Hi guys,
I have torn apart the dash to try to figure this out, maybe someone can shed some light.
When MCC is set to heat, air flow from the dash vents is low (even with high fan), when MCC is set to cool, air flow from the dash vents is high (as expected). I can observe the mixer bellcrank moving when I select any heat range. I can hear bellows "squeak" when I select footwells or dash, but seem to get air in the footwells continuously.

Any ideas?
TIA
Andrew

 

well, its all vacuum activated. my guess is you have a leak that's worse when its warm rather than cold

 

this is a 740/940, right? not a 240?

hmm, another thing. the heater valve, no vacuum == water on for heat and vacuum == water off for no heat. maybe the vacuum leak is in the control line to the heater valve, and when its set to not-heat, too much vacuum is leaking such that the system can't maintain enough vacuum to work the air control flaps?

just an offhand idea.

 

speaking of heater valves on a 740... I just replaced mine due to a coolant leak. It had the cheap 'chevy' valve in it, like this...


and I replaced it with the 'real' volvo valve (available from VDO) like...


my heat control has much smoother control now. before, a few clicks was most of my heat, and even halfway was full heat. now halfway is just nicely warm, and the range of the control is much better balanced. downside, I paid nearly $90 for the volvo branded valve at a local indie volvo shop's parts counter (but hey, it was in stock, and I wanted my car back on the road NOW).

 

Thanks for the thoughts, I have what looks like an OEM valve (in my 740) and it is oriented correctly (thanks). Is there a way to perform a vacuum test without a vacuum gauge (cause I don't have one)?

 

only way I know of is to use a mity-vac, which is a little handheld manual vacuum pump (looks a little like a gun with a pressure gauge on the end), find the main vacuum line from the engine to the MCC,and hook the mityvac up to it and pump it down a few pulls and see how fast it leaks out (in?). flip the flap controls to the various positions, and repeat. a little loss is ok but a fast leak such that you can't pump it down is not gonna cut it. and yes, when you flip a control setting that will require moving flaps, it will take a bunch of pumps to suck enough air to move the flaps.

 

thanks

 

mity-vacs usually come in a kit with a bunch of tubing, and a pile of adapters, all of which can be handy for different things.

the pro models are more expensive and are metal, but for amateur use, the plastic ones are just fine.

 

The check valve on the firewall was kaput! Almost too easy....

 

Usually is.