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Lower, Temperature probe plug, Ohms – A.O. Smith 960 through 967 User Manual

Page 24: Temperature probe, 6 temperature probe resistance

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BTH 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

LOWER

TEMPERATURE

PROBE PLUG

ohms

1

r

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
temperatures

replace temperature probe if thermistor is open,
shorted, or resistance value is off significantly.

ohm reading does approximately correspond to table at given
temperatures

continue to Step 7.

WATER TEMP

OHMS RESISTANCE

X

°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.

A. O. Smith Water Products Co.
Ashland City, Tennessee © 2004

23

Technical Training Department

TC-044 Revision 6

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