A discussion of automotive clocks two years ago drifted into a discussion of automotive temperature gauges, capped off by this explanation by the late Ric Locke:
What the temperature gauges of my recent experience have is a remarkably elastic center section. It’s always been true that everything within the white lines was “normal”, but people got worried anyway when the needle got close to an extreme. The manufacturers therefore compress most of the normal range into a few degrees of travel near the center. When the temperature gets within ten or twenty degrees of the maximum, the needle starts moving much faster.
TTAC commenter “autojim” expands the scale, so to speak:
The evolution of the temperature gauge in the US has several stages. First, there were gauges that worked. Some even had numbers on them. But US drivers, who a) can’t be bothered learning that a 50/50 ethylene glycol/water mix boils at 224F at atmospheric pressure, and b) that the boiling point goes up 3F for each PSI, would get all sorts of panicky if the temperature gauge read over 200F and bring it back to the dealer for warranty work.
So the automakers scrapped the numbers and just put cold/hot markings. Well, then customers wanted to know what part of the gauge’s range was trouble, and brought anything “too low” or “too high” by their subjective judgement back to the dealer for warranty work.
So the “NORMAL” band was added, typically with letters. Now we get into the same problem as with numbers: customers expected it to be in the middle of the NORMAL range, right between the R and the M. And never move.
So the automakers started putting huge flat spots in the gauge’s response curve. And under most conditions, that helped. Except with some heat-challenged engines in cold climates, where the coolant temp would dither around the point where the flat spot started, and the needle would move slightly in normal operation, causing customers to bring the car back for warranty work.
So the automakers did two things: one, they removed the “NORMAL” lettering again, and two, they increased the flat spot on the response curve.
And thus was gestation of the idiot light disguised as a gauge.
My old ’66 Chevy Nova had an idiot light which looked like an idiot light; however, my ’75 Toyota Celica had a real-life gauge, with calibrations at 100, 180, 210 and 250. Only once did it ever hit the top of the scale, determined to be the result of a severely clogged radiator, which was subsequently replaced.
This was followed by an ’84 Mercury Cougar with an idiot light, of which I saw entirely too much, given the Essex V6′s tendency to munch on its own head gaskets. Subsequent vehicles had gauges with tremendous flat spots. Gwendolyn’s thermostat supposedly opens at 82°C (180°F), and the gauge, sure enough, sits just a hair below the halfway point.
One gauge I’ve never had went through similar down-dumbing:
The oil pressure gauges had similar things happen, except of course the average consumer has no idea how much oil pressure is normal.
Sixty-nine kPa at idle, 390 at 2000 rpm. (I do pay attention to specs; somewhere in the glove box is the proper torque setting for the lug nuts.) Then again, Nissan wouldn’t even put a stick shift on this car, so you know they’re not interested in oil-pressure gauges.