Temp Gauge accuracy...

[alpina]

Having never really worried about the running temperature of my car. I start reading posts on this site. Lots of posts
Saying their car runs around 80C / 180F on the gauge rising slightly higher in stop / go traffic and hot weather.
(I never understand people saying 85C / 185C how can you be so precise with that gauge)
On my car the gauge needle is horizontal between the 80C and the next white square above. It can idle in the garage
For well over 30-40mins with no movement. May be a slight lift above horizontal. But not much.

Thinking this is not what others see/normal, I start to investigate what I could do. Reading posts about electric fans / water pumps etc.
I thought this would be an interesting winter project.
So recently I followed a couple of other members, and installed a heater bypass with an added Aux water pump (small Bosch unit)
I found from a scrapped MB. This is wired to a temp controller that turns it on at a set temp, I also installed an override switch.
To be quite honest I was a bit disappointed with the result. After bleeding the system the gauge once again is happy to sit
In its horizontal positon.
However having the Aux water pump run after the car is switched of helps a lot with heat soak, and the engine cools nicely.

The Temp controller I used has a probe that is inserted into the fins of the radiator (close to the top hose) and has digital
Display showing the temp. (the instructions for the controller notes that the temp reading from the probe is approx. 10C below
The coolant temp in  the rad). Now this display never showed more the 72C after running on an off in the garage for over an hour.
I checked this with an IR temp gun this gave the same reading on the top hose, top of radiator, the pod where the temp gauge
Pick up is.
The temp gauge in the car reads about 1-2mm above horizontal (between 80C and the next square)

This lead me to look at the gauge in the car. After the car had cooled overnight I had a close look at the gauge.
Something I have never really done before. Using a torch to get a better look, I can see that the needle does not
Come to rest on the bottom stop. It is about the thickness of the needle above the stop.  Given the non-linear scale of the
Gauge, can I conclude that this would be the reason that the needle is horizontal at normal operating  temps?

I have looked through the service history of the car, and found that the temp gauge was replaced in 2008.
The centre cluster has also been removed when the car was painted.

So questions, how accurate are these gauges? Can it be recalibrated so to needle starts from the stop peg ? 

It looks to me now like I never had a problem to begin with. My winter project was unnecessary. Well it does help with the heat soak.
So not all bad.

[WRe]

Hi,
normally our old instruments are very accurate, maybe yours not because it was replaced by a new one.
But nevertheless your story is interessting. In German we have a saying:    What you don't know won't hurt you! Therefore modern cars have no water temp, oil temp or oil pressure gauge even the electronic knows these dates. But we tend to know everything. When I installed an oil temp gauge in my old /8-Diesel I was always worrying because the motor became very hot or should I say what the gauge said but nothing happens.
I think it only will be dangerous when something goes into the red area, when you notice it. ;-)
It's like Pandoras Box, don't open it but I know a lot will do it and will fail.
...WRe

[alpina]

I Think you are right. I never think about the temp in my other cars. As you say they don't have them. I guess the more
I read here, and in the mood for a project lead me down this path. The car runs great they way it is. i guess i just wanted
normal (whatever that is).
I can live with the gauge reading a little high. i guess it is better than reading low.
The needle will hit the next white square after 80C when sitting in traffic on hot days. given its head start it would seem okay.
Never shown signs of moving near the red zone.
It would be interesting what temp readings others get using an IR temp gun on the radiator top hose and head when the gauge is
reading 80C.  anyone up for checking?, i would be interested in the results.

all the best DM.

[mdsalemi]

I'll certainly check my "gauge" against an IR temperature gun, which I have. I don't expect any surprises.

Just a note that our "temperature gauge" on the Pagoda isn't a "gauge" in the traditional or even modern sense, meaning a sensor/thermocouple and an electric meter. What we have is a thermometer, pure and simple--with the fluid filled bulb mounting in the engine and connected to the instrument with a capillary tube. That's why it continues to read the temperature in the engine, even after the car is shut off--it does not require any electrical power.

[alpina]

i know the 'gauge' is a thermometer. i just find it interesting that when i measure engine temp with a IR Gun,
after the car has been idling in the garage for over 30min i get readings of 72C at the top hose and radiator.
the pickup where the bulb is 75C. the needle on the gauge is horizonal, midway between the 80C and the next
white square.
There is also a probe inserted into the rad and this reads 70C. which all leads me to believe the gauge reads a little high.
i have read posts on hear with people who have a gauge that reads a little low. I'm interesed to know what IR readings
others get at the engine once warmed up.
I think it would be good experiment, and may help others.

[georgem]

So maybe whats important to us as drivers is any change from the norm.

After years of driving our individual cars we know where the guage normally sits - the "norm"  - in some ways the absolute temp is irrelevant - its this change that we need to understand.....hot day, traffic etc etc. If the needle is going north for no apparent reason - start thinking oh oh.

[Tyler S]

The problem with these temperature gauges is that a bubble can form in the fluid column and cause the gauge to read higher then it really is. Same problem with old glass thermometers. If you drop or jar one the column separates and you can see it is off reading. This can happen in shipping. New ones should be hung completely vertical for a few days to allow the column to re-connect. If all else fails you can remove it from the car and put the bulb end in boiling water on the stove while hanging it vertically to help purge the bubble back up to the expansion area in the gauge. When installed make sure the path is all uphill from the bulb to the gauge.

[Jonny B]

Hmm, I thought the gauges in the Pagoda were ether filled, not liquid.

[66andBlue]

Jonny B
correct, not all ethers are equal. I believe the W113 thermometer is filled with diisopropyl ether that has a boiling point of 69°C or 156.2°F at 1 atmosphere pressure (15 psi).

Tyler might have  thought of the stuff that is used  to put unsuspecting victims to sleep: diethyl ether with a 94°F boiling point.

Bubble formation within a closed capillary should not be a problem because the  diisopropylether will not easily undergo liquid <-> gas phase changes during normal operation (in contrast to an open system). It is a problem when you try to fill the capillary during repair: http://w113.eu/geschlossen/Reparaturen/Wasserthermometer%20reparieren%20Mercedes%20Pagode%20W113.htm

http://trerice.com/pdfs/2010/Temperature/DialThermometersDesignOperation_100_103.pdf

[alpina]

Thanks for the all the replies. I think now we are going off in a different direction, in relation to what the tube is filled with.
All I want is the simple reassurance that all Is okay. The gauge stays steady in its ‘normal positon’ of midway between 80C and the next white square.
Car can Sit idling in the garage with no problem. Taking readings with an IR gun at the thermostat casing, top of radiator, top hose and temp
Pick up range from 72C – 79C.  which I believe is in the range it should be. (I wait for someone to let us know their IR readings at the engine).

It is the gauge that reads slightly high. Which lead me to look into what was going on. I have come to the conclusion that all was okay
In the first place. Time was not all wasted, the addition of the Auxiliary electric water pump really helps the cool down / heat soak after
The engine is switched of. Nice to see the needle steadily fall.

So if you have a IR temp gun let us know your readings and where needle is on the gauge in the car.