What is Car Coolant Water Temperature Switch and its Function

“常開” 和 “常閉” (50-55 度) of the car coolant water temperature bimetallic switch. This question is very specific. It seems that users may need to distinguish between these two types of switches when repairing or modifying vehicles.
The difference between normally open and normally closed water temperature bimetallic switches: the normally closed type (marked as B) closes the contact at normal water temperature and opens when the temperature is too high; the normally open type (marked as K) is the opposite, opening at normal temperature and closing at overtemperature. This distinction standard is universal in the automotive industry, and users can judge by looking at the model identification of the switch housing.

Kawasaki water temperature sensor motorcycle temperature control switch 27010-1202

car coolant temperature control switch 17680-84000 17680-84200 water temperature sensor

0065457124, Radiator Fan Temperature Switch Solid for Car

In terms of technical principles, it is mentioned that the core of the automotive temperature control switch is a disc-shaped bimetallic strip structure, which drives the contact action through the principle of thermal expansion and contraction. This is very important, indicating that its action mechanism is a purely physical characteristic with high reliability. The definition of normally open and normally closed in the field of solenoid valves is supplemented as an analogy. Although it does not belong to the category of water temperature switches, it can help understand the general concept of “normally open and normally closed”.

Notice the deep needs that users may have: how to choose? It is mentioned that the car water temperature switch mainly adopts the normally closed design, because the engine cooling system needs to continuously monitor the temperature and only trigger the fan when it is abnormal. This confirms the design logic of the normally closed type that is also used in the micro motor protection switches such as the sunroof/wiper of the car. 然而, there is an exception scenario – if the equipment needs to be powered on for a long time, it is more suitable to use the normally open type, but this is less common in water temperature switches.

Technical characteristics and key points of distinction between normally open and normally closed types in automotive water temperature switches:
‌I. Core definition and working principle ‌ ‌Normally Closed (數控) ‌ ‌Working state ‌: At normal water temperature, the contacts remain closed to conduct the circuit; when the water temperature exceeds the set threshold (such as 95°C), the contacts are disconnected and the circuit is cut off.
‌Identification features ‌: The model is usually marked with the letter “‌B‌” (such as KSD9700-B) 或 “數控”.
‌Application scenario ‌: The mainstream control method of automotive cooling fans (accounting for > 90%), the fan is turned off and the power is stopped after the water temperature reaches the standard.
‌Normally Open (唔係) ‌ ‌Working state ‌: The contacts are disconnected at normal water temperature; when the temperature is over (such as 105°C), the contacts are closed, activating the cooling fan to run at high speed.
‌Identification features ‌: The model is marked with the letter “‌K‌” (such as KSD9700-K) 或 “唔係”89.
‌Application scenario‌: auxiliary high temperature protection, often used with normally closed switch to realize dual-speed fan control.

‌II. Parameter characteristics comparison‌

Note‌: Reset temperature difference refers to the difference between the operating temperature and the temperature of resuming work (such as 95℃ disconnection, 85℃ reset).

Water temperature sensor for Chrysler Citroen 024263 M14 thermostat

熱敏掣 195 數控 195 Degree Normally Closed, 8037024P

01102 radiator water tank thermal switch M14

‌III. Fault diagnosis and selection suggestions‌
‌Fault manifestation‌
‌Normal closed failure‌: The water temperature is too high but the fan does not start (the contact cannot be disconnected).
‌Normal open failure‌: Insufficient high-speed heat dissipation (the contact cannot be closed).
→ Detection method: multimeter continuity test + water temperature simulation (hot water heating method).

‌Selection principle‌
‌Priority normally closed type‌: Applicable to most single-speed fan systems (such as economy cars).
‌Dual switch combination‌: High-end models recommend “常閉 + normally open” graded control (95℃ low speed rotation/105℃ high speed rotation).

‌IV. Typical application examples‌

‌Hyundai Motor‌: 39230-26600 water temperature sensor switch (常閉, operating temperature 92°C)1.
‌Dual-speed fan system‌:

mermaid Copy Code
graph LR
一個[water temperature < 95°C] –>|normally closed contact closed| B(fan stopped)
C[water temperature ≥ 95°C] –>|normally closed contact open| D(low-speed fan started)
E[water temperature ≥ 105°C] –>|normally open contact closed| F(high-speed fan started):ml-citation{ref=”5,13″ data=”citationList”}

‌Tip‌: When replacing, you need to match the threaded interface (such as M18×1.5, M14 x 1.5mm) and confirm the B/K model suffix 813.