Temperature Gauges 1800E/ES Notes 
first posted Dec/2022  R. Kwas  Revisions on-going [Comments Added]

Actual experience with Water Temp Gauge

Reference Information

    Checking Temp Gauge Function and Calibration
    Troubleshooting info

    Sender Resistance Values
    Bench Gauge Calibration
    Calibration Tool Detail


The info here is applicable ONLY to the injected 1800s only, which employed Smiths supplied, all-electrically operated gauges.  Early [carbureted] 1800s had a "Filled Thermal System" Coolant and Oil Temp indicators, using capillary tube and pressure gauge technology similar to the 122, but with a different Coolant/Oil temp combination gauge on the dashboard.) Link to a notes page for 122 (becarberated 1800) Temp Indicating systems:  Temperature Gauge Notes


Oil and Coolant Temp Gauge readings, but only after a long while on the road.  Oil Temp is finally off the peg,
and it looks like this owner has a summer-weight 70C Thermostat installed.  

Oil Temp typically indicates on the very low side, with Gauge Needle not even or just barely moving from the peg.  Owners often suspect Oil Temp Gauge of not working!  See also:  Oil Temp Gauge not working?  are you sure?  


Both Temperature Gauges, (and also Fuel Gauge for that matter), are inexorably linked to the function of the Voltage-Stabilizer.  Before believing the engine is running hot, or worse, let's make certain we can trust what the gauges are telling us!  A significant part of that process means checking V-Stab output for the correct and characteristic cycling!  See also:  V-Stab function!  The easiest point of access to the (Blue) wire is behind the Fuel Gauge...accessing it there for voltage tests will spare the operator some contortions...the V-Stab is physically located behind the Combination Instrument (Speedometer).    

1800E Wiring Diagram Excerpt showing V-Stab and associated.  As can be seen, V-Stab is supplied by fused Wht-Grn wire,
 with Instrumentation Power, and it's output supplies Water and Oil Temp, as well as Fuel Gauges, by way of a Blue wire. 
[Corrected swapped Water and Oil Temp Gauge Info - Jan 2024.] 


Actual Experience with Water Temp Gauge: 

A little back and forth from a thread: 

Water Temp Gauge indication questioned by owner...is this Hot? 
Read on!


Poster Writes: 

"I've had my '73 ES as a daily driver for the past three years in Los Angeles, and it's never given be trouble. Never failed to start, and the temperature gauge has always been very close to the middle when I've been cruising around with an 80C thermostat, [That sounds absolutely correct and accurate, see below!] though it's occasionally climbed towards the red or been on the line between green and red.

Anyhow, I recently took out the gauge and sender and tested them on my stove with a thermometer, and it turns out that it's about 10C low. So When I thought I was driving around comfortably at around 80-90C, I've actually been driving around at around 90-100C. The picture is where the needle sits when at 100C.

I haven't heard any funny noises, but I'd like to check it out and see if there's anything I should pay particular attention to. (The radiator is getting cleaned out and flushed ASAP.)"

My Response:

"took out the gauge and sender and tested them on my stove with a thermometer, and it turns out that it's about 10C low. "

"I have to question exactly how you tested this Temp Indicating System "on the stove"...how did you power it??...if you didn't power it by means of a V-Stab (or alternately a 10VDC power supply), then you CANNOT expect the indicated temperature to be correct and what could be expected if it was powered by 10V, as in the vehicle! ...so then I must question the entire premise that engine is running hot!!

I'm not questioning any of the good advice about Cooling Sys maintenance and service by others, but FIRST, I'd need to hear you say that the Temp Gauge was correctly tested! See also: https://www.sw-em.com/voltage_stabilizer.htm  Cheers"

Poster's Response:

"Ah, thanks for the details! I was testing it with an adjustable digitally-controlled power supply, but you're right, I had it set to 12v and not 10V. I'll re-test it. I did check the resistance of the sender at boiling temperature, and it was around 80 ohms and not 40 ohms like the manual calls for, so it seems like something is off in the sender/gauge equation."

My Response: 

"Testing at 12V will give high reading (that is also the symptom for non-cycling V-Stab!)...testing with 10VDC PS is perfectly valid, but Sensor R should be within nominal range to be accurate so that you can trust readings...I believe the green books show a graph (or is that for the D-Jet Temp sensors)...[Green Service Manual shows no graph, but gives several Resistances at Temp values.]

Otherwise, if you do a Cal test on the bench with that Sensor, you could simply make a Cal mark (like the small tick-marks already present, which is where Smiths tested them) at the 212F point when submerged in boiling water [further details below!], and have perfect confidence in the that reading, because you will have checked it!  Cheers"


Reference Information:  

Checking Gauge Resistance and Calibration:  Gauge face on both Oil and Water Temp are the same, but Senders are different, allowing a high temp indication range for the Oil Temp Gauge. 

Coolant and Oil Temp Gauge R = 58 Ohms, measured across terminals on the back of gauges. 

Rear view of a Water or Oil Temp Gauge...note connectors for Ign Power, wire from Sender, and mount for
Instrument Lighting, but NO chassis return connection...none are required for the Temp Gauges (all of the
electrical action is strictly isolated from the housing and ground!)  Clearly evident are the two Calibration
adjustment holes (see below for more on these), between the mounting post and connector terminals.      

Water Temp Sender R = 37 - 48Ohms (at 100C / 212F).  To verify gauge calibration, disconnect Sender and replace its resistance with a fixed 47Ohm resistor (this is a standard value, 48Ohms is not!), and with Ign ON, gauge should slowly size and settle out to indicate 100C.  If indicates much higher, check V-Stab supply for cycling function.  See:  V-Stab function!

Temperatures from the Factory Service Manual added in Yellow...this certainly looks like a correct and accurate
temperature indication for having a 92C Thermostat installed! 


Oil Temp Gauge Sender:  140 - 160 Ohms (at 100C / 212F).  Test calibration similar to Water Temp Gauge. 

Typical oil Temp indication after a while on the road! 
Calibration markings added in Yellow!

Checking Temp Gauge Function and Calibration: 

This can be done with gauge in place in dashboard (so Ign ON, and powered by the V-Stab) or on the bench under 10.0VDC OR a bench Supply powering a S-Stab and that in-turn powering the gauge under test (see below!).  When doing this check in the car, checks are recommended to be preformed after the V-Stab cycling is first verified to function as expected! 

V-Stab:  Link to V-Stab function

Water Temp Gauge:  To check the Gauge, a 10 volt supply and two resistors are required. One Resistor with value of 280Ohms (Ambient/Cold R value!) and the other of value 40Ohms (Hot R value!).  Remove the sensor wire from the temp sensor (located at the top rear of Cylinder Head) and connect the 280Ohm between the sense wire and chassis, simulating a cold engine. Switch ON the Ignition but do not start and wait. The gauge should settle at the beginning of the green band.  Switch Ign OFF, and connect the 40 Ohm resistor.  After Ign is again powered ON, temp indication should slowly rise to beginning of the red zone. If actions of Gauge are as expected, R of Sender can be checked (with Sender wire disconnected) with an Ohm-meter also.

Oil Temp Gauge:  Check Oil Temp Gauge function in the same manner...by disconnecting wire at Sender (located on side of Sump) and connecting a 2.8K Ohm Resistor (Ambient/Cold R value!) and the other of value 160Ohms (Hot R value!), and checking indication under Ign ON condition.

Note:  A further section on checking the gauges using Calibration voltage has been added below!  See:  Checking the Calibration of Electronic Oil and Water Temp Gauges

Oil and Water Temp Indication in E and ES cars: 

Both Water and Oil Temp Senders (also:  Sensors) consist of a Negative Temperature Coefficient Thermister Resistive elements, meaning that when they are heated by thermal conduction at their respective sensing locations, their Resistance decreases.  Since they are both single wire Senders, with one side connected to case/housing and in-turn vehicle chassis, this decrease in Sender R allows more current to flow in the sensor path, and Gauge, resulting in the indication on the Gauge increasing.  Water and Oil Temp Senders have a different R value, but are packaged in similar threaded Housings which means that the CAN be inadvertently interchanged, so careful verification of cold Resistance with an Ohm Meter before installation is good practice and recommended!  See Reference InfoSender Resistance Values

Gauges are supplied by the slowly cycling VStab as described here, and this must be working properly (cycling!) first for Temp Gauges to function, and when they are being checked!  VStab is buried behind the combined Instrument (Speedometer) and difficult to reach itself, so checking its output on the Blue Wire at the more accessible Fuel Gauge is effective and much simpler, requiring fewer contortions (see also:  Wiring Diagram Excerpt below).  Temp Gauges are supplied by the cycling output of the VStab, but as it states in the factory Service Manual, both gauge inertias, being of a bimetal construction, have a slow enough response, that the cycling of the supply Voltage is not apparent in the indication.

This generally describes is the normal function of the Gauges/Sender combination...but consider the somewhat common failure mode of a poor connection anywhere between these Gauges and their Senders...or even from Sender to chassis.  Since the Gauges are essentially current indicating devices, gauges have no way of knowing, or telling a difference between the Resistance of the Sender alone, or Resistance of Sender PLUS some additional inadvertent resistance in the conductor connection between Gauge and Sender!  Since a higher Resistance on the NTC Sensor would normally result is a lower sensing circuit current, and an additional inadvertent R will also result in a lower indication on the Gauge, and driver thinking that temps are low, and everything is fine (ignorance is bliss!).  This is why it is not bad practice to check the calibration of Oil and Water Temp Gauges at some point, in order to be confident in what they are indicating! 

The Fuel Gauge (also supplied by the VStab!) can, no...will also be affected by an inadvertent additional Resistance in the Sender path, but that is outside the scope of the info presented here.  Link to:  1800 Fuel Gauge Info.

[Troubleshooting Tip:  Temp Gauges will function when VStab is not cycling, but since the long-term average voltage powering them is not 10V but 12V, the amount of heating of the internal bimetal element will be higher, so the indication will correspondingly be higher!  When Temp Gauges (and Fuel Gauge) all read high, check VStab first!]


Troubleshooting info: 

1800E/ES Wiring Diagram excerpt for VStab loads and troubleshooting. 


Checking the Calibration of Electronic Oil and Water Temp Gauges: 

Option A:  A calibration check will give confidence in what these gauges are displaying!  Prepare an insulated test vessel (like a double Styrofoam cup) for boiling water.  Remove Sensor from its installed location in Cyl Head or Oil pan, leave Sender wire connected, and connect an addition test ground wire between Sender brass housing, and vehicle chassis, and place Sender under test, into the test vessel. 

Heat some water to a rolling boil, turn Ign of Vehicle ON, and pour boiling water into test vessel, fully covering lower sensing area at least.  Monitor gauge indication after a few seconds of letting the gauge Needle settle and come to rest...it should read at the top of Green area.  Turn Off Ign.  If reading is very different from the expected indication for 100C/212F, troubleshoot using info here, and remedy.  Some fixed resistors of the values shown in graphic, or even a potentiometer in the resistance range, can also be temporarily be connected to check function of gauge and associated wiring.  Note:  Its OK to leave Ign ON without engine running for the duration of this test, but do not leave Ign ON only, for an extended period of time to prevent overheating Ign Coil.    

OR Option B:  ...simply disconnect Sender and apply an accurate test voltage to the wire (with Ign ON), and observe Indicator position!  See also below:  Bench Gauge Calibration


Thermister:  https://en.wikipedia.org/wiki/Thermistor


Sender Resistance Values: 

Special thanks to 1800E owner Jean-Pierre Le Lagadec for making these measurements and generating the following Excel charts for both Negative Temperature Coefficient Senders.  In both cases, the Blue highlighted (ambient temp) R value can/should be verified before installation, or with an engine temp at ambient.  Green area on chart is the area displayed on the corresponding gauge.  Note that the Senders have similar housings and threads, but very different R values...they may not be interchanged! 

Jean-Pierre points out that The Coolant Sender has a red collar around the electrical terminal, while the Oil Sender has a white collar...I personally, I wouldn't trust this as the only identifying feature, as I'm not sure, and can't guarantee, that alternate sender manufacturers would adhere to the same standard.  My advice:  Test Resistance to to be certain which Sender you are dealing with!  

Coolant Sender R.  


Oil Sender R. 


Bench Gauge Calibration:

The following info was developed in cooperation with 1800E owner Jean-Pierre Le Lagade in France, and Triumph Stag owner Sujit Roy in California, who I hereby thank for their significant contributions.  JP has given his kind permission to publish his name, and Sujit's name is out there in the public domain, associated with his YT channel already, so I figured its OK to mention it here, and I did specifically request his permission! 

This info is intended to help 1800E/ES owners with checking and adjusting the calibrating of Smiths electric gauges on the bench.  The electrical risks of working behind the Dashboard under power, not to mention the contortions necessary to perform such a service, make it prohibitively difficult to do with gauges in-situ!  Required are a bench power supply with an accurate variable voltage adjustment, test wires, a special Calibration Tool (detailed below), a steady hand and little patience!  

The Smiths Oil Temp, Water Temp, and Fuel Tank Gauges are absolutely identical from the view of the internal mechanism, which is based on a bimetal strip with heating wire, being heated by a variable current determined by the Resistance of the associated Sender.  What makes the Gauges different are strictly the makings on the face, and the Sender R values they are associated with in service.  Gauge faces do have Calibration guidemarks, which were originally used at the Smiths factory, and given the right setup conditions, these can certainly be used again to check the Cal, and to determine if a touch-up of the Cal adjustment is called for. 

In the vehicle, gauges are powered by the VStab, and this operating voltage, in conjunction with the Temp dependent Sender R value, determine the Needle deflection.  To calibrate the gauges on the bench, they are simply connected directly to the variable voltage source set to one of three Calibration Voltages, which will/should result in a the needle settling at the corresponding Calpoint. 

The mechanism has two adjustments:  [...analogous I expect, to the Span and Linearity adjustments of Bourdon Tube "Steam" gauge mechanisms...], which vary this position and which can be ever-so gently and lightly moved and repositioned with the Cal Tool, to adjust the Needle position to bring it within the Calmarks.  The process is a balancing act between the two adjustments and repeated for each of the Cal Voltages.  Gentleness on the adjustment, and Patience with the interaction of the two adjustments and repetitiveness are required!  This is not a job for the easily frustrated, and marking the factory adjustment, as a starting point is a good idea! 

Gauge with test Voltages and target Calibration marks added:  

Volvo 1800E/ES Gauge with Calmarks at the scale-ends plus mid-scale, with Cal Voltages added.


Sujit with his Stag Instrument on the bench [...does anything here look vaguely familiar?  Volvo version does have an arguably nicer instrument face!].  Screenshot from his video, showing Calibration at one of the Cal Voltages: 

Instrument at 4.8 Cal V...indicating a little high!  When changing the Cal V, a bit of time must be allowed to let the indicator needle settle at the new position...it is after all, a thermal system...this is intentional, and to prevent a 'nervous indicator needle". 


Gauge mechanism removed from housing for inspection.  End of Cal Tool, with its rectangular eccentric adjustment peg, inserted from below, is visible in one of the Cal slots, as is the bimetal element wound with the white heating wire.  Simple, repeatable and reliable technology really!  

Note:  The gauge mechanism (two sides) are riveted to the phenolic base, but not so tightly that it is not possible to slide it, and allowing this movement is by design...moving its position on the base allows slight positional changes for the purpose of fine-tuning and calibration adjustment.  It should be possible to make gentle and slight position adjustments using the Cal Tool.  Since how much resistance to movement is determined by how tightly the rivet was installed, some variation of how much force is necessary to move the adjustment should be expected!  If found to be too tight, a drop of lube might help free up the movement...conversely, if too loose, it might move on its own from vibration, loosing calibration.  A (limited!) dab of fingernail polish might be enough to secure it into place, and from moving and misadjusting under vibration. 


Calibration Tool Detail:

The dimensions given below are strictly what I estimate from carefully observing the series of videos:
1.  https://www.youtube.com/watch?v=1XvHm_cavTE  ,
2.  https://www.youtube.com/watch?v=GuGjdPFviis  , and
3.  (where he further improves the shape of the end of the Cal Tool):  https://www.youtube.com/watch?v=fKRThZiYSbA

Composite of video screenshots, plus the authors estimates of shape and dimensions!

Earlier video with a better shot of his cal tool in its entirety (Based on a 10mm bolt, the rectangular eccentric adjustment peg end is shaped as detailed...he did change the shape at the end from the first video shown to the later less rectangular shape to optimize it and allow use of the tool when the gauge is assembled...his first version of the tool worked only when the mechanicals of the gauge were removed from the housing.  Shape of the improved tool is shown.



Related Information: 

A high Oil Temp Indication...

...saw no-one...with a properly functioning Sensor, Gauge and V-Stab...dare I say:  Ever! 
This is more a symptom of a non-cycling V-Stab...or low oil level, or running on the
Autobahn at high speed for an hour...  (Senders rarely fail internally shorted!).
...or maybe a gauge measuring the temp of "liquid hot MAGMA"...
See:  Symptoms of non-cycling V-Stab



External material sources are attributed.  Otherwise, this article is Copyright 2022-2024.  Ronald Kwas.  The terms Volvo and Smiths are used for reference only.  I have no affiliation with either company other than to try to keep their 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 possibly wise-a** comment. 


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