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Message: <blockquote data-quote="jarance" data-source="post: 380848" data-attributes="member: 21">Actually, the ATF thermostat is purely mechanical device. It the same working principle as the thermostat for the engine.problem is because of it size, it get easily choked due to the white sludge whereas the bigger water thermostat do not get choked easily due to its bigger size and location. It is located higher than the bottom of the radiator. The heat exchange for the ATF is located at the bottom of the radiator and the thermostat is also located at the bottom. The mechanic told me that the coolant react with the aluminum block and cause the white sediment/sludge. When the water pump is not running, the sludge will settle and clogged up the ATF thermostat.Anyway, I am using just water and flush the radiator of until the water turn clear. it was still whitish when I drove the car back. I drain the radiator and refill until the water is clear.The workshop also checked the ATF temperature via GT1 and found the ATF oil temperature to be normal. There is only a warning that the GB auto tranny is in trouble via a Gear teeth and "!" mark. It does not tell you the cause without the GT1 tool.IMHO, I recommend that the coolant be replace every year to prevent the white sludge.</blockquote>