Proper Temp Gauge Reading
 

Here is a picture of the temp gauge from my owners manual on my 89 S4.

Notice the needle is slightly below - colder - than the square on the left side of the gauge. My needle - when the engine is cold or the ignition is turned off - is even more to the left of the indicated square. And, during normal operation the needle points to the first line - indicating the low end of the normal operating range.

How do I know my gauge is calibrated correctly? Can the whole gauge system be indicating a colder engine than I actually have? Where does the needle point when your engine is cold or the ignition is off? Why is the needle not pointing to the square to show this is the proper needle location for a cold engine?

Any thoughts you have would be appreciated.

since this is a gauge with non descript markings,
its time for you to get the IR gun out and do some testing,
see what the temps are at the top hose and the water bridge,
then you will know what the gauge is telling you.
Beyond that the needles seem to have their own stopping points and some can vary a bit.

They are not very accurate work off resistance and that changes over time and with corrosion of the electrical connections.

That suggests to me someone adjusted the needle to show slighter colder readings than it should.

I think you are exactly right. In a previous post I mentioned my temp gauge was right up against the max normal reading with the a/c on. My mechanic went through the cooling system and said all is right, but now the temp gauge reads very low - even with the ignition off it's about 1/4 inch below the left square. This tells me the gauge/needle was adjusted to too low a reading. However, I wanted to compare this reading to what others are getting.
He has the car right now, and I want him to readjust/re-look at the cooling system. It doesn't do me any good to have a low reading, and then overheat the engine with the needle in the normal zone.

Haha well if he's tweaking gauges to get the right readings you might want to consider a different mechanic :)

Paul instead of tweeking the gauge why not follow the suggestions in post 2

temps
 

87s4. i am very familiar with coolant temps as i also wrench english cars.

all your mechanic did was probably add some resistance to the line to have a lower reading. no big deal. dont touch what he did.

what you need to do is get an irr gun as suggested and measure the hose temps when engine is hot.

i get 174ish into the rad, and 160ish on the incoming to the block.....roughly 10 degrees difference should be noted, in an idle car with no wind going thru rad. if you are driving the about 35 mph, wind will help a lot....

make sure your electric fans come on.....if not you have a bad switch.

thats all you need to do and just know yours is calibrated a little lower.

+1 to Stan's post #2.

I just finished installing a 2nd hand digital cluster and noted that the temperature gauge reading was lower than my original reading, however the gauge works.

I completely stripped down my original cluster whilst chasing a PCB fault and I noted (IIRC) that the temperature gauge appears to be adjustable at the back of the gauge, so at operating temperature around 89 degrees C, checked by the IR gun and the gauge could then be adjusted to this correct setting.

Note different country code cars have different gauge plates IIRC.

Tails - that is what I think my mechanic did - he adjusted the gauge, but he adjusted it to too low a reading. My main concern is not where the gauge reads when the engine is cold, or even at operating temp, but I want to know with a high degree of certainty if it ever starts to overheat, which could obviously cause engine damage. I don't need a gauge that reads too low, only to find out the engine is toast but the gauge says it's still in the normal range.
I would think we should know the factory temp readings for the square at the left (cold) portion of the gauge, the first normal mark and the second normal mark, and then adjust the gauge to those readings. However, I have no idea what those temp reading should be. I don't think anyone else knows either.
He has the car right now, and he knows my concern, so we'll see what he does. I'll keep you posted, and thanks for all the replies.

There are different temperature gauges fitted to cars made for different countries.

My car has a degrees C temperature gauge and at the left hand side white square = 40 degrees C, the mark to the right = 80 degrees C the next mark is un-numbered, but is equivalent to 90 degrees C and the red mark on the right hand side is = to 100 degrees C.

I hope my post has an attached image

The image I have shown in post #11 was the original digital dash installed in my S4 and the temperature run around the 89 degrees C as the normal operating temperature, which is also the normal average operating temperature for cars fitted with the standards thermostat.

Todays ambient current temperature of 20 degrees C the gauge is sitting in the parked position within the 40 degrees C white block.

I hope this helps you to adjust your gauge to similar setting and then you can convert the degrees C to degrees F equivalent.

You will also note that the gauge is not linear.

BTW there is also a warning light that will come on if the car does get too hot if the gauge has been jimmied with then the system should be re calibrated so you have some idea what the actual temperatures are.
That said follow the suggestions in post#2

Tails - that is exactly the type of temp gauge I would like to have - something with numbers on it.
I told my mechanic to adjust my gauge so the needle is on the left square - as yours does - when the engine is cold or the ignition is off. If he can do that, then I'll get out the IR gun as in post 2 and see what I have.
Also, is the high temp warning light independent of the temp gauge? If so, do we know what temp it's set at so I can calibrate with the IR gun?