Low vent flow

[Mark L]

Looking at an 82 c-10 that has been LS swapped, and has AC that has been changed over to the new refrigerant. Truck has very little vent air when the fan is on max. There are other issues with the AC such as low on freon and short cycling but my concern is more the low flow out of the vents. Any ideas?

 

[Georgeb]

Start checking that the flaps are moving properly and that the fan is running on high.

 

[yevgenievich]

likely stuck/broken vent doors. Following thread has information on commonly broken part of the door opening assembly.
http://www.gmsquarebody.com/forum/showthread.php?t=4809

 

[foamypirate]

I'd also make sure that the vacuum source for the climate control is actually hooked up to something. It comes out of the firewall right by the A/C box. It should go to a Tee check valve, with a vacuum line to the engine, and a vacuum line to the little vacuum ball on the firewall. All of that is critical for the climate control to operate properly. The big side of the check valve Tee should go to the engine, while the smaller lines run to the climate control, and vacuum ball.

 

[Georgeb]

Also look through this thred.
http://www.gmsquarebody.com/forum/showthread.php?t=2875

 

[Mark L]

I'd also make sure that the vacuum source for the climate control is actually hooked up to something. It comes out of the firewall right by the A/C box. It should go to a Tee check valve, with a vacuum line to the engine, and a vacuum line to the little vacuum ball on the firewall. All of that is critical for the climate control to operate properly. The big side of the check valve Tee should go to the engine, while the smaller lines run to the climate control, and vacuum ball.

Thanks this might very well have been overlooked during the swap! Also thanks to everyone else that posted, I now have a few different directions to look in.

 

[1987 GMC Jimmy]

I'd also make sure that the vacuum source for the climate control is actually hooked up to something. It comes out of the firewall right by the A/C box. It should go to a Tee check valve, with a vacuum line to the engine, and a vacuum line to the little vacuum ball on the firewall. All of that is critical for the climate control to operate properly. The big side of the check valve Tee should go to the engine, while the smaller lines run to the climate control, and vacuum ball.

If that check valve is damaged, does it absolutely have to be replaced, or can you just do a straight shot of vacuum line? Not trying to hijack but was just curious.

 

[chengny]

If that check valve is damaged, does it absolutely have to be replaced, or can you just do a straight shot of vacuum line?

You could do that, but it would be a hassle during periods of low manifold vacuum (e.g. heavy loads/WOT).

Initially when the throttle plates are opened wide, and manifold pressure approaches the pressure in the control system, it's not too bad. The first thing you'll notice is that the various dampers/doors associated with the air handling system begin to operate sluggishly.

If the vacuum supply continues to remain unavailable for long enough, the pressure in the controls/manifold will become equalized. You'll know when that point has been reached because there will be no response to changes in the mode select settings.

Finally, when manifold pressure rises above what is required to hold the control diaphragms/dampers in their actuated positions, they will begin to shift back to their default settings.

The function of the check valve (and the reservoir tank) is to prevent the scenario described above from happening. They work together to generate and store a supply of vacuum.

When a deep vacuum is available from the manifold, it is used to operate the diaphragm controlled dampers. The tube that supplies manifold vacuum to the controls is tee'd off in the engine compartment. One side of that tee is lead over to a vacuum tank mounted on the firewall. When manifold pressure is low, a negative pressure is also developed in the vacuum tank. There's also a check valve in the section of tubing that runs from the manifold to the tee. Under ambient conditions, it is held open by a light spring. The spring allows any negative pressure that exists in the manifold to become common with the HVAC control system. Under cruising conditions, that common negative pressure rises and falls slightly - along with any minor fluctuations in engine load/manifold pressure.

But when a heavy load is suddenly applied and the manifold pressure shoots up - the check valve snaps shut. The seal created by the check valve isolates the two sides of the system. On one side is the vacuum stored in the tank (along with any vacuum currently in the control system). On the other side is the higher pressure air in the hose that is connected to the manifold. Without that check valve installed, the higher pressure air on the manifold side of the system would be able to rush in to the HVAC side - operating vacuum would be lost.

The volume of the tank is such that it is able to supply the HVAC controls with sufficient vacuum for a reasonable amount of time. Long enough to pass that slow moving semi - the one that's been blocking you for the last 5 miles! You floor it and start to pass. But it takes a longer time than you expected and before you can get in front of him, you start to sweat. You reach down, move the mode select lever to MAX AC and hope that the system responds as designed. It does. Almost instantly, you are hit with a blast of cool air. As you finish overtaking the truck, you say a silent "thank you" to the engineers at GM who had the foresight to include that check valve in the main vacuum supply line.

When you let off the accelerator, the throttle plates close down and the pressure in the manifold starts to drop. Now, with the throttle plates trimmed down, the pressure in the HVAC control system eventually rises above manifold pressure (or the manifold pressure drops below the HVAC pressure - whichever way you want to look at it). Either way, the check valve opens. At that point, the higher pressure air in the control system will begin to flow towards the engine. Eventually the pressure differential between the two systems will reach equilibrium - at manifold pressure. After that, the HVAC side will kind of float along with the manifold side - until the next WOT event.

Sometimes - because the HVAC controls are operated by vacuum rather than pressure - it's difficult to visualize the direction of flow. Also, in theory there should be no net flow, just the back and forth motion of the air in the tubing. It's kind of like AC power. The only actual flow of air - through the vacuum supply hose to the manifold - is what leaks into the system on the HVAC side. The arrow on the check valve should point in the direction of flow; towards the intake manifold.

 

[chengny]

Truck has very little vent air when the fan is on max.

If the air flow appears to be normal from all the other vents and seems to move from zone to zone as directed by the mode select lever.

And, the only real noticeable lack of flow is from the dash vents when the select lever is in the AC position and the temperature control is all the way hard over to cold.

And, the flow from the dash vents increases noticeably when the mode is left in the AC as above, but the temperature select lever is now moved away from the hard over cold position.


If this is the case check the cab recirc air inlet damper (in the RH kick panel) for proper operation.

With these systems, if the mode select lever is in the AC position and the temperature select slide is moved all the way over to the COLD end - three things happen:

1. The blower is automatically shifted to HIGH speed - and cannot be overridden.

2. The outside air damper (under the windshield wiper cowl far RH side) is pulled closed.
In this position, only 20% of the normal outside air volume can flow into the cab.

3. The inside air recirc valve is pulled wide open. The plenum behind the damper is
connected to the blower inlet - just like the outside air.


So, in the A/C - full cold position, instead of 100% of the air supply to the blower
coming from outside, only 20% is outside air. The other 80% is drawn from inside the
cab.


But if your recirc damper is not opening, only the 20% from the outside air
damper will go to the blower. Total flow will be reduced by 80%.

From the 1973 service manual:

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[1987 GMC Jimmy]

If that check valve is damaged, does it absolutely have to be replaced, or can you just do a straight shot of vacuum line?

You could do that, but it would be a hassle during periods of low manifold vacuum (e.g. heavy loads/WOT).

Initially when the throttle plates are opened wide, and manifold pressure approaches the pressure in the control system, it's not too bad. The first thing you'll notice is that the various dampers/doors associated with the air handling system begin to operate sluggishly.

If the vacuum supply continues to remain unavailable for long enough, the pressure in the controls/manifold will become equalized. You'll know when that point has been reached because there will be no response to changes in the mode select settings.

Finally, when manifold pressure rises above what is required to hold the control diaphragms/dampers in their actuated positions, they will begin to shift back to their default settings.

The function of the check valve (and the reservoir tank) is to prevent the scenario described above from happening. They work together to generate and store a supply of vacuum.

When a deep vacuum is available from the manifold, it is used to operate the diaphragm controlled dampers. The tube that supplies manifold vacuum to the controls is tee'd off in the engine compartment. One side of that tee is lead over to a vacuum tank mounted on the firewall. When manifold pressure is low, a negative pressure is also developed in the vacuum tank. There's also a check valve in the section of tubing that runs from the manifold to the tee. Under ambient conditions, it is held open by a light spring. The spring allows any negative pressure that exists in the manifold to become common with the HVAC control system. Under cruising conditions, that common negative pressure rises and falls slightly - along with any minor fluctuations in engine load/manifold pressure.

But when a heavy load is suddenly applied and the manifold pressure shoots up - the check valve snaps shut. The seal created by the check valve isolates the two sides of the system. On one side is the vacuum stored in the tank (along with any vacuum currently in the control system). On the other side is the higher pressure air in the hose that is connected to the manifold. Without that check valve installed, the higher pressure air on the manifold side of the system would be able to rush in to the HVAC side - operating vacuum would be lost.

The volume of the tank is such that it is able to supply the HVAC controls with sufficient vacuum for a reasonable amount of time. Long enough to pass that slow moving semi - the one that's been blocking you for the last 5 miles! You floor it and start to pass. But it takes a longer time than you expected and before you can get in front of him, you start to sweat. You reach down, move the mode select lever to MAX AC and hope that the system responds as designed. It does. Almost instantly, you are hit with a blast of cool air. As you finish overtaking the truck, you say a silent "thank you" to the engineers at GM who had the foresight to include that check valve in the main vacuum supply line.

When you let off the accelerator, the throttle plates close down and the pressure in the manifold starts to drop. Now, with the throttle plates trimmed down, the pressure in the HVAC control system eventually rises above manifold pressure (or the manifold pressure drops below the HVAC pressure - whichever way you want to look at it). Either way, the check valve opens. At that point, the higher pressure air in the control system will begin to flow towards the engine. Eventually the pressure differential between the two systems will reach equilibrium - at manifold pressure. After that, the HVAC side will kind of float along with the manifold side - until the next WOT event.

Sometimes - because the HVAC controls are operated by vacuum rather than pressure - it's difficult to visualize the direction of flow. Also, in theory there should be no net flow, just the back and forth motion of the air in the tubing. It's kind of like AC power. The only actual flow of air - through the vacuum supply hose to the manifold - is what leaks into the system on the HVAC side. The arrow on the check valve should point in the direction of flow; towards the intake manifold.

Thank you for explaining. Mine disintegrated and was then duct taped so I chunked it. I'll get a new one at some point.

 

[RockyGryphon]

If I'm not mistaken (at least on my 85), there are varying gap spaces between vent conduits in the center dash vents that let the air leak out behind the dashboard guts. putting duct tape to seal/close the gap(s) helps air flow to the vents if this is your case. I think this is showing the point:'

https://postimg.org/image/dfmkicann/full/

 

[chengny]

Closing that gap will only increase air flow to the defrost vents. Why it even exists is yet to be explained. Some discussion on the subject here :

http://www.gmsquarebody.com/forum/showthread.php?t=8031&highlight=defrost