Engine Coolant Temperature Gauge question

[syljay]

Patient is a 1990 100Q. The wife's car.

The coolant gauge always seemed to be reading on the low side.
I drove the car today with the outdoor temperature hovering around 
15F. Seemed to take a long time for the gauge to start registering.
I'm wondering if the thermostat is stuck in the open position.

For the purpose of discussion, I'll provide a textual reference for 
the coolant gauge markings.

[ ][][]  |  [ ]  |  |  [ ]  |
. .                  . .
  C                            H
  1 2 3   4   5   6  7   8   9

Marking sizes are:
single wide marker = 4, 6, 7, 9
double wide marker = 2, 3
Triple wide marker = 1, 5, 8

The heater came on after the usual delay . .which takes about a mile 
of travel.

While driving down the highway at 65, the gauge stayed barely above 
the "3" indicator mark.

When I arrived at my destination, I left the engine running and 
watched the temp gauge. After a few minutes, the gauge crept up to the 
"5" indicator marker, and the radiator cooling fan kicked in.

Back on the highway, the gauge stayed barely above the "3" marker.

Question 1:
Should the thermostat keep the coolant temperature relatively 
constant? Even at low temps and highway driving?

Question 2:
What is the usual position of the needle when driving? And what is the 
position of the gauge when the radiator fan kicks in?

Question 3:
I think the dots for positions 1 and 7 are calibration test marks. If 
I remember correctly, the Bentley shows usage of a "Special Tool" to 
test the gauge. More than likely the tool is just a rheostat. Does 
anyone know the resistance values at the two test positions?

Question 4:
Does anyone have a simple method of measuring the coolant temperature 
with an external thermometer?
I was thinking of taping and then insulating my electronic cooking 
temp gauge probe to the Upper radiator hose. With enough insulation 
the outside of the hose should be at the same temperature as the 
coolant inside.
Shhhhhhh . .don't tell the wife!!


SJ in NJ