Checking Temperature Sensors - Weil-McLain WM97+ Manual

WM97+ -
gas fired water boiler
Troubleshooting
Check control module fuses
ALWAYS check control module fuses before replacing control
module or any major components (blower, etc.). If one of
these fuses is blown, it can prevent control module or other
components from operating.
1. Turn OFF power to boiler at external line switch. Then remove the
jacket door.
2. Access the control module fuses.
a. See Figure 84. Rotate item 1, the inlet air silencer, about 45 degrees
so it will be out of the way.
b. Remove the two screws, item 4.
c. Note the three slots (1, 2 and 3) on either side of the jacket. Lift
the control housing (item 2) until the top support arms are aligned
with slot 2, as shown. Pull the top of the housing forward so the
upper arms swing free. Continue rotating the control housing until
it rests on the jacket lower flange as in Figure 85.
d. Disconnect the ignition wire (at item 7). Remove the control hous-
ing back cover (item 6) by pressing the retainer clips (item 5), then
lifting and pulling cover forward.
3. Remove and inspect the two fuses (items 8 and 9, Figure 86).
4. If necessary, replace one or both of the fuses:
a. Low voltage circuit fuse is 3-amp fast-blow (Littelfuse 326012P).
b. Line voltage circuit fuse is 12-amp slow-blow (Littelfuse 257003).
Do not jumper either fuse or replace with any fuse except as
specified. Failure to comply could result in severe personal
injury, death or substantial property damage.
5. Reinstall control housing cover and return control housing to normal
position. Replace screws (item 4) and reconnect ignition cable at item 7.

Checking temperature sensors

1. The boiler temperature sensors (flue, outdoor, return water and supply
water) are all resistance-type devices. Figure 83 shows the correct value
for the sensor at various temperatures.
2. Use the resistance values at 32°F, 60°F, 70°F and 212°F to measure the
sensor resistance at known temperatures (ice point, room temperature
and sea level boiling point). For ice point and boiling point, insert the
sensor in water at that temperature. Use an ohmmeter to read resis-
tance value.
3. To check whether the control module is correctly sensing temperature,
you can use a resistance decade box. Connect the decade box temporar-
ily in place of a sensor and read the corresponding temperature on the
control module display. The temperature should be close to the value
corresponding to the input resistance.
Figure 83
Sensor resistance values
Temp Sensor ohms Temp
(°F) (°F)
Min Max
32 90
34265 37871
40 100
27834 30764
50 21630 23907 110
60 16944 18727 120
70 130
13372 14780
80 140
10629 11747
Part number 550-142-058/0811
— Boiler Manual
(continued)
Sensor ohms Temp Sensor ohms
(°F)
Min Max Min
150
8504 9399 2517
160
6847 7568 2091
5545 6129 170 1744
4517 4992 180 1461
190
3698 4088 1229
200
3043 3364 1038
Figure 84 Control housing lifted to slot 2 (jacket
door removed and inlet air silencer
rotated out of the way)
Figure 85 Control housing rotated forward
Figure 86
Control housing with back removed
— close-up of control module fuse
location on the control board
Max
2782
2311
1928
1615
1359
1147
83