut OFF.

[This info is also found on Service Notes Page.]  



Reference Information:

Bourdon Tube:  From a popular reference site ( https://en.wikipedia.org/wiki/Pressure_measurement#Bourdon) :  The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its circular form in cross-section when pressurized.  Although this change in cross-section may be hardly noticeable, and thus involving moderate stresses within the elastic range of easily workable materials, the strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized.

Thread Size for Temp Sensing Bulb (TSB) Over-Nut into B18/20 Cylinder Head:   5/8" UNF (or 18TPI). 

Best Practice for Installing Temperature Sensing Bulb into Cylinder Head (Anti-Seize and Gasket Sealer are both required!):  Generally, capillary line should be routed carefully, with no sharp bends and with vibration loops, but specifically at the Sensor, Anti-Seize can be optionally applied to threads of over-throw nut at TSB, but more importantly between interfacing surface of nut and bulb so that nut is able to turn with respect to bulb, in the future...uniting them with gasket sealer is not only totally unnecessary, but will cause bulb and capillary line to turn with nut, when nut is next loosened, a certain death sentence for the capillary line!  Apply the slightest bit of Gasket Sealer to the conical sealing surface of the Sensing Bulb.  The conical sealing surface of Cylinder Head to Sensor is a very positive seal, but having a look into the hole in the Head to inspect how clean it is, before assembly, is not such a bad idea...  

Temperature Sensing Bulb with Areas which should be lubed Green, to allow turning the overnut without turning the Bulb,
and sealed Blue conical sealing surface, to prevent leaks, highlighted. 



An electronic temperature gauge replacement option is available (I have not had one of these in my little hands for evaluation, so below is strictly presented because I am aware of it.  This is not a specific endorsement.  Readers who have first-hand experience with Mr. Basart's work are welcome to e-mail me with their experience!  It looks like he's in Holland)   http://www.hukebasart.nl/huke/en/?Electronic_temperature_meter

Pictures of the Basart conversion: 

Front appearance of the gauge is unchanged from stock.  The sensor is an electronic resistance element type treaded into an adapter which threads into the cylinder head.

Rear appearance reveals a standard push-on terminal for a common connecting wire.  The gauge internals are apparently replaced with an electronic gauge, and the original  pointer is reused.  Although no second connection is evident in the picture, I expect the electronic version must be supplied with Ignition Power.  


A couple of pictures by "Niveketak" of a conversion to the Basart electrical gauge:  http://www.volvoforums.org.uk/showthread.php?t=233203

The electrical sender must be installed with a threaded adapter.  Thread for B18/20 Temp Sensor:  5/8" UNF (18TPI) The electrical gauge is epoxied in place, on the old gauge frame, and connections are made to studs. 


http://65brick.blogspot.com/2008/05/temperature-gauge.html  a good documentation of grafting an electrical gauge into the original gauge, by "J. Jai" on his blog, but who I cant seem to find any contact info for, and who has not updated his postings in a while...I invite him to contact me!


Link to another electric conversion of an Amazon Temp Gauge:  https://forums.swedespeed.com/showthread.php?357201-A-solution-for-a-bad-temp-gauge


Repair of a similar style Temp Gauge for a '33 Plymouth.  This procedure includes refilling of the system.  Special techniques must be used to handle the volatile ether to keep from incinerating oneself!   http://www.ply33.com/Repair/tempgauge


External material sources are attributed.  Otherwise, this article is Copyright 2015-2019.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep its fine vintage products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 


You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you dont, youre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 


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