HVAC Question - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 8 (permalink) Old 04-27-2015, 03:27 PM Thread Starter
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HVAC Question

Not a big concern now that it's warming up.

I went through and replaced the vacuum lines connecting the HVAC system so I no longer have heat coming out at weird times. AC works great and blows ice cold with the new R134 system.

But....I never had any heat. I can get heat if I disconnect the vacuum line that comes through the fire wall...it blows hot gangbusters through the defog vents, so I know the heat has the potential to work.

It seems the temp slider (non digital base model) has no effect. Can anyone help me understand how the temp selection works with this system?

1985 300ZX
1998 Ridiculously built solid axle 4x4 Frontier
2012 Murano LE AWD
2007 Ram 2500 Cummins 4x4
2008 Boulevard C90T
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post #2 of 8 (permalink) Old 04-27-2015, 07:29 PM
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if you would really like answers that would help you and avoid flames give the following info when you post: year of car, n/a or turbo engine, stick or slushbox. Most of us can tell by the year what model you have ie 85T is a turbo z31. Z cars come in many flavors and they are all different. example 84,85,86 Z31's pretty comparable. there were changes in 87,88,89. in addition 300ZX can refer to two completly different cars the Z31 and the Z32. No mindreaders here the more info you can post the more likely you get a cogent useful answer. The cars with L engine (straight six) changed almost yearly from their introduction. A 240Z and and a 280Z

your heater control valve is usually operated by vacuum closing it off. need to know type of z to determine. do you have the fsm from xenon. helps a lot especially when you read it. also there are sensors that affect operation due to outside temp

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post #3 of 8 (permalink) Old 04-28-2015, 11:53 AM Thread Starter
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Originally Posted by rogerz View Post
if you would really like answers that would help you and avoid flames give the following info when you post: year of car, n/a or turbo engine, stick or slushbox. Most of us can tell by the year what model you have ie 85T is a turbo z31. Z cars come in many flavors and they are all different. example 84,85,86 Z31's pretty comparable. there were changes in 87,88,89. in addition 300ZX can refer to two completly different cars the Z31 and the Z32. No mindreaders here the more info you can post the more likely you get a cogent useful answer. The cars with L engine (straight six) changed almost yearly from their introduction. A 240Z and and a 280Z

your heater control valve is usually operated by vacuum closing it off. need to know type of z to determine. do you have the fsm from xenon. helps a lot especially when you read it. also there are sensors that affect operation due to outside temp
85 base model. Signature gives the year. Content of thread indicated base model.

Non turbo. Figured it was obvious that it's a Z31 since it's posted in the Z31 forum.

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1998 Ridiculously built solid axle 4x4 Frontier
2012 Murano LE AWD
2007 Ram 2500 Cummins 4x4
2008 Boulevard C90T
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post #4 of 8 (permalink) Old 05-02-2015, 09:07 AM
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Originally Posted by Desert Rat View Post
It seems the temp slider (non digital base model) has no effect. Can anyone help me understand how the temp selection works with this system?
The temp slider just changes the position of the air mix doors. The air mix doors control how much air passes through or bypasses the heater core.
So that the control unit knows the position of the air mix doors, there is a Potentio Balance Resistor attached to the linkage.
The 86 FSM I'm looking at has pretty good diagrams and trouble diagnoses for checking the abnormal condition of "outlet air will not become warm".
Hopefully it is just the solenoid valve pack that is faulty, specifically SV6 and SV7 which are combined into a Double Sided Solenoid Valve ("DSVV"). The DSVV turns vacuum on and off to the actuator that works the air mix door linkage.

The air mix door linkage could just as easily have been operated by a cable, but DSVVs, Potentio Balance Resistors and electrical sliders are more elegant.

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post #5 of 8 (permalink) Old 05-02-2015, 11:42 AM
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It is a lousy system because as the cars get older the vacuum actuators crack and fail along with the vacuum lines. Critical ones are no longer available and to work on the system the dash has to be removed. A cable system would have been the way to go and would have lasted forever but Nissan loves to over engineer things.
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post #6 of 8 (permalink) Old 05-02-2015, 08:08 PM
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A cable system would have been the way to go and would have lasted forever but Nissan loves to over engineer things.
Can you name a new car that has cable-operated HVAC controls?
I had a 73 Chrysler with vacuum pushbutton HVAC controls and a 69 Buick with vacuum locks and boot release. There is nothing unique about Nissan "overengineering" using vacuum. The electric servo motors in Z32 HVAC systems don't always last 30 years. I've seen electric servo motors in GM HVAC just a few years old fail.
Would customers buy a new car with cable-operated HVAC controls? The same customers that could not change a flat tyre if their life depended on it?

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post #7 of 8 (permalink) Old 05-03-2015, 09:24 AM
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Can you name a new car that has cable-operated HVAC controls?
I had a 73 Chrysler with vacuum pushbutton HVAC controls and a 69 Buick with vacuum locks and boot release. There is nothing unique about Nissan "overengineering" using vacuum. The electric servo motors in Z32 HVAC systems don't always last 30 years. I've seen electric servo motors in GM HVAC just a few years old fail.
Would customers buy a new car with cable-operated HVAC controls? The same customers that could not change a flat tyre if their life depended on it?
I think back to my Austn Healey 3000 and MB 190sl days, when they were cable operated and never had problems...You are right though, today most people want automatic systems and cable can not do that.......
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post #8 of 8 (permalink) Old 05-04-2015, 10:49 AM Thread Starter
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They didn't exactly make all this stuff easy to get to either. pulling the whole dash is not something I'm really interested in doing.

I know there's the potential for heat...disconnect that vacuum line coming through the firewall, and I have heat blowing full blast through the defog vents.

I guess that's the redneck fix on those rare cold mornings that I drive the car here in Arizona.

I've chased all the vacuum lines on the system and replaced them all. I had a few leaks thanks to cracked lines, but feel pretty good that those are chased down. The ones in the car are still nice and soft, and in good condition, but the ones under the hood were toast.

1985 300ZX
1998 Ridiculously built solid axle 4x4 Frontier
2012 Murano LE AWD
2007 Ram 2500 Cummins 4x4
2008 Boulevard C90T
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