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Is there some special logarithm driving the needle on the temp gauge, instead of it being a direct linear reading? Just curious. The reason I ask is I've noticed the needle is always centered directly in the middle of the gauge after warmup. It never varies one side or the other. It's as if there was a hidden peg to the right of the needle. Except for one instance last week (low coolant due to a leak, since repaired), I had never saw the needle move even a hair away from center once the engine was warm.
Just for giggles, I've been reading the coolant temp on my scan tool. Once the coolant reached approximately 180F, the needle was centered. The normal operating temp is around 210, but the needle never moved to show that 30 degree difference as warmup was completed. The needle just stayed fat, dumb, and happy in the middle.
When I had the low coolant, the needle only moved to the 5/8 position at most. The scan tool showed about 230F, so it seemed to be accurate for that portion of the range. But in the range of 180 to 210ish, the needle parks exactly in the middle.
Pretty much all modern cars have similar strategy behind the coolant temp gauges.
'Back in the day' cars had gauges that displayed actual temps. OEMs found that very few owners knew enough about cars (and physics) to interpret the readings appropriately. Log on to any muscle car forum today and you'll see owners panicking about coolant temps increasing while stopped in traffic.
Conversely, it's not rare to read about an owner not taking the appropriate action even when the car has sounded all the warnings and alarms it has. There's been multiple threads here about owners continuing to drive until the engine seized despite glowing red lights and pegged gauges. 'I was just trying to get home' is a common justification.
Oil pressure gauge readings are just as subject to misinterpretation.
Gauge is programmed that way. I had old 300E with true reading gauge and it took some getting used to "normal high" readings almost to red. I think it scared/confused too many owners. Hence dumbed down idiot gauge. I watch Torque Pro true coolant reading, programmed to flash above 210°just so I have some warning.
Last edited by Quantico; Oct 25, 2015 at 10:55 AM.
The combination of that temp gauge design and old coolant hoses is a disaster waiting to happen. Spring a leak and the "Low coolant" message is almost immediately followed by the gauge going into the red zone, leaving you just a few seconds to shut down and avoid cylinder head damage.
What's an owner to do? Pray for good luck, monitor the temp with an ODB scan + alarm, replace 5-year-old hoses before they burst, or save up for a new cylinder head.
Temp Gauge Computation?
