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STEP
Reconnect PL3 to the variable speed furnace control and
connect a DC voltmeter across terminals PL3-3 YELLOW
(+) and PL3-2 GREEN (-). Does the voltage appear to fluc-
27
Note: Typical voltmeters will show a fluctuation of .2-vdc to
1-vdc. The amount of fluctuation is not important and you
could see even more fluctuation depending on the voltmeter
Check the blower motor serial output signal. To do this dis-
connect PL3 from the furnace control and connect a DC
voltmeter across terminals PL3-4 (+) and PL3-2 (-) on the
variable speed 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.
28
Note: Some voltmeters will not sense this fluctuation at all.
Test your voltmeter on a known good furnace prior to ser-
Note: You can also make a simple blinky light with a 1 KW
resistor and an LED (RED works best). These parts can be
purchased at a nearby Radio Shack. The schematic is
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 clos-
29
Does the voltage fluctuate as described in step 28?
Reconnect PL3 to the variable speed furnace control and
connect a DC voltmeter across terminals PL3-4 BLUE (+)
30
and PL3-2 GREEN (-). Does the voltage fluctuate as de-
Disconnect PL13 from the blower motor and connect a DC
voltmeter across terminals PL13-10 BLUE (+) and PL13-1
31
GREEN (-). Does the voltage fluctuate as described in step
You have an open wire or bad terminal on the BLUE wire
32
between connectors PL3 and PL13. Repair it or replace
Disconnect PL13 from the blower motor and connect a DC
voltmeter across terminals PL13-10 BLUE (+) and PL13-1
33
GREEN (-). Does the voltage fluctuate as described in step
You have short to ground on the BLUE wire between con-
34
nectors PL3 and PL13. Repair it or replace harness.
The blower motor and furnace control are communicating
35
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.
ACTION
tuate more than it did in step 26?
you use.
vicing this product.
shown in Figure 1:
est to the flat side goes to PL3-2.
scribed in step 28?
28?
harness.
28?
but the RPM must be below 250.
1KΩ
PL3-4
Fig. 1—Blinky Light Schematic
YES
20
30
31
15
15
PL3-2
30
NO
GO TO
28
29
20
33
32
8
34
8
14
A03094
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