andrews 32/143 Installation Manual, Operation And Service Manual page 80

APPENDICES
Conditions: Upper or Lower Sensor Reading
Faulty, High Water Temperature, or suspect
thermostat is not accurate.
Sensor Resistance Testing
Upper Sensor
1. Determine resistance value of upper sensor. Test across
grey wires.
2. Draw quart of water off T&P valv e. Using a thermometer,
determine water temperature.
3. Use table below to verify correct resistance per water
temperature measured.
Lower Sensor:
1. Determine resistance value of lower sensor. Test across
center wire (common) to each outside wire. Resistance of both
thermisters in the lower sensor should be close to each other.
If the resistance values for both thermisters are not close to
each other, replace the lower sensor. The dual thermisters are
used to provide high limit protection in case the thermostat
circuit fails to shut off the water heater.
2. Draw quart of water off Drain Valve.
Using a thermometer, determine water temperature.
3. Use table below to verify correct resistance per water
temperature measured.
Are readings N
correct?
Y
Check harness
N
continuity.
Continuity okay?
Y
Replace
control board
Example: If water temperature is 26.7
°C 0
4.5 26109
10 19906
15.5 15314
21.1 11884
26.7 9299
32 7333
37 5827
44 4663
49 3758
55 3048
60 2488
65.5 2043
71 1688
76 1402
82 1170
88 9 82
93 828
Replace
thermister
Replace
harness
Be Carefu l When Making Resi st ance
Measurem ents Not to Damage or
Deform Conne ct or s o r Co nne ct or Pi ns .
Sensor Resistance at Various Temperatures
° C, then the resistance through the sensor would be 8449 (see shaded area).
NOTE: Sensor resistance increases as the temperature falls.
In Degrees C
1 2 3
25400 24712 24045 23399
19383 18876 18383 17905
14925 14548 14180 13823
11592 11308 11032 10763
9078 8862 8653
7165 7000 6839
5697 5570 5446
4562 4464 4368
3679 3602 3527
2986 2925 2866
2439 2391 2344
2004 1966 1928
1656 1625 1595
1376 1351 1327
1150 1129 1110
965 949 933
814 801 788
Page 26
SERVICE PROCEDURE D24-I
Thermostat Circuit Testing
Upper thermister location
(applicable models)
10
Checking resistance of
upper sensor
Lower thermister
access located inside
control box.
11
Checking resistance of
lower sensor (center and
outside pins)
CAUTION
4 5 6
22771 22163 21573
17440 16990 16553
13477 13140 12812
10502 10248 1000
8449 8250 8057 7869
6683 6531 6383 6238
5326 5208 5094 4982
4274 4183 4094 4006
3453 3382 3312 3244
2808 2752 2697 2643
2298 2253 2209 2166
1891 1856 1820 1786
1566 1537 1509 1481
1303 1280 1257 1235
1090 1071 1053 1035
917 901 886
775 762 749
80
7 8 9
21000 20445
16128 15715
12494 12185
9760 9526
7685 7507
6098 5961
4873 4767
3922 3839
3177 3112
2590 2538
2124 2083
1753 1720
1454 1427
1213 1191
1017 999
871 857 842
737 725 713