Board/ Fuse Question?????

[MB905]

Hello, my name is Mike, im an apprentice in the Hvac field and a ride along...

Question> York Affinity Furnace with a 33" single speed psc ventor. It was a no heat call, the gentlemen installed his own t-stat, either tried changing on the fly or shorted wires after the fact. Once i got there i was sent in to investigate (no knowledge of tstat change, possible short, or that he tripped main breaker) Turn furnace switch on, call for heat, blower comes on right away, unusual prepuge then four flashes. Figured H/L, or Ventor TCO, for some weird reason before anything i decide to check for 120 from board to ventor terminals!

This is my question- There was 115v from the inducer board terminals connecting to my ventor, from there i spun the ventor (cooling fan i think?) and it Wasnt seized. Okay, open blower door, and sure enough 4 red flashes is (low air flow, fuse ) check fuse blown. Sorry and damn (customer reiterates)..
        How could 115vAC be at my inducer without it moving??? I think the fuse either causes some sort of dead leg not allowing voltage to return. Although it was a very simplistic inducer, hot in- common out- ground, how is it even diffrent with a good fuse<( I should of checked) Maybe it flipped polarity!
I love this site thanks in advance!!!!!!

[Admin]

What model of York Affinity were you working on?

Does the control board look like the one in this Igntion Control P/N S1-3310102956000 manual?

4 RED FLASHES: This indicates that a primary or auxiliary limit switch has opened its normally closed contacts. With this fault code the control will operate the supply air blower and inducer. This condition may be caused by: dirty filter, improperly sized duct system, incorrect blower speed setting, incorrect firing rate or faulty blower motor.  Also, this fault code could be caused by a blown fuse located on the control board.

It may be normal that both the inducer motor and fan blower motor are energized when the 24V fuse is blown.  I'm not sure why the inducer motor wasn't running when you measured 120V going to it.  There could very well be a version of control board that is doing what you are describing and opening the neutral side of the circuit.

[MB905]

Ya ive done some more research and basically from my understanding is that its a PWM pulse with modulation, so the 115v potential is there put ultimately the meter is fast enough to pick them up but the duty cycle is so small that it's providing basically no power to the motor. PWM, pulse width modulation. With a scope you would see very thin slivers at 60 hz. Thanks for the help, even though it may have been a dumb question, so thanks again!!