Conditions: Upper or Lower Sensor Reading Faulty, High Water Temperature, or suspect thermostat is not accurate.
SERVICE PROCEDURE PDV24-I
Thermostat Circuit Testing
Sensor Resistance Testing
Upper Sensor
1.Determine resistance value of upper sensor using an ohmeter. Test across grey wires.
2.Draw quart of water off T&P valve. 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 thermistors in the lower sensor should be close to each other. If the resistance values for both thermistors are not close to each other, replace the lower sensor. The dual thermistors 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.
Upper thermister location
(applicable models)
Checking resistance of
upper sensor
Lower thermister
access located inside
control box.
Are readings N | Replace |
| |
correct? |
| thermister | Checking resistance of |
Y |
|
| |
|
| lower sensor (center and | |
Check harness |
| Replace | outside pins) |
N |
| ||
continuity. |
| ||
| harness | CAUTION | |
Continuity okay? |
| ||
|
| ||
Y |
|
| Be Careful When Making Resistance |
|
| Measurements Not to Damage or | |
|
|
| |
Replace |
|
| Deform Connectors or Connector Pins. |
|
|
| |
control board |
| Sensor Resistance at Various Temperatures | |
|
|
Example: If water temperature is 84°F, then the resistance through the sensor would be 8449 (see shaded area).
NOTE: Sensor resistance increases as the temperature falls.
In Degrees F
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°F | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
40 | 26109 | 25400 | 24712 | 24045 | 23399 | 22771 | 22163 | 21573 | 21000 | 20445 |
50 | 19906 | 19383 | 18876 | 18383 | 17905 | 17440 | 16990 | 16553 | 16128 | 15715 |
60 | 15314 | 14925 | 14548 | 14180 | 13823 | 13477 | 13140 | 12812 | 12494 | 12185 |
70 | 11884 | 11592 | 11308 | 11032 | 10763 | 10502 | 10248 | 10000 | 9760 | 9526 |
80 | 9299 | 9078 | 8862 | 8653 | 8449 | 8250 | 8057 | 7869 | 7685 | 7507 |
90 | 7333 | 7165 | 7000 | 6839 | 6683 | 6531 | 6383 | 6238 | 6098 | 5961 |
100 | 5827 | 5697 | 5570 | 5446 | 5326 | 5208 | 5094 | 4982 | 4873 | 4767 |
110 | 4663 | 4562 | 4464 | 4368 | 4274 | 4183 | 4094 | 4006 | 3922 | 3839 |
120 | 3758 | 3679 | 3602 | 3527 | 3453 | 3382 | 3312 | 3244 | 3177 | 3112 |
130 | 3048 | 2986 | 2925 | 2866 | 2808 | 2752 | 2697 | 2643 | 2590 | 2538 |
140 | 2488 | 2439 | 2391 | 2344 | 2298 | 2253 | 2209 | 2166 | 2124 | 2083 |
150 | 2043 | 2004 | 1966 | 1928 | 1891 | 1856 | 1820 | 1786 | 1753 | 1720 |
160 | 1688 | 1656 | 1625 | 1595 | 1566 | 1537 | 1509 | 1481 | 1454 | 1427 |
170 | 1402 | 1376 | 1351 | 1327 | 1303 | 1280 | 1257 | 1235 | 1213 | 1191 |
180 | 1170 | 1150 | 1129 | 1110 | 1090 | 1071 | 1053 | 1035 | 1017 | 999 |
190 | 982 | 965 | 949 | 933 | 917 | 901 | 886 | 871 | 857 | 842 |
200 | 828 | 814 | 801 | 788 | 775 | 762 | 749 | 737 | 725 | 713 |
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