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Discussion Starter #1
Hello everyone,

In recent days I have phosphorus a lot on the measurement of T° of oil and water on our cars.
On the strength of measurements made by Roland (Spidernuts) I was able to modelize through curves the behavior of our temperature sensors, our galvanometers on the dashboard and see how to correct the reading when it is false.
The data are related to a 2000 Touring (type 102) but I am convinced that the galvanometers of the 101, 105 and 106 types are the same (in any case the sensors offered by our suppliers are common to all these models).
I have collected my ideas which I propose to share with you below :

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Discussion Starter #3
On my car, I have provided mini variable resistances that are available at different ranges : 200 Ohms, 400 Ohms, 2kOhms, 5 kOhms and for a power of 0,25 W.

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The setting with the mini screw is very accurate.

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To compare with original components :

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Series circuit (too increase the resistance):

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// circuit (too decrease resistance)

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On my car, the measurements behave super well.

Full load engine, in a large climbing highway , I manage to raise the water temperature to 90 °C and oil to 95 °C.
Then downhill, without accelerating, with the air still fresh in the morning, we see the water drop to 80 ° C. The oil has more inertia but returns quickly to 90 °C.
In quiet regime, we see the small oscillations of the water T ° according to the regulation of the calorstat.
It's just great and very reassuring!

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Thanks to variable resistance I'll be able to set the measurement if i have to change one of the sensors in the futur.

Hope this will help.

Serge
 

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I suspect being stranded alone due to quarantine has left you with much free time.

My approach to this problem has been a little different...

The original water temperature sender typically does not need correction. The oil temperature sender does.

It is a characteristic of Alfa’s that after a period of driving, the water and oil temperatures will equalize. At least this is true in a car with no regulated oil cooler.

Thus, I will drive my car until all of the temperatures stabilize, return home, and insert a fixed resistor in line, testing different values, until the oil and temp gauges are equal. If this takes too long, I take another test drive to confirm the values.

You will note in the wiring schematic that a resistor is depicted in line for oil temperature. In fact, the wiring harness I purchased from Lionel Velez included an in-line capsule in the oil temp wire to allow for easy exchanges of resistor values to achieve the right correction. I bought a box of low-current resistors online, and can make small corrections to achieve a good calibration.

My final test is to use a laser/infrared thermometer to read the temperature of the oil sump. If the warming drive is long enough, the temperature of the oil and engine block and sump will have all equilibrated.
 

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Don,

A few comments :

The sensors offered by our resellers are from different brands (VDO, VIBOR, ...) and have therefore different characteristics. if you change them, there is little chance that you will have a correct reading of the T°. You will need a correction for sure.

With the work done above, you may learn that if the reading of your T ° is too low then you must put the resistance in parallel. This is never provided in the cabling.

I do not agree with you, once the car has been driven well, the oil temperature is always higher than the water temperature: Around 85 °C for water /around 90 ° C for oil (may be 95°C).

If you need to lower the temperature that is read, the range of the series resistance will be from 0 to 100 Ohms.
If you have to increase the T ° that is read, the range of the parallel resistance will be from 0 to 11.000 Ohms. There is little chance that you will achieve a result with conventional resistors. Hence the use of variable resistances.

There is no way that you can determine the oil temperature using a laser thermometer pointed at the engine block.

Finally, I often hear people explain that the temperature measurement on our cars is imprecise.
The work carried out above shows in reality that once the galvanometers and the sensors are well calibrated by the addition of a simple well chosen resistance, we have very precise measuring instruments.
Be aware, the instruments are more than 60 years old but the reading can be exact at 90°C and elsewhere error is less than 5 ° C.

Awesome no?

Serge
 

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Great overview, Serge -- this may help a lot of owners!
 

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Hmm. Once again.... it will be fun.

On a level surface, driven modestly, the oil and water temps will equalize. It requires time to heat-soak the entire power-unit. It is also true that climbing a long hill at elevated power will drive the oil temps higher than the water. The two temps will equalize so long as there is excess heat transfer capacity in the water coolant system. Above that point, both temps will climb until A new limit is reached.

I have personally tested these observations, including a direct measurement of drained oil taken immediately upon return from such a calibrating drive. I have observed the same temp, measured by a laser thermometer, of the radiator, engine case, cylinder head, and drained oil. I also took measurements around the head, noting higher temps of the exhaust manifold, but very nearly the same at all points of the block, head, and radiator.


I take your point about aftermarket senders, although the one I recently used in the first engine for 1488 displayed a very similar curve to the factory sender. However... I have not yet finished my research, but my current suspicion is that it was the larger diameter of the aftermarket oil temp sender that obstructed the flow of oil, ruining all of my bearings. Thus, until I’ve determined otherwise, I won’t be using non-original oil temp senders.

The water system/radiator will cool more quickly once the engine is shut off. However, my real point was not to seek perfection in the actual temperature measured, it was to calibrate the two systems so that the oil gauge reads mid-span, just like the water, when the power plant is in a temp-equilibrated state.
 

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Discussion Starter #9
Philippe,

This is the source :


And this is the set I bought :

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Good post Serge!
Probably later this year I will start my car and drive it for the first time. And yes I realized already that there are different senders around. I hope I have the correct ones. But if there is an inaccurate reading you have given us the tools to simply adjust our instruments.
Thanks Mike
 
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