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How can it go faster?
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74 chevy pickup. Defrost quit working so I did some poking around under the dash this afternoon and came to the conclusion that the heater is operating fine and the fan is blowing fine. What isn't working well is the vacuum actuated valves to direct the air through the proper ducts. It seems that there are several of these valves (ran off of vacuum lines). There is a big conglamaration of hoses that all mount to the back of the switch in the dashboard. It doesn't seem that any of the hoses have vacuum so I am suspecting either (1) a big leak somewhere, (2) no vacuum at all or (3).......

I was gonna test for vacuum but none of the hose connections wanted to come loose very easy so I gave up before I broke something more and made a bigger problem.

I guess my question is how much vacuum is this system supposed to have? Where is the vacuum supplied from (the engine I know) but where does it come off of the motor at? Any help would be much appreciated....till later, heater only on the floor.....haha

Thanx all!!
 

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NYOFP4RJ3CHRIS
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The vacuum line you're looking for is easiest to trace starting from the passenger side firewall to the source (engine). Make sure that line is hooked up to a vacuum source from the manifold. A very common problem when this symptom exists is when the vacuum reservoir/cannister is cracked or damaged and leaks. It's probably gonna look like a black ball with one or more vacuum lines attached to it. It's generally located on the fender well or firewall in proximity to the heater housing.You can bypass this reservoir and not even use it in a pinch. The only thing that'll change is that when the engine is under load, your heat may blow from say the defrost instead of the dash vents (which ever is "default mode" so to say).

If you have vacuum hoses that are very soft or brittle, start replacing them, that alone may fix your problem. HTH
 

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How can it go faster?
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Discussion Starter #3 (Edited)
thanx...I found it!! the line is going to a vacuum source coming from the base of the carb. Zero vacuum at idle and increases with RPM's. The "black ball" thing actually acts like a check valve I discovered, only allowing air through in one direction. After learning about the vacuum increase with the RPM's (and attaching the hose back to the carb, it musta slipped off when I had to pull the power to the coil at work one day. My ignition went **** up and wouldn't turn off suddenly) Anyways, rehooking up the hose and giving the motor about 3000 RPM's actually provided enough vacuum ot kick in the defrost. YEAH!!....oh yeah, I had some duct work that had came apart up in the dash....that sucked but I got it taped up and fixed....should be better now!!

Am I connected to the appropriate vacuum for the heater system? ported vacuum vs. manifold vacuum??
 

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heater vacum

hey fellas,
i have a 68 chevelle and my heater is not working, but i read in your threads something about the vaccum canister, I had one of those on the passenger side firewall I removed during the engine removal. With engine back in and running, do I need this vaccum canister to make the heater work? I wasn't sure what this black round canister was to tell the truth, but maybe this is the problem. The hose that went to it is unsed now, so I need to look what I did with that can. Pls let me know if this may my problem. If so should it be connected to full manifold vaccum or ported? Thanks
 

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Let's move this to General Rodding Tech.

Dan
 

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Magikal said:
74 chevy pickup. Defrost quit working so I did some poking around under the dash this afternoon and came to the conclusion that the heater is operating fine and the fan is blowing fine. What isn't working well is the vacuum actuated valves to direct the air through the proper ducts. It seems that there are several of these valves (ran off of vacuum lines). There is a big conglamaration of hoses that all mount to the back of the switch in the dashboard. It doesn't seem that any of the hoses have vacuum so I am suspecting either (1) a big leak somewhere, (2) no vacuum at all or (3).......

I was gonna test for vacuum but none of the hose connections wanted to come loose very easy so I gave up before I broke something more and made a bigger problem.

I guess my question is how much vacuum is this system supposed to have? Where is the vacuum supplied from (the engine I know) but where does it come off of the motor at? Any help would be much appreciated....till later, heater only on the floor.....haha

Thanx all!!
Possibly the rotary vacuum valve behind the selector switch is bad.

Very common problem and they're servicable. It's the piece all the vacuum hoses plug into and is actuated by the dash selector lever.

Very easy repair. I think GM still sells them. Probably less than $15. One hose on the valve has vacuum all the time.

Make sure it does otherwise you have a vacuum source problem.

There are also vacuum actuators that operator the mode doors.
 
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