The blower started acting up for few weeks. There was a several consistent short rumblings of the blower, something like half second on and half second off after ignition. Once the rumbling is over, the blower fan just works fine. I can also hear the relay inside the control box clicks when it rumbles. It looks like the control box is trying to give power to the blower but shuts it down almost immediately and re-apply the power again. That caused the blower to labor on and off.

If I switch the fan control from auto to on, it's just working fine.

The rumblings has been getting longer and longer duration, it lasts from half a minute and it could be for minutes before it will stop.

Tried a new blower motor and a matching capacitor, changed the control relays on the control card, problem is still there.

Running the unit with the door open, no difference.

-Flame sensor is good,
-Vacuum sensor working fine and tested by covering the air in-take, flame will shuts off by itself.

The only way to keep the unit running here in the Great White North is to turn the fan "on" permanently.

I need professional advice badly... please help.

Many bad blower motors have this type of issue. Since you replaced the capacitor I suggest you replace the motor AND get a new capacitor of the correct rate for the new motor. Not all motors use the same capacitor and just because you bought a new one for the old motor does not mean it is the correct one for the new motor.

Many bad blower motors have this type of issue. Since you replaced the capacitor I suggest you replace the motor AND get a new capacitor of the correct rate for the new motor. Not all motors use the same capacitor and just because you bought a new one for the old motor does not mean it is the correct one for the new motor.

Tried a new motor and a new matching capacitor. It doesn't seem to be a motor problem. Tried to switch speed winding tabs, the control board relay still clicks - interrupting the power supply to the motor. It seems the control board sense something and turning off the power supply to the motor but re-apply the power all immediately and then off again...

Since you have tried the motor which was a expensive no cure I would concentrate on the low voltage end. Start by checking for 24 volts at the transformer. Many times a problem with low voltage will have a adverse effect on controls for blowers and such.

One question does this unit have the new style variable speed blower motors?

It's not the new DC motor with the variable speed control. It's just an AC 4 speed motor. The 24V seems to be fine. I just don't understand the relay on the control board clicks like.. click click click click, then sotp for couple second and then click click click click repeating the same cycle... Just wonder there must be something wrong with the control board.

Good thing is when the fan is set to "on" rather than "auto", the furnace just working fine.

Could be the control board but before replacing check to make sure all grounds for the board are good and that the ground (not neutral) is good and tight from the main electric panel to the unit. A bad/loose ground plays he$$ with circuit boards.

Sounds like it but because of spending $150 for a motor and $12 for the relays while problem is still exist.. I have to be very careful before jumping to a conclusion. Still trying to search I fI have missed any other possible cause...

The new DC motors are 500.00 plus the end bell electronics at 350.00 plus the special board at 250.00.

I tend to think there might be something wrong with the 24V.

Just checked the ground and it's good. However, noticed the LEDs are kind of flickering at 60HZ which may mean the 24V not stable. It could be related with dry out capacitor in the regulator circuit.

My next try is to connect the system with my own regulated 24V supply. Hope that will be the case... and then it'll be an easy fix.

Thanks for the suggestion in checking out the 24V!!

It seems I have found the problem. It seems the 24V is not stable. My guess was the 24V instability resets the control card and hence creating the clicks... on-off-on-off.. supply to the motor.

In searching the 24V supply, I have found there are three wires connected from the AC (just replaced with a high efficient Carrier 3.5ton machine). C-R-Y. I thought it's worth while a try to disconnect it as C-R may draw current from the 24V and making the circuit marginal.

The good news is the clicks has gone and the unit functioning normal now. I have trying to adjust the temperature up/down to force the unit cycled for 5-6 times now and it just working fine!

I have to call the contractor - Sears, now to see what's wrong with the new AC.

According to Carrier, this what in the installation manual:
---
Use furnace transformer, fan coil transformer, or accessory
Transformer for control power, 24--v/40--va minimum.
NOTE: Use of available 24--v accessories may exceed the
Minimum 40--va power requirement. Determine total transformer
Load and increase the transformer capacity or split the load with
An accessory transformer as required.
---

In fact 40VA is quite a bit and that's why the furnace just stalled.

Thanks a million!

Sears = major problem. LOL

Contractor... not the first time. Had a similar problem in the past, the same furnace was not turning on due to the contractor did not re-connect the thermostat control wire correctly after installing the electronic air filter. It was not Sear but similar...