Also, the air pressure governor receives it's pressure from the "wet" tank just after the air dryer.
If there is a leak in the wet tank or lines leading to it, the wet tank pressure will drop quickly, causing the air governor to cycle the compressor on, then when the pressure reahes 120 in the wet tank, the air governor cycles the compressor off by sending pressure to the compressor unloader. The control line to the air dryer purge valve is connected to the unloader line and this pressure also causes the air dryer to purge each time the governor cycles the compressor off.
You would not notice a wet tank pressure loss on your dash gauges because there are check valves at the primary and secondary air tanks "downstream" to keep the air pressure from leaking back toward the wet tank, and the gauges read the pressure in the primary and secondary tanks.
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If you can't shift it smoothly, you shouldn't be driving it.
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