Refurbishing the HVAC vacuum selector valve in a 1978 280Z -- useful info

[FastWoman]

Hi all,

This weekend I finally got around to tracking down that last vacuum leak -- in the pneumatics of the HVAC control apparatus. (I had previously identified a leak through the vacuum line leading through the firewall from the vacuum solenoids or "magnet valves" -- righthand/passenger side of hood compartment.)

The larger source of the leak was immediately apparent. In their infinite wisdom, Nissan put a splice in the vacuum line in the righthand/passenger dash area. It's shown in the factory service manual diagrams. It seems like that's just more to go wrong, but I'm sure there was an assembly line rationale behind the splice. In my own case, I had three splices, apparently from repairs by the previous owner. The tubing was starting to rot and had split over one of the splice connectors, then falling off. So I did a quick fix of the splice and found that I still had a vacuum leak. Not surprising. Anyway, I had to tear into the dash to track down the remaining leak and to replace old tubing with new. (Joy!)

The center console on my '78 didn't come out as nicely as I had remembered on my '75, but I got it out. No leaks in any of the vacuum actuators in the system. The leak was definitely in the vacuum selector valve. It looked as though I could simply remove it from the bottom of the control assembly, but I wasn't sure. I figured I'd be better off taking the conservative approach and removing the entire control assembly before worrying about the vacuum selector valve. This gave me the opportunity to clean and lube it, but in hindsight I would have left it in place, only removing the vacuum selector valve. There are too many things to take loose and reconnect, and the assembly doesn't really come out without a bit of a fight.

Once I had the vacuum selector valve out, I was able to confirm that, yup, it leaked. It seemed like a rather simple construction -- and was. It consists of two metal plates faced against each other, sealed with vacuum grease. The design of the valve is such that it vents air to the actuators to allow them to relax. The problem is that dirt is sucked into the valve and combines with the grease over time, eventually resulting in accumulated gunk that doesn't seal the two plates together very well.

Anyway, the valve disassembles very easily. Just remove the little c-clip on the shaft, and the valve disassembles into 5 parts -- the two plates, the shaft, the clip, and a spring (on the outside of the assembly, pressing the plates together). There was a bit of scoring on mine from the dirt, but it was very minor. I just cleaned the two plates, lubricated lightly with silicone vacuum grease, and reassembled. Note that there is a recess in the middle, right around the shaft. I packed that recess with extra grease to feed out over time. In the original assembly process, Nissan had apparently not packed this area. After reassembling, to my delight, the assembly was vacuum-tight. I reassembled the whole mess, properly adjusting the cables, and found that the HVAC system worked perfectly and my engine had no remaining vacuum leaks. Yea!

Here's what I would have done differently -- things that the factory service manual doesn't tell you:

Rather than taking out the entire HVAC control assembly, I would have removed the vacuum selector valve from the bottom of the assembly. The cleaning/lubing of the remainder of the control assembly simply isn't important enough to go through the headaches of R&R.

To do this, I recommend the following procedure, and I'd advise penciling it in on the appropriate page of the factory service manual (page AC-31 of my 1978 manual):

(1) Turn the mode lever to "defrost" (far right position).

(2) Remove the two screws retaining the vacuum selector valve, and remove through the bottom of the assembly (very easy). Be careful not to turn the two plates (also very easy). Note that the selector valve could install either the way you took it out or 180 deg turned. I'm pretty sure from the construction of the valve that it would make no difference how it is turned during reassembly. Note also that the drawing of the vacuum selector valve in the factory service manual is not accurate, at least on my car. The nipples are located in different positions. Perhaps there is more than one configuration of valve.

(3) Note when you remove the valve that the top and bottom plates have alignment pointers on their outer radius. In the "defrost" position, these pointers should be aligned. At least that's how it is on my valve. Again, there may be different configurations. If the arrows don't align, then make your own alignment marks.

(4) Disassemble, clean, re-lube, and reassemble valve. Use silicone vacuum grease or (silicone brake grease?).

(5) Realign the plates using the alignment arrows.

(6) Without rotating the plates, reinstall into the assembly, gently meshing the gears together.

(7) Check the final alignment with the air selector lever now in the A/C position (far left). In this position, the S nipple should be completely vacuum tight, and the other three nipples should vent to the outside. I believe this is the only position where this would be true.

One final note: One of my "magnet valves" is bad in the engine compartment. It is the primary one that evacuates the reservoir when the HVAC system is turned on. When the system is turned off, it is supposed to vent air back into the HVAC pneumatic system (note: edited as per below), thereby relieving the vacuum on the entire system. (There is a little filter on the bottom of the valve.) In my car, someone had bypassed that valve. I was worried that with a healthy vacuum and no leaks that the vacuum actuators might remain actuated when the system was turned off. However, the design of the vacuum selector valve does vent air back to the actuators when the system is off. I suppose the vacuum reservoir does hold a vacuum when the system is in the A/C position and the HVAC system is shut off. I presume that's not a problem.

Anyway, I hope this posting saves someone a few headaches.

Peace,

Sarah

Edited August 17, 2009 by FastWoman
small error corrected

[sblake01]

(There is a little filter on the bottom of the valve.)

If I'm properly pictureing what you're describing, that's actually a check valve which would keep the vacuum from backing up into the valve and also keep the vacuum in the resivor bottle which would hold the vacuum in the system even when the system is off. Also a part of the infinite wisdom of Nissan's design.

[FastWoman]

I'm not talking about the check valve. I mean the "magnet valve" (i.e. one of the two doodads connected to two tubes and two wires and mounted to a bracket on the fender). Anyway, I remembered its hookup wrongly. The way it was originally configured, it vented to the vacuum selector valve side, not to the vacuum bottle side (see pp AC12-13). Thus the vacuum bottle would stay under vacuum either with the original configuration or with the way my system has been reconfigured. The vent to the HVAC side seems to be redundant with the vent passages in the vacuum selector valve. I'm wondering why they even have that "magnet valve" in the design.

[sblake01]

Oh, okay. I'm familiar with the magnet valves, I simply misinterpreted your description. From what I recall, one magnet valve controls the operation of the mode doors and the other is part of the fast idle control. There are simpler ways to accomplish both of those functions as you see in more modern systems. I'm a licensed HVAC/MVAC tech. I no longer have my 78 (or my 810 for that matter) so I don't have a visual but I do recall parts of those systems being redundant or perhaps not even necessary.

[zcarnut]

The vent to the HVAC side seems to be redundant with the vent passages in the vacuum selector valve. I'm wondering why they even have that "magnet valve" in the design.

I had the same conclusion. That’s why I removed the magnetic control valves from my 260Z (which has a transplanted 1978 ac system). I considered the “fast idle actuator” an eyesore with my aftermarket triple carbs.

I also determined that the plastic vacuum reservoir tank was also redundant and I use the brake booster as a steady vacuum source.

I’m still working on configuring the selector valve so I can run the heater and the ac at the same time. This would help defrost the windshield much faster in cold weather and most post 1980 cars have this feature. I would use the (useless) BI-LEVEL switch position for this.

[FastWoman]

Cool ideas. I've found that my engine hardly notices when it's pulling the compressor, so I don't think the idle increment is all that necessary anyway. Your use of the power booster as the vaccum reservoir is very appealing. I can understand why Nissan wouldn't do this, though, as it could expose them to safety liability claims, should a leak be introduced from the HVAC system.

I’m still working on configuring the selector valve so I can run the heater and the ac at the same time. This would help defrost the windshield much faster in cold weather and most post 1980 cars have this feature. I would use the (useless) BI-LEVEL switch position for this.

Having just been into the control assembly, it seems to me that you could simply wire a microswitch in that would engage the compressor relay when the lever is in the bi-level position. I'm not sure EXACTLY where you would put the switch, but there's plenty of room in/on that thing. I'd probably put a small bolt with a rounded head through the lever to create a "bump" that would trip the microswitch. That could be wired in parallel with the other microswitch. I think it would also be possible to attach an extra bump or nub on the lever to engage the microswitch that is already there when in the bi-level mode. That would probably be more elegant.

[Powerglide]

After reading these posts with intrest, I thought I'd ask a question pertainting to the factory HVAC in my '76. The systems works as designed with one exception. When the a/c compressor cycles off---disengages---the ficd (or fast idle actuator) remains engaged. Even after shutting the a/c off the fast idle remains engaged. Even shutting the motor down does not release the vacuum in turn releasing the idle compensator. After 1/2 hour or so with the car off the vacuum releases and idle returns to normal base setting. What is particularly perplexing is that for a brief period---maybe 2 weeks---the ficd would in fact realease the fast idle when the compressor cycled off---and then increase the idle speed when it re-engaged. What's happening?:stupid:

[FastWoman]

It sounds like you might have the vacuum lines reversed. One nipple is slightly lower than the other on the magnet valve. The lower one should go to the fast idle actuator. If the upper one is connected to the actuator, that line will not vent when the magnet valve is shut off. Furthermore, you'll have a vacuum leak when the HVAC system is running without the A/C compressor clutch engaged.

[grantf]

Way old thread: Where did you find the grease? what brand? would "super lube" work?

Edited March 7, 2012 by grantf

[Willoughby Z]

Way old thread: Where did you find the grease? what brand? would "super lube" work?

Yes, if you mean Super Lube Silicone Hi-Dielectric grease.

[grantf]

Yup, thanks!

[cozye]

Thanks for digging this up. On my to do list. I was hoping this would fix my air mixture. It's cold when the ac is on, and flaming hot when the heat is on. The air mixture lever doesn't do anything.