- 33 -
STEP
ACTION
YES
NO
GO TO
28.
Check the blower motor serial output signal. To do this disconnect PL3 from the furnace
control and connect a DC voltmeter across terminals PL3-4 (+) and PL3-2 (-) on the
modulating furnace control. The voltage should be near 0-vdc but it will fluctuate briefly
several times a second. If you have an analog voltmeter the needle will briefly go high
several times a second. If you have a digital voltmeter with a bar graph it will show a large
change in magnitude on the bar graph several times a second. If you have a plain digital
voltmeter it will show a brief fluctuation in voltage and the magnitude may vary depending
on the volt meter used.
Note:
Some voltmeters will not sense this fluctuation at all. Test your voltmeter on a known
good furnace prior to servicing this product.
Note:
You can also make a simple blinky light with a 1 K
resistor and an LED (RED
works best). These parts can be purchased at a nearby Radio Shack. The schematic is shown
below:
29
When using the blinky light the LED will flash briefly several times a second when the
blower motor serial output signal is working properly. LED’s are directional and the lead
closest to the flat side goes to PL3-2.
29.
Does the voltage fluctuate as described in step 28?
30
20
30.
Reconnect PL3 to the modulating furnace control and connect a DC voltmeter across
terminals PL3-4 BLUE (+) and PL3-2 GREEN (-). Does the voltage fluctuate as described
in step 28?
31
33
31.
Disconnect PL13 from the blower motor and connect a DC voltmeter across terminals PL13-
3 BLUE (+) and PL13-4 GREEN (-). Does the voltage fluctuate as described in step 28?
15
32
32.
You have an open wire or bad terminal on the BLUE wire between connectors PL3 and
PL13. Repair it or replace harness.
8
33.
Disconnect PL13 from the blower motor and connect a DC voltmeter across terminals PL13-
3 BLUE (+) and PL13-4 GREEN (-). Does the voltage fluctuate as described in step 28?
15
34
34.
You have short to ground on the BLUE wire between connectors PL3 and PL13. Repair it or
replace harness.
8
35.
The blower motor and furnace control are communicating but the RPM must be below 250.
36
36.
Disconnect jumper wire across R and W/W1 thermostat terminals.
37
37.
Remove tape from door switch and turn power off at main disconnect.
38
38.
Does the blower wheel rub against blower housing?
12
39
39.
Does the blower wheel turn freely?
40
15
40.
Is blower wheel firmly mounted on motor shaft?
15
12
STEP
ACTION
YES
NO
GO TO
41.
Remove tape from door switch and turn power off at main disconnect.
42
42.
Disconnect both multi-pin connectors from blower control module attached to the blower
motor. Be sure to depress release latches on connectors or they may get damaged.
43
43.
Remove blower assembly from furnace.
44
44.
Remove two phillips head or two ¼-in. hex head bolts from blower control module attached
to blower motor.
45
45.
Carefully lift blower control module off blower motor. Depress latch on internal connector to
disconnect blower control module from motor portion of blower motor. DO NOT PULL ON
WIRES. GRIP PLUG ONLY.
46
WARNING: Wait at least 5 minutes after disconnecting line voltage from equipment before opening blower motor to
prevent electric shock which can cause personal injury or death.
1K
PL3-4
PL3-2
440 08 4001 00
