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I changed a head gasket on the 1978 due to wrong heat signal and a slight oil seepage problem above the oil filter at 200k. I convinced myself that age and miles was worrying enough. Wrong!
To my dismay the wire was more the gauge problem and not a head gasket.
Don't beat yourself up about this too hard, it might not have been neccessary right there and then but who knows what was in the making.
In the photos below is the very cylinder head I mentioned in my previous post that needed to be replaced because of corrosion of the water channels.
This was right after cleaning the area above the oil filter with brake cleaner.
And subsequently after running the engine. You mentioning this exact spot rang a bell.
I kept on running the engine in this state for over a year and lot of distance, and just kept topping up the coolant regularly.
I now wonder, because of Dave's explanation on deionized water, CO2 and the formation of carboxylic acids, if my procrastination may have led to the early demise of the head.
The 1978 car is strictly an analog gauge.
The later 1985- electronic speedo's use a digital stepping compensation board to raise a reading at a certain heat levels.
Strictly speaking, the temp comp board is a fully analogue device based around a quad opamp. And around opamps, quite a few different circuits can be constructed.
Probably due to the resistors or the capacitors dry out or oxidize internally.
Small clocks and maybe tachometers can too?
At this age, electrolytic caps would be the usual suspects as they do literally dry out. Resistors, diodes, and ceramic or film capacitors would probably be good for another few decades or so. Solder joints can crack and connectors oxidize.
Shorting the lead of the temp sensor to ground wouldn't really bother me as long as it's being done just long enough to see what the meter does.
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