Engine Coolant Temperature Gauge question

[Huw Powell]

> 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.

Heat after a mile at 15 dF ain't bad.

> 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.

Sounds sort of right - my 89 90Q has two "operating" temps - 1/3 up is 
"normal" when it's friggin' cold (t stat open a bit, no need for rad 
fan), and 2/3 up is "normal" when it's warm out - that's where the rad 
fan stops it from going higher.

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

It should stop it from dropping below its setting (82 or 87 C, IIRC)

> 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?

Don't know your car, see mine above.

> 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?

Infrared temp "gun" - any decent shop will have one.

> 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.

That sounds like fun, at the very least.

> Shhhhhhh . .don't tell the wife!!

Does she, um, have access to teh innertubes?

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/