Carrier 58MVP Troubleshooting Manual page 23

STEP
At start of cycle as inducer is ramping up, does low-pressure switch LPS make
17.
then break?
18. Does fault occur 45 sec after call for heat?
Do you have 24v between C (Common) on low-pressure switch LPS and Com
19.
on furnace control board?
Do you have 24v between connector terminal PL2-1 and Com on furnace con-
20.
trol board?
21. Turn power off.
Double check continuity between C (Common) on low-pressure switch LPS and
22.
connector terminal PL2-1.
The YELLOW wire to low-pressure switch LPS is not making good connection.
23.
Repair or replace it.
Do you have at least 0.95 in. wc pressure drop (0.75 in. wc on trophy unit)
across heat exchangers when fault occurs? Determine this by connecting 1 side
24. of a slope manometer with a tee to collector box pressure tap. Connect other
side with a tee to gas valve air pressure fitting. Refer to pressure check dia-
gram in Appendix C.
25. Is inducer motor MTR1 rotating in direction indicated on inducer housing?
26. Replace inducer motor or inducer motor assembly.
27. Fix problem.
28. Turn power off.
29. Is inducer blower wheel okay?
If possible, replace inducer blower wheel. Otherwise, replace inducer motor as-
30.
sembly.
31. Is there any frost build-up on combustion-air inlet?
The problem is caused by reversed vent and combustion-air pipes or some
32.
other moist air entering combustion-air pipe.
33. You have excessive restriction in combustion-air or vent pipe.
34. The problem is either a sag in vent pipe or a clogged drain trap or line.
Turn power off and disconnect jumper wire across R and W/W1 thermostat ter-
35.
minals.
Turn power on and wait several sec, then jumper R and W/W1 thermostat ter-
36.
minals on furnace control board.
Do you have 24v between connector terminal PL2-2 and Com on furnace con-
37.
trol board when fault occurs?
38. Turn power off.
Do you have continuity across terminal PL2-2 and N.O. (Normally Open) con-
39.
tact on low-pressure switch LPS?
The ORANGE wire from low-pressure switch LPS to furnace control board is
40.
not making good connection. Repair wire or replace harness.
Turn power off and disconnect jumper wire across R and W/W1 thermostat ter-
41.
minals.
Connect 1 side of a slope manometer with a tee to collector box pressure tap.
42. Connect other side with a tee to gas valve air pressure fitting. Refer to pressure
check diagram in Appendix C.
Turn power on and wait several sec, then jumper R and W/W1 thermostat ter-
43.
minals on furnace control board.
Monitor slope manometer. Does pressure reading appear to be stable when
44.
fault occurs?
Replace pressure switch assembly. If problem persists, check wiring for an in-
45.
termittent connection in low-pressure switch LPS circuit.
46. Check for a sag in vent pipe. If problem persists, replace inducer motor.
Do you have 24v between connector terminal PL2-2 and Com on furnace con-
47.
trol board?
48. The main harness is miswired.
Check Inducer PW line. To do this, disconnect PL7 from inducer motor and
connect a DC voltmeter across terminals PL7-2 BROWN (+) and PL7-10 YEL-
LOW (-) then run COMPONENT TEST by turning setup switch SW-6 ON. Does
49. voltage across PL7-2 and PL7-10 change between states as shown below?
• State 1—OFF (-0.1 — 0.1 vdc)
• State 2—LOW (6.0 — 7.0 vdc)
• State 3—HIGH (10.5 — 11.5 vdc)
50. Does inducer motor MTR1 turn on?
51. Do you have less than 90 vac between I1 and I2 on furnace control board?
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YES NO GO TO
34 18 —
50 41 —
24 20 —
21 13 —
— — 22
— — 23
— — 10
35 25 —
28 26 —
— — 10
— — 10
— — 29
31 30 —
— — 10
32 33 —
— — 27
— — 10
— — 27
— — 36
— — 37
13 38 —
— — 39
16 40 —
— — 10
— — 42
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— — 44
45 46 —
— — 10
— — 10
48 13 —
— — 27
26 13 —
19 51 —
53 52 —