1990 740 Very little air blowing on any floor settings.

[Filadog]

Hello Guys,
I am new to the forum scene although I have used advice from from several Volvo forums through my many Google searches. Thanks in advance for allowing my question(s) and I hope that I may one day contribute through my repair experiences as well.
I have owned my 1990 740 Turbo Wagon for more than 15 years and recently revived it after it sat idle for more than 5 years. I have sorted out the serious issues but one small problem that still requires attention is my heater vent control. Air blows fine on the defrost or upper level settings but on any setting which includes the floor it barely allows air to blow through the upper vents and almost none down below (over time the car warms enough to get by in North Carolina but I am planning a trip to Indiana and heating requirements will be greater). I did notice that I can get a bit more air above if I use the A/C setting which includes the floor. Perhaps someone who knows the inner workings of this system could lead me to the specific problem rather than my tearing the dash apart for no reason. I know there is good vacuum coming through the valve in the engine compartment and I can hear the flap operating for the upper vent on that setting alone. Thanks, Matt

[93Regina]

Perhaps someone who knows the inner workings of this system could lead me to the specific problem rather than my tearing the dash apart for no reason.

Well now, remove the dash, and install ECC control panel, then it would be justified.

It has been suggested that there is a vacuum diaphragm via driver's side center console, which is accessed via removing driver's side steering wheel facade; and of course, access to heater mechanism is via radio removed.

So, you trace vacuum from engine compartment to vacuum-dampers, and check dampers.

See: Heater, Vents, Vacuum Controls, and Blower Motor:

Also, here's an experienced hand's comments: Hello, the correct way to test the vacuum bellows is with a vacuum pump pump them up and see if they hold pressure. My experience has been that the vacuum bellows on the drivers side with 2 lines going to it is usually the one that fails, but ANY of them that go bad can cause your problem. including the one up top, and the recirc fresh air door motor behind the heater case on the passenger side, The flaps that direct air flow are internal in the heater case assembly. And, the vacuum motor can be replaced without removing the dash and the entire heater case assembly, however, The heater case has to be cut with a die grinder or a hot knife to access the small nuts that hold the vacuum bellows in the heater case. Volvo didn't give much thought when they sent that one down the line. We charge 5 hours at the dealership because it does require some work and laying in the floorboard for a while.

Air blows fine on the defrost or upper level settings

That's great news....and all that is needed in northern climates....its wise to wear warmer clothing in a northern climate.

[Filadog]

Thanks for the links and suggestions. Seems a bit of work so I must wait until I return after the holidays. But I am confident with this info I can now tackle it more efficiently...... and successfully. Again, much appreciated. Happy Holidays to all!

[jimmy57]

The floor/defrost door on that HVAC case is vacuum pull to go to defrost and vacuum on the other side of diaphragm to move to floor. The floor side of diaphragm has to be sealed to the rod that goes to door and the seal fails. It is a hard job to replace. I suggested blocking the defrost vents to a customer that did not want to spend the several hundred dollars to repair. She made a couple of long bean bags and that works well.

[93Regina]

I suggested blocking the defrost vents to a customer that did not want to spend the several hundred dollars to repair.

In northern climates, defrost must be working in snow/humid/ice conditions.

Before 1960s, it was not uncommon to find vehicles, trucks/cars, without heaters in southern states.

But, up north, its a given, defrost must work...it might not be comfortable for city daily drivers, but eventually on road trip, the vehicle will warm up in vehicle.

[jimmy57]

I assumed the OP understood the concept of a rectangular bean bag or other similar item moved over the defrost vents to partially restrict air flow, or fully if they desire, the defrost air so the airflow would increase out the floor vents. I never mentioned using adhesive to stick them there permanently.

Taking apart a dash on a 24 year old car that has spent its life in the sunbelt is a recipe for a destroyed dash and some shops would not even attempt the task.

Another solution if passengers rarely ride in rear seat is to stuff foam or rags into the floor vents for rear heat that are under the front seats. That might increase the front flow enough to make you warm.