Thanks guys for your help, Arthur you hit the nail right on the head.

Just for the record, if you have a 190E 2.6 1991 there should be 3 coolant sensors on the left side of your motor head.

If we start from the firewall to the front of the car:

4 pin sensor = Igniton and injection sensor
1 pin sensor= motor temp gauge in the car
2 pin sensor(blue)= aux. fan/ac

I just verify this with my test lamp to the ground, when I touch the 1 pin sensor connector the temp gauge rise to 80c and when I touch one of the prong of the 2 pin sensor connector my aux. fan start.

Now my question, the 4 pin sensor have small difference on is two circuit but I mean small.
This morning it is -10C over here if I refer to the NTC grahp for engine coolant sensor the reading should be around 8000 ohms mesure in diagonal, my sensor is in the spec., reading on one set of pin is 8400 and the reading on the other set of pin is 8450.
A difference of 50 ohms is it enough to make the car stall when I decelerate and the motor just reach around 85c????
Should I redone the test with the motor at 85C and see what will be the difference ???

Thanks again
Max