Blower Motor Making Abnormal Noise

[LTZ C20]

Hey all, I believe my blower motor is making an abnormal noise but I can't figure out why or how it's doing it. It's making fairly loud rumbling, like it's off balance and rubbing on something in one spot.

It used to make the same noise only when I turned the truck off and the motor was free spinning down to a stop, now it's doing it when the truck is on and it's pretty loud and annoying.

What I have done:
-removed blower motor and inspected HVAC case for debris, none found.
-during inspection of case with motor out, inspected motor and fan for damage or debris, none found.
-inspected for signs of rubbing on fan, none found.
- checked power and ground, both check good.
-checked to make sure blower motor was completely bolted down and not loose, checked good.
-spun the blower motor as fast as I could with my hand to see if it is wobbling, checked ok.
-turned the ac on and listened to each speed one at a time and they all sound normal.

It's when you have the ac switched to off so that the blower motor is doing its slow constant spin just to keep air moving, I believe it's called the anti-mildew function, or when you shut the truck off and it free spins down. On all 4 speeds with the ac on it sounds normal.

I've had this thing out 2 or 3 times to see if it wasn't installed right or was rubbing on something or shaking but everything seems ok.

Note: it's been doing this intermittently for about 6 months, only at key off. Now it's doing it key on and off, with key on you have to have the HVAC turned off. The rest of the HVAC system is functioning as designed.

[bd]

How much end play on the armature?  Did you inspect the squirrel cage carefully for a cracked vane or tossed weight?  Absolutely positive nothing is hiding in the duct that might be playing roller ball with the fan?  Replace the motor with one for a 94 1500 with A/C.

[LTZ C20]

How much end play on the armature?  Did you inspect the squirrel cage carefully for a cracked vane or tossed weight?  Absolutely positive nothing is hiding in the duct that might be playing roller ball with the fan?  Replace the motor with one for a 94 1500 with A/C.

Ok questions.

1 - which part are you referring to as the armature? Is that the shaft the fan barrel rides on?

2 - Yes I inspected the cage pretty good, I didn't seen any signs of a tossed weight or a cracked blade. I can always check again of course if needed.

3 - I'm about 95% sure nothing is in there. Again, I can remove and re-inspect again if needed.

4 - If everything is ok and I do replace the motor with a 1994 unit, what is so special about the 94 unit compared to the unit for my truck? Do I replace the squirrel cage also or just the motor?

[bd]

The shaft assembly is the armature.  The motor probably has a bad bearing.  Unfortunately, the bearings aren't serviceable. 

The 1994 blower motor produces greater air volume out the dash nozzles.  If the updated motor comes with a new cage, compare the new unit to the old one.  If it comes w/o the cage, transfer the old cage over to the new motor.  Before you install it, however, make sure the cooling ports in the motor housings are the same diameter.

[LTZ C20]

Ok I understand on all points. I'll double check everything and then make a decision based on my findings and report back at my earliest opportunity.

The bad bearing was the direction my diagnosis was leaning, as always, a second or third opinion is always good. My blower has some pretty good velocity currently on high speed, I can't imagine what it will be like with even more flow. Does the 94 model happen to draw less current on the system during operation?

[bd]

Expect current draw to be similar to a new motor that is cataloged for your year/model.  However, "new" motors often draw a little less current than "worn out" motors.

Another thing to look for is the lower stop for the cage.  Make sure the cage is properly seating on the keyed stop that slips onto the armature below the cage and properly positions the cage on the shaft.

[LTZ C20]

Got it. The lower stop, is that the metal ring with the allen head set screw in the side that tightens onto the shaft?

On a side note, the controls for these trucks are cable operated for the temp door, vacuum operated for the mode positions and an electrical switch for the fan speed. The 87-95 models that have the push button HVAC control head with the green LCD display, with the yellow "off" and blue "ac" buttons, does that style control head operate the same or do they used electric door actuators for the mode positions and temp door?

[bd]

Does your blower look like the one imaged below with the plastic cage?

[LTZ C20]

Uhhh, I don't remember if it has a plastic cage or metal cage. It's been awhile since I had it out, I can't recall which it was.

[bd]

The 'stop' I referred to is a roughly 1/2" square, stamped steel support that slips down the motor shaft and keys into the base of a plastic cage to support and locate the cage on the shaft.  The Allen setscrew collar you referenced sounds like the upper anchor for a steel cage.

[LTZ C20]

Ok. Well that makes more sense. I'm leaning towards mine being a steel cage then because I seem to remember something about checking for tightness on a collar with set screw. I'm almost back to the truck, I'll take a look with my flashlight and see if I can see it thru one of the un-used holes in the mounting flange.

[bd]

On the "push-button' A/C control head... you are probably thinking of those used in the newer C/K body style that had motorized actuators.

[LTZ C20]

On the "push-button' A/C control head... you are probably thinking of those used in the newer C/K body style that had motorized actuators.

So we are on the same page, this unit is when they started using electric actuators.


As for the blower motor, I found a small hole to probe thru the mounting flange, the cage is a steel one. When I tapped on it, it sounded tin like, as metal tapping on metal. It did not sound like metal tapping on plastic.

[bd]

So we are on the same page, this unit is when they started using electric actuators.

Yes.

NAPA markets the 1994 blower as a complete assembly under a couple of different P/N's.

[LTZ C20]

If I end up replacing the motor, I will go to work first and try to get an AC Delco unit, if not then I will look to an auto parts store or possibly online for a Delco unit.