There are a couple things that come to mind. First it is unlikely that the switch is the issue. However, I attached the testing of it. The way that operates is it senses the need for assistance and the ECM then ups the idle speed just a bit to spin the pump faster and increase the pressure.
It sounds like either you have air in the system and when it pumps air rather than fluid it gets stiff. It could also be a loose belt as well and when the assist is needed the belt slips and that causes the stiff feeling you are getting.
Run through this info and let me know what you find and we can go from there. Thanks
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Tuesday, March 3rd, 2020 AT 4:55 PM