Before I go ripping the center dash apart again, is there any " common " problem that would cause a Mode door to get jammed in the Vent position?

 

Probably once I take it apart ( Again.... Grrrr ) , the answer will be staring me in the face.. but just want to know if there are any Common assembly errors or such that I may have made.

 

The mode vacuum pot itself is actuating... but the linkage only moves a short distance before it jams. It's not like it's seizing or binding either... it's like the door is  hitting something inside.

 

Didn't worry about it this Summer... as door was stuck on Center vent and with AC that was fine. But now I'm taking a trip to the West Coast of Vancouver Island and I know that I will need defrosters.

 

Any " Tips " fellow Z drivers? TIA.

Edited September 3, 20159 yr by Chickenman

You don't say which year you have, but the early year s30s had a problem with the control cable on the drivers side bending over. A service bulletin was issued that modified the configuration so that the cable pulled rather than pushed.

Sorry, 1976 280Z with factory AC. All cables are operating smoothly.

Remember, the air intake door (which brings in either fresh air or re-circulated air) and the “mode” (heat or ac) door are vacuum controlled. Look for cracks in the vacuum lines on the solenoid valves and the vacuum bottle in the engine compartment.  A defective check valve can also cause this.

A follow-up to zcarnut - AC mode is all spring-actuation and cables.  Defrost requires the vacuum pots to overcome the springs.  Maybe you've got a stuck magnet valve or split hose, and the springs win the battle.  

 

Does the idle RPM increase when you go to Defrost mode, or switching between other modes?   That was one of the signs of a split hose for me, the vacuum leak increased my idle RPM.  

 

All vacuum lines have been checked and replaced. New one way check valve as well. Magnetic switches are working  Edit: All vacuum pots are working via selctor switch  ( See solution ). Selector valve is working properly and has been serviced. I've checked each Vacuum Pot with a Mighty Vac. Pretty sure it's not a Vacuum issue.

 

Vacuum Actuator for Mode door is physically jamming. I can apply vacuum to the Mode door pot, and linkage moves a bit as described in first post...then it jams solid. Pot piston only moves full distance if I disconnect linkage from it.

 

Guess I'm just gonna have to dig into it again tonight. Nuts!!

Edited September 6, 20159 yr by Chickenman

The mode door is hidden behind your evaporator core, isn't it? If that's the case, you won't be able to do much from the front. You can get a little access to the mode door if you pull the blower assembly out of the passenger footwell. There just isn't much to it though... Pivots on two pegs on the bottom. I think I got some pics around here somewhere....

 

If you take the linkage off the drivers side peg, you should be able to move the door smooth through complete range of motion. Might tell you if it's a linkage issue or something inside the box.

 

Maybe a critter dropped a couple nuts into your air intake and they came through the blower and are jamming the door?

And BTW, aren't all the doors on the A/C cars tied together with the vacuum actuators? Are you sure your problem is with the mode door and not with the heat/defrost door down closer to your feet? The linkage for those floor heater doors is kinda kludgey.

The mode door is hidden behind your evaporator core, isn't it? If that's the case, you won't be able to do much from the front. You can get a little access to the mode door if you pull the blower assembly out of the passenger footwell. There just isn't much to it though... Pivots on two pegs on the bottom. I think I got some pics around here somewhere....

 

If you take the linkage off the drivers side peg, you should be able to move the door smooth through complete range of motion. Might tell you if it's a linkage issue or something inside the box.

 

Maybe a critter dropped a couple nuts into your air intake and they came through the blower and are jamming the door?

 

Good point about the Mode door being hidden behind the Evap core. That means it wasn't anything I did while replacing the ducting foam strips.

 

I'll try disconnecting the linkage and wiggling things back and forth. Maybe I can " un-jam " it.

And BTW, aren't all the doors on the A/C cars tied together with the vacuum actuators? Are you sure your problem is with the mode door and not with the heat/defrost door down closer to your feet? The linkage for those floor heater doors is kinda kludgey.

I don't think they are interconnected. Floor flap opens and closes normally and I can get heat out of it. Just can't get air to the Defrost ducts. Airflow is stuck on center vents. Air Intake door on passenger side seems to be working fine as well.

 

Wratched my right shoulder up playing Tennis a while ago... so it's literally going to be a PITA getting under there. Point about Mode door being behind Evap core saved me some time though. Thanks for that.

Glad to (try to) help. I'm operating off obsolete brain cells with the details of the A/C stuff. Been a long time and I don't remember all the details.

 

I do think disconnecting the linkage off the vacuum actuator is a good idea. Might be able to figure out if the problem is inside or outside the heater box. But be warned... there really isn't a lot inside the box. Unless some piece of foam has fallen out of place and is jamming up the works I'm not sure what else could have gone wrong in there. One thing I've seen in some other cars is that when the original foam degrades over time, sometimes it turns into a sticky gooey mess. I don't remember if that happens with old Datsun stuff, but if so, your mode door might just be "glued" into place by sticky residue left over from failed antique sealing foam.

 

Also, here's some pics I took when I had my air box out of the car for some work. I don't have A/C, but the basic system parts (like the air box) are the same for all the cars. You've just got the vacuum stuff bolted onto yours that I don't have:

 

 

 

 

 

 

The selector valve directs vacuum to all three actuators in Defrost mode.  Water ****, fresh air door and mode door.  A leak in any might weaken another.

 

Actuating by hand would at least answer the question of if it's catching on something.  You can only generate so much force with a vacuum actuator.  Air pressure limited.  Maybe a little extra force to break it free would let it work afterward.  You don't need to disconnect it either, just give it some help.  If you can reach it.