S U F S E R V I C E H A N D B O O K

T R O U B L E S H O O T I N G

TEMPERATURE PROBE

STEP 6

TEMPERATURE PROBE RESISTANCE

UPPER

LOWER

TEMPERATURE

TEMPERATURE

PROBE PLUG

PROBE PLUG

STEP 6: CHECK RESISTANCE OF THERMISTOR IN UPPER AND LOWER TEMPERATURE PROBES.

Condition:

Disconnect CN3 and CN8 plugs from control board.

On/Off switch is off.

Multimeter is set to lowest ohms scale above expected resistance.

Check resistance between black wire connects on CN3 and CN8 plugs as shown in illustration above.

IF…

THEN

 

 

 

 

ohm reading does not approximately correspond to table at given

replace temperature probe if thermistor is open.

temperatures

shorted, or resistance value is off significantly.

 

 

ohm reading does approximately correspond to table at given

continue to Step 7.

temperatures

 

 

 

WATER TEMP

OHMS RESISTANCE

 

 

°C

°F

 

 

 

 

 

 

 

3

40

26,435

21

70

11,974

 

 

 

37.5

100

5,862

49

120

3,780

 

 

 

54.5

130

3,066

60

140

2,503

 

 

 

71

160

1,698

82

180

1,177

 

 

 

NOTE: The upper and lower temperature probes contain thermistors which are heat sensitive resistors. The control board interprets changes in thermistor resistance as changes in water temperature.

These thermistors are very reliable and should only be replaced when:

The resistance test indicates an “open” (infinite resistance) or a “direct short” (no resistance) circuit.

The nature of the service problem is temperature control and the resistance readings are significantly off when com- pared to the values in the table here at the given tempera- ture.

State Water Heaters

23

Technical Training Department

Ashland City, Tennessee © 2004

STC-077

Page 24
Image 24
State Industries STC-077 Temperature Probe Resistance, Upper Lower Temperature, Probe Plug, Water Temp Ohms Resistance

STC-077 specifications

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