Volvo 940 air conditioning fan blow to floor when accelerating

You probably notice that it blows through the dash UNLESS you accelerate hard, then it's floor and defrost vents that gets the air.

It's a ~$40 dual action bellow sort of up over the gas pedal that is malfunctioning. You can put a band-aid on it by removing the kick panel over the pedals and then plugging the yellow line going to the bellow.

To replace it (per Volvo) you have to remove most of the dash! That's expensive and a pain in the behind, plus you probably will have rattles in the dash after.
OR you can cut a hole in the air box where the baffle connects and replace it, then patch the hole with the 'lid' you cut out and duct tape it back on.
Nobody will ever see it. :D

check your heater coil

check the 2 fuses that are in the circuit
one for the ECT and one for the fan relay

i have heard somewhere that this is caused by a faulty vacuum check valve which is black and white plastic and is located near centre above the rear of the engine up against the firewall. vacuum drops when you accelerate so there is not enough vacuum to keep various vacuum operated heating/cooling system valves in their selected position . the check valve keeps sufficient vacuum in a reservoir tank when vacuum at the engine manifold drops.

Welcome to the club :). This is a common issue. A lot of people thought it was just the pressure valve under the hood, but they have had limited success. If I figure it out, I'll let you know, and if you do, be sure to let me know. Thanks.

Sounds like it could be the mass air flow sensor.
Have vehicle scanned for codes and use shop manual to conduct the correct tests on that sensor .

Black smoke points to unburnt fuel. I would start by replacing the spark plugs and wires if you have not done so in the last couple of years.

could be a fuse they are located behind the ash tray. could be the fan motor you can test this by finding the connection for the fan motor and connecting a positive and negative wire to the positive and negative on the connector. it could also be a relay or loose or bad connector . start at the fuses check all of them. then check to see if your fan works then check all your connections if that is not it try to see if any of your relays look burned. im not sure whicjh one controls the fan controls. good luck if you need more help or specifics i will look into it more i have a friend with one of these vehicles that i can look at.

I HAVE REMOVED TEMP CONTROLL UNIT FROM DASH,TESTED CIRQUIT BOARD AND FOUND
ONE SOLDER SEPARATED FROM BOARD AT THE SMALL RELAY'S LEG. PUT IT BACK TOGETHER AND NOW IT WORKS LIKE A CHARM. REMOVING THE CONTROLL UNIT WAS THE MOST DIFICULT JOB TO DO.LOTS OF THINGS TO REMOVE,LOTS OF SCREWS AND SHARP EDGES.IT IS DONE.

2 way vacuum servo under dash, very difficult to replace, suggest dealer install, about $300, lots of labor, part $50.