Last night i discovered my 1972 Delta has a bit of an issue with its vent system.
For the most part everything seems to work ok.
The slider is at "off" on the furthest left setting, then moving across the settings seem to work ok. Things whir behind the dash, and air moves between the different vents.
However, the problem is that i'm getting no air firing up to the windscreen. None of the settings seem to get air to the screen no matter what the combination. I'm guessing the "defrost" setting should point air directly at the screen?
The fan operates perfectly on all settings, and the heater works fine. A/C is ok, albeit, short of gas.
With winter around the corner, there will be the need to de mist and at times defrost the screen, so i need to get it sorted sooner rather than later.
I'm not sure of how the system works. Is this a common issue?
Can anyone point me in the right direction of how i go about fixing it. Is it going to be a nightmarish "remove dashboard" kind of problem..?

 

The Chassis Service Manual has a good description of the operation of the ventilation system.
If you read that, it will point you in the right direction.
The three greatest possibilities are an air deflector door mechanically stuck, a vacuum diaphragm with a hole in it, or a bad vacuum hose.

The1971 and 1972 CSMs are available for free from WildAboutCars, but the scans of both of these are not great, so it would absolutely be best to buy a genuine copy from a literature dealer or eBay (no, the shipping to the UK will not warm your heart).

- Eric

 

Lol. Thank you Eric.
Yes shipping costs are proving a headache. I am finding mechanical bits are in reasonably good supply here, but anything other than generic B body parts have to come on a plane.
I just bought some roof rail weatherstrips, and they cost more or less the same to ship as they did to buy! I did get them cheap though. Car Parts Discount ( carpartsdiscount.com ) were much cheaper than anyone else, and they are decent quality Metro parts too.

 

Yeah, pretty much every single B/C-body part will be different from the A-body parts in these years, and if you need any, you will probably need to get them from a member here, or from a US breaker.

It has been my impression over the years that Metro has some of the best quality rubber parts, and their prices are less than many.

- Eric

 

As noted, start with the Chassis Service Manual. Airflow in the HVAC box is directed using internal flapper doors controlled by vacuum actuators. Hopefully this is simply a case of a vacuum hose that's come loose or developed a crack. Other potential causes are a bad vacuum actuator diaphragm, a bad selector switch on the dash control head, or a sticking flapper door. The latter is a possibility since GM used spongy foam tape to seal these flapper doors. The foam deteriorates over time and gets sticky.

 

I can hear some mild hissing from behind the vent controls on a couple of the settings when i move the slider...

 

Well, for those that may be interested, i solved the problem.
The door that points air to the single, floor directed outlet is the one that needs to shut in order for air to be directed at the windscreen vents.
This door was being held by quite a strong spring. The vacumn wasn't strong enough to move the door and overcome the spring. I removed the spring, and all settings now work perfectly. I cannot see why this spring was in there
I'm sure there must have been a reason originally, but all its doing now is hindering operation.

With the dash out (pain in the ***), all hoses and connections were found to be sound, with no leaks or cracks.