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When I first started my new engine I ran a mechanical guage along with the rally guage. Readings were similar to the OP's experience. But boy was the ralley guage slow. The mech guage would show some pressure during cranking and jump right up as soon as the engine fired. It seemed like the rally guage took 5 minutes to catch up. I don't know if they all do or if my new parts store sender is just slow?
The rallye dash gauges are slow due to their design. Current from a constant voltage source flows into the gauge. The other end of the gauge goes through the sender to ground. The gauge itself is hooked to a bimetalic strip. Current flows through the strip causing it to heat up. Because of the dissimilar metals, the strip bends because one metal expands at a different rate and distance than the other. The bending metal moves the pointer, the distance dependent on the amount of current and heating of the strip.
Very, very simple and very easy to manufacture. And very slow to react. Mechanical gauges are much quicker and more accurate. They cost an order of magnitude more to manufacture so they were not used by manufacturers.
Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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