Very little HVAC airflow - SOMETIMES

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slovcan

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Hi folks,

1997 K2500 Extended cab Silverado

For a long time now I have noticed unreliable operation of the mode selection (Defrost, Floor, Vents). Sometimes after a while I'll get the mode I want - usually the defrost/floor combo. Where it really showed the problem was trying to get good flow through the dash vents in summer. Sometimes when I start the truck I can hear a bit of a thump as an air door closes (or opens). Sometime the flow will increase or come out the right place after I've been driving for a little while.

Last spring I had a new compressor installed and that gives nice cold air now.

Now in the dead of winter I get nice warm/hot air so the heater core is doing its job.

The real problem with all the above has always been the lack of airflow. I changed the blower motor/fan yesterday. The old one started making some dry squealing noise for a few minutes when first started cold. The new one is nice and quiet and smooth running. When I first tried it yesterday after install I got good airflow to the floor and OK airflow to the windshield as selected.

Today, however, I am getting very little airflow anywhere. It does seem to go where directed, but there is so little it likely won't clear the windshield or heat the truck.

I am hoping I have a couple of worn out mode/blend motors, but NOT the one that requires the removal of the dash. I do know the recirc has always worked as it should.

When I had the blower motor out yesterday I did sweep my hand around inside the heater box and it was clean - no leaves or crap to feel.

So, can anyone give me a reason to continue living?

Thanks,
Glenn
 

east302

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The actuator by the gas pedal would be the one to check. The one behind the ashtray is for the heater core.

There’s a silver position indicator dial on the actuator and lines on the actuator itself. From floor to defrost, it will rotate between those marks at the 11 and 1 o’clock positions. See if it does.

If it doesn’t, see if the two actuator arms and springs are intact. If so, take the wiring connector off and there are three wires. I forget the colors, but one is 12V (battery), one is ground and the other is the signal from the panel. The signal voltage should modulate smoothly as the dial is turned. See what range you get. It’s either 1 to 5V or 1 to 12V from
one end to the other. If it’s constant, then the panel isn’t signaling right.

If all that passes, best guess is a damper issue - somehow dislodged or something blocking movement. Pull the dash to get to that. The air is going somewhere if the blower is good. See if it’s coming from the center floor vent at all settings. The damper there had a rubber border that can rot and not seal correctly.

If it’s an overall lack of airflow, take another look at the evaporator core (pull the blower and it’s to the left). Fan speed audibly changes correctly? If not, pull the voltage wire off the blower and see what you get from low to high. High would be battery voltage. If there’s a lack of airflow only after driving with the a/c on for a while, the core could be freezing up - check the refrigerant charge.
 
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slovcan

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Thank East302. I'll reply a bit below.

The actuator by the gas pedal would be the one to check. The one behind the ashtray is for the heater core.

There’s a silver position indicator dial on the actuator and lines on the actuator itself. From floor to defrost, it will rotate between those marks at the 11 and 1 o’clock positions. See if it does.


I will check this today. It does seem to be intermittent with no seeming reason for the system to perform better at one time or another.

If it doesn’t, see if the two actuator arms and springs are intact. If so, take the wiring connector off and there are three wires. I forget the colors, but one is 12V (battery), one is ground and the other is the signal from the panel. The signal voltage should modulate smoothly as the dial is turned. See what range you get. It’s either 1 to 5V or 1 to 12V from
one end to the other. If it’s constant, then the panel isn’t signaling right.


I will check this, too, but something tells me the panel is OK. Does the panel ever fail intermittently?

If all that passes, best guess is a damper issue - somehow dislodged or something blocking movement. Pull the dash to get to that. The air is going somewhere if the blower is good. See if it’s coming from the center floor vent at all settings. The damper there had a rubber border that can rot and not seal correctly.

I am seriously hoping it is not a damper issue. I don't think I am up to pulling the dash. Also, I would think (wish?) that a damper issue would not be intermittent. If it was out of place, would it sometimes go back into place and work correctly? Here's where you say no, not intermittent. ;)

If it’s an overall lack of airflow, take another look at the evaporator core (pull the blower and it’s to the left). Fan speed audibly changes correctly? If not, pull the voltage wire off the blower and see what you get from low to high. High would be battery voltage. If there’s a lack of airflow only after driving with the a/c on for a while, the core could be freezing up - check the refrigerant charge.

Yeah, the fan definitely has 4 speeds. I have seen the A/C pipe under the hood covered in frost, but this time the A/C has not been turned on. I suspect I may have to pull the motor again. I swept my hand around in the round section where the fan lives, but not up and over to the left where there may be a buildup of stuff by the evaporator. Maybe this stuff is being blown around and covering some of the evaporator and preventing the air from passing through. Could this be?

In relation to the mode actuator and doors, can these doors stop moving because of the actuator in a position that actually severely reduces airfow? For example, mostly blocking both floor and vents or something like that?


Thanks,
Glenn
 

slovcan

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OK, I did some testing and took some pictures. First of all, I always set my HVAC on the Defrost-Floor split. That's where it was when I shut the truck off last night and it was still there today before I started it - according to the index marks on the actuator.

Today everything is working great. It stayed working great for the hour or so that I was testing and driving. Every mode selection I made - Defrost>Defrost-Floor split>Floor>Floor-Dash Vents split>Dash Vents worked fine. The Recirc button also worked fine.

What didn't work fine ------- the A/C. It is always on. When I had the compressor replaced last spring it worked as designed. Press the A/C button ON and the compressor clutch would engage and you'd have air conditioning. Press the A/C button OFF and the compressor clutch would disengage and the air conditioning would be off.

Today, though, the air conditioning is ON. Always. Pressing the A/C button to ON simply turns on the indicator light in the button. Pressing the A/C button to OFF simply turns off the indicator light in the button. The compressor clutch is engaged all the time regardless.

So, what's up with that?

Defrost-Floor split position before starting from the night before.
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Defrost-Floor split rods look fine.
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Defrost position index mark
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Defrost-Floor split index mark
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Floor position index mark
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slovcan

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A few more pictures:

Floor-Dash Vent split index mark
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Dash Vent position index mark
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A/C ON even though not selected on
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someotherguy

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Sure sounds like a bad control unit. I'd wait for other opinions though before grabbing another; it's been a while since I've messed with them. Pretty classic failure mode IMO though.

Richard
 

slovcan

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Thanks Richard. Yeah, it is possible. The other thought I had was in view of the constant A/C running maybe the evaporator froze up and blocked airflow.. That still sort of says the A/C was on when it shouldn't have been, though.
 

slovcan

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OK, update. After owning the truck for 12 or 13 years, is it possible I just had finger trouble? Maybe.

I have discovered that with the A/C button selected OFF, any Mode setting from about halfway between Floor and Floor-Defrost split and all the way up to full Defrost, the A/C will operate. The indicator light in the A/C button stays off.

At any setting from about halfway between Floor and Floor-Defrost split all the way down to Dash Vents only, the A/C will stay off. If I push the A/C button in any range, the indicator light will come on and the A/C will come on (compressor engaged).

It almost seems as though it could be normal operation or, at least, now I know how to control it.

I have not lost airflow since this all started the day after I changed the blower motor. I still suspect either the evaporator froze up and blocked airflow or there is debris in the recess by the evaporator that gets blown up onto the evaporator sometimes and blocks airflow. I will monitor that situation.

Cheers,
Glenn
 

east302

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The position indicator marks in your photos look good. Defrost will engage the compressor so that’s normal as others said. If there was rust on the blower motor shaft when you pulled it, look for a water leak at the cowl vent screw hole directly above it. Put a mirror in with the blower removed and you’ll see the screw sticking though. Put a hose to the windshield and see if water comes though. Goop the hole and screw with RTV and that should fix it. Wet carpet is another indicator.
 
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