Hey guys,

I had an issue with my 78 280 where the defrost didn’t work and I had no heat. I knew my heater core was plugged up. So I took my dash off, ready to do a heater core swap with a 01 escort described here on the fourms. That all went well and the heater core is in. But before re installing the dash I decided to test it, but the problem has changed. 

There is still cold air coming out of the center vents. But I stick my hand down into the defrost tube and I feel hot air! So I felt the heater core tubes and it’s getting coolant just fine. And according to the attached diagram when the heat is on, only the floor vents and Defog is supposed to work, but air is still flowing from the center. So doesn’t this mean the mode door isn’t working? If so what can I do to start troubleshooting the mode door. I know it’s vac actuated. 

Is it possible that it is because the dash isn’t installed? 

You've probably lost vacuum at your vacuum tank.  I had a hose that kept splitting at the tank's supply side.  Cold air was the sign that it had split again.  The mode door is vacuum operated.  No vacuum, you're stuck in cold mode.  You can see it in  the illustration.

3 minutes ago, Zed Head said:

You've probably lost vacuum at your vacuum tank.  I had a hose that kept splitting at the tank's supply side.  Cold air was the sign that it had split again.  The mode door is vacuum operated.  No vacuum, you're stuck in cold mode.  You can see it in  the illustration.

Funny thing is all my vacuum lines were re done in there. I’ll check around in there. How many psi should you have going out of the vacuum tank? 

As much vacuum as your engine makes.  (Make sure that check valve is working.)

Check if that first magnet valve is normally open or closed.  Might be an electrical problem.  Either way, if it's not open there's no vacuum supply.  It's all right in front of you in the engine bay, the bottle, the hoses, and the magnet valves.  Should be able to disconnect some hoses and see where the vacuum is not happening.  Also possible that it's not making it through your selector valve.  It's a pretty long path from the intake manifold to that mode door actuator.  Or even that the vacuum actuator is holed and inoperable.  Or that it's disconnected from the mode door.  I think that it's a piece of wire that ties them together.  Or a mouse built a nest in the ductwork.  They love Z ductwork.  Many possibilities.  You found the right picture in the FSM.  It's quite a system that Nissan put together.

11 minutes ago, cgsheen1 said:

As much vacuum as your engine makes.  (Make sure that check valve is working.)

The brake booster check valve? Not sure how to test it. 

Well I gotta start hunting for vacuum leaks in the engine bay it seems. I’ve already done some checking around I can’t see anything obvious. I’ll start looking. 

I traced it back to where vacuum is lost. At those two little solenoids I think? I’m not sure what they do to be honest but just before them I have vacuum then after then I do not. 

Do these need to have 12v at them? 

They do if you want them to pass vacuum.  Looks like they're normally closed.  You might double-check your hose connections too.  Can't see what's connected to what in your picture.

11 minutes ago, Zed Head said:

They do if you want them to pass vacuum.  Looks like they're normally closed.  You might double-check your hose connections too.  Can't see what's connected to what in your picture.

Ran through all the connections and they are correct. 

 

Ok so I do NOT have vacuum after the first magnet valve. I have 8 volts on that connector (top two in the picture) is 8 volts correct and I need to get a new magnet valve or am I looking at a wiring issue? 

8 implies a voltage drop due to current flow.  There's power there.  Might be that the solenoid piston is just stuck.  Tap on it, maybe, when it's powered, and see if it comes unstuck.  Injectors get stuck often and a tap will free them up.  Same basic concept as those magnet valves.

Welcome to the world of old crusty parts.

On 12/3/2018 at 9:14 PM, Pochie45566 said:

I traced it back to where vacuum is lost. At those two little solenoids I think? I’m not sure what they do to be honest but just before them I have vacuum then after then I do not. 

Do these need to have 12v at them? 

i just fixed my heating issues and traced the vacuum lines. The left solenoid in the picture has a hose connected at bottom left  of this picture the where the strut tower is. If you unbolt the bracket you can see it. I don't see your vacuum lines connected to it from the angle of the picture. You should check there.

Edited January 16, 20196 yr by lsoz

There is a check valve between the vac reservoir bottle and the S1(maybe) vac line that goes through the firewall. The hose connection on the inside of that check valve has a different OD than the other hoses and sometimes if it is not fitted properly it can cause that very problem.

Edited January 16, 20196 yr by gwri8