Very little HVAC airflow - SOMETIMES

[eran tomer]

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.

what's that damper?

 

[Sprinter77]

Hi Sprinter,

The screw is under the hood. Open the hood and it is about the second closest to the right side fender. The screw hole goes into the top of the airbox where the blower motor lives. Water starts to leak around that screw. Doesn't affect airflow unless water kills the motor, of course.

Maybe you have an actuator issue. That doesn't require pulling the dash. Another possibility I learned about here is the possible collection of leaves or other debris in front of the evaporator - in through the blower opening and up over to the left.

hi slovcan
I think that the most likely issue is the doors in the a/c unit in the dash are not attached somehow as ehall8702 said. that's what he found in the dash. thanks for the info on the screw under the hood into the airbox. in this desert environment we don't get very much rain and i have changed blower motors and made sure there is no debris in front of the evaporator. thanks for your info and reply.

 

[Sprinter77]

i'll write a report on what I find (if I get that far) right now I'm working on a brake issue. i'll update that issue in the brake thread later (if I can correct that)

 

[east302]

what's that damper?

There are three inside the box. The lower one is visible if you pull the cover off the floor outlet. Odds are that the rubber seal is rotted (red arrow) so if all your air is going out the floor vent then that would reduce flow from the dash.

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[slovcan]

Well, I'm back. I am still getting reduced airflow sometimes. It seems to be reduced regardless of which output is selected - Defrost, Vent or Floor - after being on Defrost or Defrost/Floor split for a while. I think it may well be the evaporator icing up. When it happens I select Floor only and after a little while strong airflow is restored. I had a new A/C compressor installed last spring. No leaks, but maybe it was slightly underfilled. What do you all think?

Thanks,
Glenn

 

[someotherguy]

If the evaporator is icing up, turning off the A/C but leaving the fan on (with temp selector on hot, since it's likely cold up there in NS right now!) should melt the ice pretty quickly. If you are unsure the A/C compressor is actually turning off (controls may not be functioning properly) - you can unplug the clutch connection on the compressor. Obviously be careful; don't be tempted to do this with the engine running. Don't trust those 95-up controls; they can frequently do the opposite of what you are expecting them to do.

You probably already know this and I can't remember if I mentioned it in this thread - all airflow through a GMT400 HVAC system passes through the evaporator core, regardless of control settings. If it's iced up, yes, you will absolutely lose significant airflow.

Richard

 

[SS Performance]

Last summer in my 96 I was having trouble with my AC. It would be blowing cold air then after awhile I could hear the blower working real hard but no air out of the vents. I added freon and it would work for a while then the problem would recur and I would add more freon. I took it to a shop and they discovered I had OVER filled my freon.

They reset the freon to the proper level. After that the AC would work for awhile then would slowly the air out oof the vents would get warmer and stop blowing.

I have driven the truck for several months so I don't know how the defrost or heat are.

Thanks
Craig

 

[slovcan]

If the evaporator is icing up, turning off the A/C but leaving the fan on (with temp selector on hot, since it's likely cold up there in NS right now!) should melt the ice pretty quickly. If you are unsure the A/C compressor is actually turning off (controls may not be functioning properly) - you can unplug the clutch connection on the compressor. Obviously be careful; don't be tempted to do this with the engine running. Don't trust those 95-up controls; they can frequently do the opposite of what you are expecting them to do.

You probably already know this and I can't remember if I mentioned it in this thread - all airflow through a GMT400 HVAC system passes through the evaporator core, regardless of control settings. If it's iced up, yes, you will absolutely lose significant airflow.

Richard

Hi Richard,

Thank you. I think you have confirmed my suspicions. I'm sure the system is working as designed and the controls are OK.
anytime Defrost or Defrost-Floor is selected the A/C comes on. I can see the pipes and receiver/dryer can all completely covered with a thick white frost after a while. It makes sense that the evaporator would also be. When the airflow all but disappears, I select Floor only (stops the A/C) and the heat clears the evaporator and airflow resumes. That's how it seems, anyway.

Last summer in my 96 I was having trouble with my AC. It would be blowing cold air then after awhile I could hear the blower working real hard but no air out of the vents. I added freon and it would work for a while then the problem would recur and I would add more freon. I took it to a shop and they discovered I had OVER filled my freon.

They reset the freon to the proper level. After that the AC would work for awhile then would slowly the air out oof the vents would get warmer and stop blowing.

I have driven the truck for several months so I don't know how the defrost or heat are.

Thanks
Craig

I don't know, Craig. It sounds like you have a leak and when it gets low it does what mine does and ices up. Maybe it was just overfilled that one time when you took it to the shop?

 

[slovcan]

One more thought on this...

Even if my problem is an underfilled system, the clutch cycle switch should be stopping the compressor before everything ices up, shouldn't it?

I'm getting the charge vacuumed out and weighed today. It was empty or nearly so when I had the new compressor and dryer installed, so there is a chance it was a bit underfilled. I will order a new clutch cycle switch anyway, because the original now has 283,000 kms and a failed compressor on it.

 

[slovcan]

Well, that didn't go entirely to plan.

The refrigerant was down to 1.5 lbs from the initial charge of 2.3 lbs last spring (new compressor and dryer). It held vacuum then without issue.

There is a small leak at the hose fitting on, I think, the right side of the condenser. The condenser itself shows no sign of leaking.

BUT, there is a bigger leak around the switch in the back of the compressor. The tech said this was a low pressure switch. Is it? Today it would not hold vacuum.

He told me that when he installed it he moved that switch over from the old compressor and figures the leak is either a damaged o-ring or the plastic housing of the switch is cracked.

We agreed he would save my refrigerant and leave the system empty for now. As soon as you good folks tell me that it IS the low pressure switch, I'll get one and change it. Once that's done I'll get the system recharged.

Oh, he suggested a Wurth sealer that he figured would take care of the small leak at the hose fitting. He said he has used it in a few cars including his own and it does hold up OK. I asked if it was the same as the evil cooling system/radiator sealer, and he said no, agreeing that stuff is truly evil. Wurth products are good quality - at least I've never heard anything bad about them.

If it's the liquid hose it's cheap to buy, so maybe I'll just get one. Unless, the truck has to be disassembled to change it.

So, low pressure switch? Held in with a circlip. I should be able to unbolt the compressor and tilt it forward for access to change it. I think.

Cheers,
Glenn