Engine Coolant Temperature Gauge question

[syljay]

Wendell Roy wrote:
> 
>>Question 1:
>>Should the thermostat keep the coolant temperature relatively
>>constant? Even at low temps and highway driving?
> 
> 
> Yes. But the thermostat is only one part of the equation. At low temps it is
> restricting coolant flow to the radiator in order to keep the engine warm.
> At high temps it is all the way open and then the radiator fan and its
> control system is in charge. At highway speeds the fans will never come on
> but in traffic there is quite a temperature delta between the normal coolant
> temp and the one that causes the fan to kick on. This is to prevent rapid
> cycling of the fan.
***** But, ideally, the thermostat should control the coolant 
temperature by opening and closing itself . . or partially opening and 
closing. No fan in the picture yet if the outside temp is low enough 
to permit the thermostat to control the temperature by regulating 
water flow thru the radiator, and assuming there is airflowing thru 
the radiator.

The question is: How close can the thermostat control the temps? All 
such devices have a hysteresis loop built into them. In other words, 
if the set point is 182 C, you can expect the device to open at 186 C 
and close at 178 C, or something close to that. Also, when I tested 
some thermostats in a pot of water on the stove, I did notice partial 
openings and closings depending on the water temperature. So, this 
partial opening also aids in leveling the temp swings that would be 
caused by abrupt opening and closing.

Therefore, I should expect to see a small variation on the temperature 
gauge . . . which led to my question #2 . .what is the normal variation?
> 
>>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?
> 
> 
> On mine, cruising, it's at 1/3rd scale. Temperature climbs to 1/2 scale in
> traffic before the fan comes on, and 2/3rds scale is fan high overheat mode.
> Just a hair above that is boil over. If the thermostat is stuck open then
> the gauge won't register at all.
***** Seems like a big swing to me. But, I'm basing my judgement on my 
85 Dodge D250 Truck, where the gauge works with but little variation.
I'll do a check on my 88 5kq and see how that gauge behaves.

> 
> Keep in mind that these gauges are not the greatest. They consist of a
> heating element wrapped around a bimetallic strip that bends to produce
> needle movement. Not only is there the opportunity for mechanical errors,
> the temperature of the car interior and therefore the instrument cluster has
> some effect as well.
> 
> I think that troubleshooting this boils (ha ha) down to one simple test: If
> you are getting good heat during highway cruising then the thermostat is
> working.
**** Well, I do get heat . .and the wife aint complaining . . .so the 
Audi Gods must be sleeping on the job.

SJ in NJ