AC/ cold to hot driving.

Most likely a vacuum leak. Does it turn hot when under acceleration? If so it's a common symptom of a leak. One of the (many) hoses are probably cracked and causing loss of vacuum to the actuator(s) that control the doors of the climate system, or could be a leaky vacuum actuator that controls the air blend doors. I replaced all my vacuum lines and still had the issue so I had to pull the dash and climate box behind where the radio is and replace one of the actuators that was dry rotted and wouldn't hold constant vacuum.
Have you replaced all the rubber vac hoses under the hood? There's one hose that goes from the tank near RT headlight thru firewall to a bank of solenoids near gas pedal. If the lines are ok then most likely one (or more) of the rubber bladders in the actuators are cracked and leaking. There are 3 on the side of the heater box that you can somewhat see above the gas pedal, and there are two behind it and to get to those you have to pull the dash and that box behind where the radio is. People usually put a one way valve on the line that goes thru the firewall and they say it helps. Turbo cars have a vac pump to increase the amount of vac and I'd say to check to make sure it is working, but you have an N/A so that won't apply to you.

Thanks for all the advice and help. I will pull the dash sometime next week.

Fun stuff. If you have a good dash, be careful removing the defrost grilles to get at the 10mm nuts, they're pretty brittle, and mind the little sunload sensor. Also, the rubber grommet around the heater core tubes at the firewall is NLA but there is a toyota part that will fit, I will have to look for the P/N for you if it comes to that.
I couldn't find the post I made when I did mine but below is a pic of behind the heater box with the heater core removed from my 86T GLL. The larger black actuator on top controls the main blend door above the heater core and when there is no vac it diverts air thru the heater core. Any one of those rubber hoses, accordian-style actuators on the side, or the ones pictured with air bladders in them (there is also one more above the glovebox that controls fresh/recirc air) are susceptible to cracking/dry rotting and ANY leak in them or any of the other vacuum controlled stuff will cause loss of vac resulting in your condition. On mine, the smaller actuator on the bottom had a leaky bladder. None of these are available anymore but I was able to use a MotorCraft part (YH-1463) with minimal alteration and it works great. I used a Mity-Vac to isolate the line that had the leak.



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… and to be clear, just because the actuator that controls the blend door that gives heat loses vacuum, it DOES NOT necessarily mean that it is bad. A leak anywhere in any of the hoses, the rubber bladder in the hot water cock, cruise actuator or the actuators on the side of the heater box will cause that actuator to lose suction and since there's the blower blowing right against the diverter the moving air will help open the diverter to force air thru the heater core.
So unless you like fun projects, I'd first replace all the rubber hoses in the engine bay and the one going thru the firewall up to the T connector under the glove box and test. This is where using something like a Mity-Vac comes in handy.

AZinMA: Could U send this post with comments to my e-mail? I have tried to save to my desktop, but I lose it every time? This post is JUST WHAT I NEED. I sure appreciate your willingness to post this info with pic. It should make this project much easier. I want to save it for future reference as well . Thank you - Thank YOU !! - Jerry

BTW: My '87ZX IS a turbo.

#5 Hey I'm glad you mentioned the cruise control, because mine isn't working. I believe I will start there!!!!!

Hey I think I found my problem, I used a Mity vac and the three servos all lost vac. They would not pull the doors half way. All my hoses looked good none cracked or dry rotted.

Oh yea is there anywhere that I can new servos. Thanks!!!

None of the actuators are available anywhere. You will have to figure out which one(s) aren't working and find something to adapt, or attempt a repair. Like I said above I used a Ford part for the smaller actuator on the bottom of the back of the box. I have heard of people using silicone to seal the leaks on certain actuators, maybe something like FlexShot would work?

I used a mity vac to check the servo's and could not get them to hold any pressure. Does this sound like they are bad to you or what would you consider a bad servo?Thanks!!!!!

If you checked for vacuum on any of the lines going to any actuator or servo and it did not hold vacuum then either the rubber hose or the actuator / servo is bad. Everything under there should hold vacuum. When the system is working, it is under constant suction. What gets the actuators to move is the bank of solenoids with the maze of rubber hoses coming off of it just above the gas pedal. Those are valves that release the vacuum to the the right hose that controls the actuator. So regardless of where you did that check, if you're still not holding vacuum then either the hose or the actuator on the end of that line is bad.

Ok thanks for all the help!!!!!!!

Got cold air without heat, I had three bad servos, the 2 behind the air box and the rec. Thanks for all the help!!!!!!!!