I have a strange issue with my P38 HVAC. The car was off the road for a couple of months due to a blown head gasket, and after the the HVAC was not blowing properly. The blend motors were replaced and calibrated, but now although the HVAC starts OK, after a while there is no air coming through the vents even though I can hear the fan running. Diagnostics do not show blend motor faults and the internal ambient and evaporator temperature sensors are new.

The system seems to work longer when cooling than heating but recently, at best, it will operate for about 60 minutes before the airflow stops - the fan still running.

I am at a loss, can anyone suggest what the problem and solution might be? Many thanks for your time, Simon

Are both fans running? If only one runs it draws air in from outside but rather than it going through the AC evaporator and heater box, the air just goes across the car and out the other side. If you poke the recirc button, it closes off the inlets so the air will then come out of the vents as it can't go anywhere else. If you take the pollen filters out, you can see the fans by looking down the hole. With the ignition in position 1, they should both start spinning.

Gilbert,

Thank you very much, that sounds highly likely, I'll try the re-circ button when it happens next. I'll let you know the result.

Thank you again, very much appreciated!

Simlon

Hi Gilbert, I had a couple of long drives yesterday and on both teh air stopped flowing into the car after about 50 minutes. Sadly pressing the recirc button didn't change anything - though considering what a lemon the car turned out to be it is very likely that the button doesn't work and the inlet vents didn't close. I'm going ahead with changing the fans and will update in due course. Thank you again, Simon

Make sure you an see both fans spinning by looking down the pollen filter housings first.

I'll do that today - though with a query as the system works perfectly to start with, the problem only arises after some time.

The motors for the blowers are standard brush and commutator DC motors. I've had a couple where one winding or one segment of the commutator fails so they don't work if the brushes are on that segment. If you take them out and power the motor on its own, rather than running smoothly, they vibrate.

Also, check that when the airflow stops it has genuinely stopped and not that it is blowing on the screen. By default, initially the air is blown out of the dash vents and into the footwells but after a while, when the aspirator is telling it that the interior is close to the desired temperature, the distribution motor changes so the airflow comes out of the dash vents. If the distribution motor or the gears aren't properly aligned, it may be directing it all to the screen vent instead. This assumes that the blowers are still working though and the air is just going where you don't want it to go.

I checked after a trip yesterday and the flow is not being directed to the screen - or anywhere else in the car, though I can feel little 'seeps' of air around joints in the dash, so teh A/C is working, just not going in the right place. Stupidly I forgot to check that the fans were both running after the trip when the problem was evident.

This still sounds a very likely explanation to me so I've ordered some replacement fans and will get them changed; I'll update when done. Thank you again for your time.