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AC / vent vacuum distribution problem on 1992 740

You can pull out the fan and at least see it with a endoscope, possibly get a vacuum in there. If its holding vacuum, its not a vent issue.
 
One of the most common 700/900 air vent problems is that middle actuator (#25). It's a compound actuator with two vacuum chambers, one on each side of the diaphragm, while the other two are only single chambers on the outer side of the diaphragm. Its single pushrod controls two of the vents depending on whether it's pushing or pulling. The rear chamber of that actuator is usually the first to start leaking. The flexible rubber seal around where the rod exits starts to split and leak. There's normally a yellow vacuum line for that back chamber. Removing that yellow line is similar, but a bit different from the others lines as the rubber loop on the end is over a knuckle on a straight nipple, not just over a bent nipple. If the seal is leaking, but the diaphragm isn't leaking then only the rear chamber function is affected, the outer chamber will still function normally.

The way to test this is to see if that rear chamber can hold vacuum. Although you can do it sucking on the end of a vinyl hose attached to the nipple, the best way to deal with these problems is with a vacuum gauge, such as a Mityvac brake bleeder that many of us have. If it can't hold normal vacuum (10-20 psi) then the simplest fix is to remove and plug the yellow line with something like a golf tee, nipping off the excess. You don't need to plug the nipple. You will still have floor vent control, just not the recirc function. Many people don't use the recirc function anyway so you can consider this bypass a fix.

The diaphragms in all the actuators can eventually start leaking, so while you're there check all the actuators. If they can't hold vacuum for a good few seconds then simply disconbect and plug off its hose knowing you will lose its function, but the rest of the system can now work normally if there are no more leaks.

Now having said all that, with these aging cars this will normally restore a significant amount of vent function, but it still may not be all that great. At that point you need to start vacuum testing the entire system in order to isolate the problem areas, starting with the vacuum lines off the manifold, the little check valve between there and the firewall, and the check valve on the brake booster (making sure it's properly seated and sealed).. One of the hardest areas to test is the control panel mechanism. You'll need to identify the hoses and which funtions they control to know which lines should be open or closed in the various positions. They usually don't fail, but do eventually start to slowly leak.
 
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945T said:
One of the most common 700/900 air vent problems is that middle actuator (#25). It's a compound actuator with two vacuum chambers, one on each side of the diaphragm, while the other two are only single chambers on the outer side of the diaphragm. It's single pushrod controls two of the vents depending on whether it's pushing or pulling. The rear chamber of that actuator is usually the first to start leaking. The flexible rubber seal around where the rod exits starts to split and leak. There's normally a yellow vacuum line for that back chamber. Removing that yellow line is similar, but a bit different from the others lines as the rubber loop on the end is over a knuckle on a straight nipple, not just over a bent nipple. If the seal is leaking, but the diaphragm isn't leaking then only the rear chamber function is affected, the outer chamber will still function normally.

The way to test this is to see if that rear chamber can hold vacuum. Although you can do it sucking on the end of a vinyl hose attached to the nipple, the best way to deal with these problems is with a vacuum gauge, such as a Mityvac brake bleeder that many of us have. If it can't hold normal vacuum (10-20 psi) then the simplest fix is to remove and plug the yellow line with something like a golf tee, nipping off the excess. You don't need to plug the nipple. You will still have floor vent control, just not the recirc function. Many people don't use the recirc function anyway so you can consider this bypass a fix.

The diaphragms in all the actuators can eventually start leaking, so while you're there check all the actuators. If they can't hold vacuum for a good few seconds then simply disconbect and plug off its hose knowing you will lose its function, but the rest of the system can now work normally if there are no more leaks.

Now having said all that, with these aging cars this will normally restore a significant amount of vent function, but it still may not be all that great. At that point you need to start vacuum testing the entire system in order to isolate the problem areas, starting with the vacuum lines off the manifold, the little check valve between there and the firewall, and the check valve on the brake booster (making sure it's properly seated and sealed).. One of the hardest areas to test is the control panel mechanism. You'll need to identify the hoses and which funtions they control to know which lines should be open or closed in the various positions. They usually don't fail, but do eventually start to slowly leak.
Click to expand...
Well that's a lot of good information. So if I yank the yellow (which I did) are you saying it loses recirculation on all vent positions? Because that might suck given my car is inhaling very humid 100º+ degree heat index air 200+ days a year.

The ideal situation for me would be to block off any leak and have recirc permanently functioning because I rarely use the AC and the outside air at the same time. If it's ever cool enough to get outside air I just roll down the windows. How could I do that?
 
isaac said:
Well that's a lot of good information. So if I yank the yellow (which I did) are you saying it loses recirculation on all vent positions? Because that might suck given my car is inhaling very humid 100º+ degree heat index air 200+ days a year.

The ideal situation for me would be to block off any leak and have recirc permanently functioning because I rarely use the AC and the outside air at the same time. If it's ever cool enough to get outside air I just roll down the windows. How could I do that?
Click to expand...
Yes, the recirc function affects air intake and thus all the air outlets. Hmm, I've not had to think before about forcing the recirc function to stay enabled.

Before I get started on an idea, I'll mention the actuators are not repairable and no longer available unless you can find a good used one. I once saw that a person did try to repair the split rear seal of the middle actuator, but I don't recall the details and how well they succeeded. There were similar GM Delphi actuators that could be adapted, but they're also NLA. The actuators are also difficult to replace as they fasten from inside the air box (two threaded studs) and their rods have to be disconnected from their vent flaps. You need to either open up the air distribition box to get at their fastening nuts or else use a shortcut to cut them out of the side of the air box. It's a major pain to access and open up the air distribution box, requiring removal of the whole center console, tunnel console and seemingly half the rest of the underdash, so the shortcut route is a viable option. If you want to know more about this, it can all be found in the old 700-900 FAQ on the UK Volvo Owners Club website in the Vacuum Servo topic of the Heating and Air Conditioing section. Complete and detailed dissassembly instructions to access and open up the airbox can be found at the beginning of my lengthy Heater Core Replacement article (schematic diagrams are at the end), which can also be found from the 700-900 FAQ home page in the feature descriptive article pull down list. In that pull down list is also the article fully describing how to replace the actuators by cutting them out.

Now it's been over 20 years since I last had a 700/900 air box open and looked at how the recirc function works, so my memory is a bit shady here. First off, the recirc flap doesn't fully close, it always lets in 15%-25% fresh air from outside to prevent CO poisoning, that I do remember. The middle actuator has a single rod that connects to the recirc flap. In the rod's neutral position, the recirc vent is open letting in outside air from the cowling below the windhsield. When the rod extends it closes the recirc flap (80% closed) and allows air from the forward floor vent area to be pulled back up into the air box for distribution. When the rod is pulled back from the neutral position it opens up the floor vent. I'm pretty sure I've got this part right.

The thing I'm having trouble remembering and can't tell from the diagrams is whether the recirc flap is also the floor vent flap or if there's a linkage from the recirc flap to a separate floor vent flap. If I could properly visualize that then I'd know whether this next suggestion will work or not. At any rate, my thought for you would be as an experiment to simply permanantly block off 75% of the outdoor air coming in through the windshield cowling, effectively trying to mimick the recirc function. It would be very easy to do, just remove the cowling, remove the mushroom cap over the air intake above the blower motor and temporarily duct tape 80% of the opening closed to see if this will work as being equivalent to the recirc function. My concern is that the air won't flow correctly in the air distribution box and the cabin air will have trouble recirculating, so it would only be partially effective and may cut down on the volume of air out the vents. Others reading here who may recall this area and the recirc function better should quickly be able to say how well this might work, if at all.

Removing the cowling is fairly easy. Remove the wiper arms and undo the cowling bolts on either side under the hood. There's a third bolt you need to remove in the middle of the firewall, below the rubber trim and behind the wiring harness bundle. That third bolt engages a thumb tab sticking down from the front of the cowling. To remove the cowling you pull it forward to disengage side slots from their studs on the body while lifting up the front to get the tab clear of the firewall. Be sure to use a blanket between the cowling and the paint otherwise guaranteed you'll scratch it. The hood needs to be at the partially open position to move the cowling all the way forward.

If you want to try this then please post back to say how well it works.
 
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945T said:
Yes, the recirc function affects air intake and thus all the air outlets. Hmm, I've not had to think before about forcing the recirc function to stay enabled.

Before I get started on an idea, I'll mention the actuators are not repairable and no longer available unless you can find a good used one. I once saw that a person did try to repair the split rear seal of the middle actuator, but I don't recall the details and how well they succeeded. There were similar GM Delphi actuators that could be adapted, but they're also NLA. The actuators are also difficult to replace as they fasten from inside the air box (two threaded studs) and their rods have to be disconnected from their vent flaps. You need to either open up the air distribition box to get at their fastening nuts or else use a shortcut to cut them out of the side of the air box. It's a major pain to access and open up the air distribution box, requiring removal of the whole center console, tunnel console and seemingly half the rest of the underdash, so the shortcut route is a viable option. If you want to know more about this, it can all be found in the old 700-900 FAQ on the UK Volvo Owners Club website in the Vacuum Servo topic of the Heating and Air Conditioing section. Complete and detailed dissassembly instructions to access and open up the airbox can be found at the beginning of my lengthy Heater Core Replacement article (schematic diagrams are at the end), which can also be found from the 700-900 FAQ home page in the feature descriptive article pull down list. In that pull down list is also the article fully describing how to replace the actuators by cutting them out.

Now it's been over 20 years since I last had a 700/900 air box open and looked at how the recirc function works, so my memory is a bit shady here. First off, the recirc flap doesn't fully close, it always lets in 15%-25% fresh air from outside to prevent CO poisoning, that I do remember. The middle actuator has a single rod that connects to the recirc flap. In the rod's neutral position, the recirc vent is open letting in outside air from the cowling below the windhsield. When the rod extends it closes the recirc flap (80% closed) and allows air from the forward floor vent area to be pulled back up into the air box for distribution. When the rod is pulled back from the neutral position it opens up the floor vent. I'm pretty sure I've got this part right.

The thing I'm having trouble remembering and can't tell from the diagrams is whether the recirc flap is also the floor vent flap or if there's a linkage from the recirc flap to a separate floor vent flap. If I could properly visualize that then I'd know whether this next suggestion will work or not. At any rate, my thought for you would be as an experiment to simply permanantly block off 75% of the outdoor air coming in through the windshield cowling, effectively trying to mimick the recirc function. It would be very easy to do, just remove the cowling, remove the mushroom cap over the air intake above the blower motor and temporarily duct tape 80% of the opening closed to see if this will work as being equivalent to the recirc function. My concern is that the air won't flow correctly in the air distribution box and the cabin air will have trouble recirculating, so it would only be partially effective and may cut down on the volume of air out the vents. Others reading here who may recall this area and the recirc function better should quickly be able to say how well this might work, if at all.

Removing the cowling is fairly easy. Remove the wiper arms and undo the cowling bolts on either side under the hood. There's a third bolt you need to remove in the middle of the firewall, below the rubber trim and behind the wiring harness bundle. That third bolt engages a thumb tab sticking down from the front of the cowling. To remove the cowling you pull it forward to disengage side slots from their studs on the body while lifting up the front to get the tab clear of the firewall. Be sure to use a blanket between the cowling and the paint otherwise guaranteed you'll scratch it. The hood needs to be at the partially open position to move the cowling all the way forward.

If you want to try this then please post back to say how well it works.
Click to expand...
Jesus Christ you have a good memory. Alright I'll start trying things, other than the yellow hose on the dual-acting vacuum diaphragm, which I'm pretty sure totally shut off the floor vents when I did that.

Incidentally weirdly the right side vent of the 4 front vents blows by far the hardest and I don't know why.
 
isaac said:
Jesus Christ you have a good memory. Alright I'll start trying things, other than the yellow hose on the dual-acting vacuum diaphragm, which I'm pretty sure totally shut off the floor vents when I did that.

Incidentally weirdly the right side vent of the 4 front vents blows by far the hardest and I don't know why.
Click to expand...
If pulling and plugging that rear yellow line causes the floor vents to no longer work then it may be that the diaphragm in that actuator is leaking and not the rear seal, or possibly both. Try it the other way around putting the yellow line back and plugging the front hose to see how much of a difference that makes. The proper way to diagnose these issues it to use a vacuum guage on the each nipple and see if it can hold vacuum. When I last dealt with this problem earlier this year I finally broke down and got a cheap vacuum gauge from China adding it to another order. As popular as they are, I didn't have a Mityvac (or similar) to use as a vacuum bleeder for the brake system. I always use a pressure bleeder at the master cylinder and never the pedal.

If you're getting significantly more air out of the right face vent then that suggests the duct for the left face vent (that goes over the steering column) and the combined duct for the centre face vents (connects to the top of the air box) may be dislodged from the air box and leaking under the dash. If someone has had to have the air distribution box apart before then it's not that hard to do, especially when you're in a rush. Fitting those ducts back together can turn into a bit of a wrestling match. It could also be the vent actuator or vbent flap. By now the odds are reasonable your 740 has needed a new heater core, which requires removal of the entire air distribution box right back to the firewall. I'm guessing that's when this started. It's one of the worst, if not the worst job you'll ever have to do on a 700/900.

As for my memory, I've replaced a number of those heater cores over the years and had to remove the airbox. That's why I wrote that long Heater Core Replacement article in the 700-900 FAQ (Ari took the pictures). I just checked my files and that article dates back to 1996. They say as you age old memories are the last to go. :lol:
 
945T said:
If pulling and plugging that rear yellow line causes the floor vents to no longer work then it may be that the diaphragm in that actuator is leaking and not the rear seal, or possibly both. Try it the other way around putting the yellow line back and plugging the front hose to see how much of a difference that makes. The proper way to diagnose these issues it to use a vacuum guage on the each nipple and see if it can hold vacuum. When I last dealt with this problem earlier this year I finally broke down and got a cheap vacuum gauge from China adding it to another order. As popular as they are, I didn't have a Mityvac (or similar) to use as a vacuum bleeder for the brake system. I always use a pressure bleeder at the master cylinder and never the pedal.

If you're getting significantly more air out of the right face vent then that suggests the duct for the left face vent (that goes over the steering column) and the combined duct for the centre face vents (connects to the top of the air box) may be dislodged from the air box and leaking under the dash. If someone has had to have the air distribution box apart before then it's not that hard to do, especially when you're in a rush. Fitting those ducts back together can turn into a bit of a wrestling match. It could also be the vent actuator or vbent flap. By now the odds are reasonable your 740 has needed a new heater core, which requires removal of the entire air distribution box right back to the firewall. I'm guessing that's when this started. It's one of the worst, if not the worst job you'll ever have to do on a 700/900.

As for my memory, I've replaced a number of those heater cores over the years and had to remove the airbox. That's why I wrote that long Heater Core Replacement article in the 700-900 FAQ (Ari took the pictures). I just checked my files and that article dates back to 1996. They say as you age old memories are the last to go. :lol:
Click to expand...
Thank you to 945T for sharing this excellent detail on the 411 of the 7/9 HVAC system! I own an 90 and it also suffers from "no flow in the center vents" until cruising at stable speeds with high vacuum. My AC is still working but I get air flow out of the center vents sporadically... but specifically at cruise when vacuum is peak. Crack open the throttle and it's gone.

I did one heater core in my old 92 Sedan.... not something I want to do again! But I will be dealing with restoration of functional AC specifically. Fortunately the AC season is now ending here in east TN... so perhaps this winter I'll dive into it. I'm hopeful but uncertain if any parts remain in the supply chain for the GM Harrison design system.

Isaac - did you find a remedy for your situation?
 
There is a GM part that with a little work can replace the dual actuator. It was still available as of a couple years ago. I bought one for a 960, I'll see if I can hunt down that part #
 
My 1991 740 Turbo will cut the dash vents when accelerating. I remember pressurized smoke test was inconclusive. I can search, but what's the common point of failure on that? Is it like this thread?
 
ZVOLV said:
My 1991 740 Turbo will cut the dash vents when accelerating. I remember pressurized smoke test was inconclusive. I can search, but what's the common point of failure on that? Is it like this thread?
Click to expand...
Is the vacuum check valve by the firewall working?
 
ZVOLV said:
My 1991 740 Turbo will cut the dash vents when accelerating. I remember pressurized smoke test was inconclusive. I can search, but what's the common point of failure on that? Is it like this thread?
Click to expand...
Yep, leaking vent actuators along with the reduced manifold vacuum when accelerating can make this very noticeable, especially with a turbo under boost. I've got two 940s, one NA, one turbo. Both now have the yellow line plugged. All the other actuators are able to hold vacuum, so not leaking. Both still have weak vents and the turbo is by far the worst on hills. Although the check valves seem okay when tested, with age I suspect they may not be sealing as well under lower vacuum conditions, so I should probably put them on my replacement parts list. Some day I'll try to chase the leaks further. I highly suspect the heater control panel will have slight issues. I need to pull it out to replace a dead bulb, so will try to check it then if I have time. Do check the rubber fittings on the manifold nipples as they can become a bit loose with age and also have hidden splits.
 
PS -
OP, adding to the good advice already provided by 945T - If you can dig up a copy of the TP 31127 Green Book (Body Fittings), you'll find the HVAC unit and vacuum diagrams are covered in the latter part of the book.

The later editions should show your system; if not, your system is very similar to the ECC system insofar as where most of the vacuum lines go and what the actuators do, but without all the fancy electronic temperature sensors and controls.

The earlier non-ECC system in 740s used cables and levers, and the first version of climate control in 700s (known as ACC) is quite different.
 
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