For those who might be interested, if you have a 940 or 740 where the air vents are either not working or working poorly then this post is for you. Heading into a "who knows how hot" summer what with climate change, you at least want good air vents, if not a working air conditioner.

A fix may be a lot easier than you expected, so keep on reading here as I just successfully did this job again in a third 700/900 and wanted to document it while fresh in my mind.

The 700/900 heater systems use vacuum servos (pneumatic motors) to operate the vent flaps in the air distribution box. There are three large, hollow hockey-puck sized vacuum servos mounted on the left (driver) side of the air box, above and to the right of your gas pedal foot. Each has an internal diaphragm with a rod attached that sticks into the air box and connects to its vent flap lever. The heater control knob selects which servos are getting vacuum and retracting their rods to operate the vents. Additionally, the air recirculation button (on the right) also operates one of the servos. Some of the details I'll be mentioning here are for the standard 940 AC system. There are slight variations between the different AC and climate control systems over the years, but the principle behind this problem is the same.

If there is a vacuum leak anywhere in the system then the flaps won't operate properly and when the leak gets large or when manifold supplied vacuum gets weak (such as when accelerating up a hill) they may not work very well and eventually won't operate at all.

Our RWDs are now coming up on 30+ years old with most of them suffering from this problem. It can often end up high on your list of wanting to trade in your Volvo for something more reliable, especially if you're not overly handy at digging under your dash to try isolating and fixing the problem.

Although this topic has been discussed here before and is documented in the FAQ, including a separate write-up there on replacing the servos, it makes the problem sound much worse than it usually is, plus it's documented poorly, hence this new post as we can no longer modify the FAQ.

Although it can be a problem with the manifold vacuum check valve under the hood or even a leak at the heater control panel, the problem is normally a vacuum leak in one of the servos.

Most people assume this is a diaphragm leak, which it occasionally can be, but much more commonly it's a leak where the rod comes out of the servo. Fortunately, a leak at a rod only affects the operation of one servo, the middle one for the combined floor vent and air recirculation flap. If any of the other two servos leak at their rods then it doesn't matter (it won't affect their operation or the rest of the system).

The rod exits the middle servo through a small flexible rubber disk (like a small diaphragm, but not the main diaphragm in the middle of the servo that the end of the rod attaches to). That rubber disk can start to get ripe with age and develop a leak. Initially it's a small leak that only affects normal vent operation when accelerating, but eventually it starts to split and the vents hardly work, starting with the floor vent. Because the servos share the same vacuum source, any loss of vacuum in one line can affect the whole system and makes the other vents weaker.

Now the middle servo is slightly different from the other two. Knowing how it works will allow you to do a bypass that normally restores full function to all the vents (floor, dash and defrost), but will leave you without the air recirculation function (which many of us rarely use).

The middle one is a compound servo with two vacuum nipples, one to the outer and one to the inner side of the diaphragm, basically being able to use vacuum to pull the rod in two directions. The vacuum line connected to the face of the servo (normally a blue line) is for the floor vent function. The vacuum line connected to the nipple on the backside of the servo at the rear (normally a pale yellow line) is for the recirc function.

When air leaks around the rod, the yellow line loses vacuum, which can affect vacuum in the whole vacuum system. You might expect when the knob is in the defrost-only and the dash-only positions that the yellow line wouldn't affect vacuum, but because the recirc function is a separate button, it can affect everything.

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So, knowing all that, the bypass is to simply disconnect the yellow line from its nipple and plug the hose so the system can't lose vacuum. There's no need to plug the nipple. Now all the vent servos can work normally as long as there are no other significant vacuum leaks, including all the three servo diaphragms, which is fortunately the typical case. All you'll lose is the recirc function!!!
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It's less than a one-hour job and the procedure is as follows:

1) Remove the driver side under dash panels and the knee bolster metal bracket.

2) With a good light, look up above the console side panel and you'll see the middle servo with the blue line on the middle nipple. Behind it and sticking straight out the back toward the firewall is the nipple with the pale yellow hose on it. It's a bit awkward to see and get at, so you may want use a mirror.

3) It's best to remove the middle blue line for easier access to the nipple at the back. There's a round ear on the end of the rubber fittings that you need to slip over the corner of the nipples to be able to pull the tube off the nipple.

4) Optionally, you can attach a vinyl tube to the middle nipple and apply vacuum to verify that the servo can hold vacuum and the diaphragm itself isn't leaking (if it is then you have a bigger problem than what we're doing here). If you've got a vacuum brake bleeder (MityVac) then you can use it (pump it a few times to get enough vacuum), or you can simply use your mouth to suck on the tube, sealing the end of it with your tongue for a few moments then remove your tongue to see if it's still holding suction.

5) Now remove the yellow line from the back nipple. You may need a light, mirror and long screwdriver blade to get the round ear over the knuckle of the nipple.

6) Use a vinyl tube on the nipple to verify that the back side of the servo leaks and can't hold a vacuum. If it's leaking badly you'll even be able to blow in it and possibly hear the shutter start to move. If the back side of the servo can hold vacuum then your problem is somewhere else in the system, so go to the FAQ for further reading on these issues.

7) Plug the yellow line with something like a wood screw to seal it off. Screw threads will allow a minor leak, so the seal needs to be the shank of the woodscrew or the head of the screw. For the benefit of future owners, maybe fasten a taped note to the loose yellow tube and tuck it out of the way. Now start the engine and give everything a test before you button it all back up.

8) As an added note, if you have trouble seeing what you're doing and getting your hand up to remove the yellow line, you can remove the ignition controller off the side of the brake pedal assembly (a single Torx screw at the front), pull it back off its upper two pins and drop it down so you can open up the connector, remove the controller and tuck the connector out of the way, up and off to the left.

I was so happy to have everything back and working that I decided to celebrate and buy a new (rebuilt) AC compressor from Rock Auto to replace the original compressor that has a leaking shaft seal behind the clutch. The shaft seals aren't easy to come by and it's easy to damage them, so a decent rebuilt is the most expedient solution. Four Seasons is the goto brand for rebuilt AC compressors. Check fitments carefully as even if the compressor is the same model, the clutch pulley sizes can vary between turbo and NA engines (turbos used a smaller pulley, but can still work with the larger pulley if you go with the slightly longer NA compressor belt.)
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now