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AC Vacuum Necessary?

Hi all,

I decided to do some work on the engine bay of my '77 Datsun 280Z. I would like to know whether it is possible to remove the vacuum lines from the system, and if so, what would be the effect on the AC system? If I understand it correctly, the AC selector and mode doors are mechanically operated and vacuum lines assist in opening (i) the air intake door and (ii) the mode door? Please advise.


If you’re able to identify exactly which hoses are for only the ventilation system, then I’d disconnect and plug them off, see if everything works as it should. Problem is most late '70s cars had a rat’s nest of hoses, hard to tell which one did what.

That’s definitely encouraging to hear there’s a possibility the AC will still function!

I’m actually doing a V8 swap and wanted to clean the engine bay of the vacuum canister, hoses, and the related wiring to the vacuum system (i.e. the magnet valve fast idle control device and magnet valve vacuum source). Although in terms of wiring it looks like I’ll need to retain the compressor relay and pressure switch on the receiver/drier.

Most vehicle HVAC systems default to the defrost position as a safty feature. There is a spring in the defrost vacuum actuator to keep it in the defrost position if there is a vacuum supply failure. Your car may be too old to have this feature, I don’t know what year this was required. Hook up a battery and turn on the blower to see where air is blowing from. You can always wire tie the mode doors where you want them to stay.

I have no idea if it’ll still function. The vacuum hoses won’t likely control the compressor, but may affect getting air blown past the evaporator. Just try it and see, as long as you plug off the hoses you disconnect (both ends) it shouldn’t hurt anything.

Good luck grafityflyer. I had a V8 240Z about 30 years ago. It was called a “Scarab” back then.

What V8 are you using?

@missileman: That’s awesome, definitely heard of the old scarabs! I’m going with JTR’s setup which seems to be what most folks use.

@keith: Dropping a 355 SBC with Edelbrock Performer RPM intake and Quick Fuel 600 cfm carb. If vacuum lines are required to keep the AC functional I suppose I could always find vacuum sources on the SBC (i.e. carb / intake)? But the original vacuum lines ran to (i) the fast idle actuator and (ii) the check valve on the Datsun engine. I wonder whether this mean if I retain vacuum lines that I have to install a fast idle actuator and check valve on the SBC?

For reference, here is a copy of the vacuum hose diagram from the original service manual.

I hope those with more expertise than myself on AC systems can make some sense of it?


Looks like all the under-hood stuff (to the left of the ‘grommet’) is either for adjusting up the idle speed when you turn on the a/c (something your SBC probably won’t need) or to the vacuum tank, to supply a relatively constant source of vacuum to both the underhood stuff and the ventillation system. It looks like you will need vacuum to operate the various doors inside the car. If you just hook it up to the intake manifold without the tank you’ll probably have doors opening or closing as you hit the gas. Maybe find another place for the vacuum tank?

So if I understand it correctly, it would be best to keep the vacuum canister then hook the hoses to the SBC? I suppose the ideal vacuum sources on SBC would be either the intake manifold or carb? Do I somehow need to replicate what was on the Datsun engine, i.e. the magnet valve fast idle control device and magnet valve vacuum source?

Thanks again!