1993 Heat/Defrost Issue
 

Not to long ago I bought a 1993 940. I am having issues with getting a strong current of air from any of the ducts. Whether floor, defrost, or dash, I seem to just be getting a radiating heat but not an actual air movement. If I were to take a piece of toilet paper and hold it to the vent I would get no movement at all, yet I can feel heat in the car.

So I have checked all the vacuum bellows for operation.. they work. I checked the servo motor for the heat control and the recirculation bellow. All good there. There are no vacuum leaks or issues with any of the diaphragms.

So here is my question. Since the vehicle is older could the filter after the evaporator be so clogged that there is entirely no air movement through the matrix. I have checked the blower motor and it works fine. I know that this is a significant amount of work to change this filter so I was wondering if anyone else has had or heard of an issue like this.

Also the water valve has been bypassed. Meaning there is not one on the vehicle. Plus it is manual control and not electronic.

Any help or advice would be greatly appreciated.

How can you have any temperature other than HOT without the water valve? How do you control the heat level? Are your VENTS operating? Are you able to change the AIRFLOW from the DEFROST (windshield) setting to the MIDDLE vents and to the FLOOR?
I have never heard of a filter after the evaporator; you may have the fan intake plugged by leaves which is the only thing that would reduce/stop airflow. Oh, and if you have MANUAL controls (sliders), this must be a non US version 940, right?

I have a new water valve. The previous owner bypassed it. I plan on installing it once I get this issue resolved. By manual, I meant that there is not electronic cabin control IE where one sets temperature and and computer takes over. You will have to excuse my ignorance on Volvo's in general. The shutter's that control where air flow is routed does work. However there is no flow. I will check the fan intake.
It is a US version and there is indeed a filter after the evaporator. Why it is there is beyond me.

Fan intake is clear. No debris at all. Back to the evaporator?

OK, so what you are saying is that the fan runs at good speed, but you get no airflow out of ANY vents, no control with the knobs? BTW, your system is electronic. I'd say that you have a bad CONTROL UNIT, the electronic control box with all the knobs. It costs TOO MUCH new from Volvo, it's not available aftermarket BUT plentiful used on Ebay OR easily repairable. If you take it out and open it, usually a burned spot will be obvious on the circuit board--repair that, and all will be well.

(A plugged filter, if there was one, would rarely stop ALL FLOW.)

I don't understand how it could be a control unit. All the routing is controlled by the "mixer" which I am assuming is what you are calling the control unit in the middle of the dash. For instance.. when I turn the knob to the dash vents I can physically see the "gates open to route flow to the dash. I have a good amount of the dash open at this point. When they open I should be able to feel something. The control unit as far as I can decipher is properly controlling all the gates which would route air flow to a particular location. Also I took the ballast resistor out and could feel the air flow blowing strongly from that opening. The blower is under the passenger dash. It ducts past the evaporator or through it to a middle box with the gates. Unless I am unable to properly envision the routing. It seems that no air is making it out of the area where the blower is. Which would be through the evaporator. This is my thinking with the filter being clogged but now I am starting to wonder if air is getting past the evaporator itself. But given the amount of debris there was, I find this difficult to digest. I don't want to condemn the control unit right off, cause everything mechanically it controls seems to be in order. Is there anything else I might be over looking?

I appreciate your response lev.

Here is a decent diagram of the routing and controls.
Heater unit With atc (auto. Temp. Control) combu 940. Combu. Fits: Volvo | VolvoPartsWebstore.com

Checked the circuit board in the control unit. No burns or broken connections. Looks good.

Good job, you are certainly on it!
Hm, well, something is preventing the air from coming out of the housing and going to the vents... Must be a gate stuck shut somewhere, that's all I can think of at this point. The filter, I see there is one, has never been an issue before... Keep posting, this is interesting.

the non-ECC controller used on US model 740/940's is vacuum activated, not electronic. only the ECC (Electronic Climate Control) that has degrees on the temp knob is electronic.

vacuum comes from the intake manifold, goes through a 1-way valve, to a storage bottle tucked deep under the dash somewhere, to the vent control box. various vacuum lines come out of the vent controller and go to the flap actuators. no flaps at all probably means no vacuum there, check the main vacuum hose.

even with all the flaps in the neutral position, most of the air should be coming out of the main dashboard vents when the fan is on.

there's 3 sets of flaps. 1) redirects air to the defroster vents on top of the dashboard. 2) redirects air to the heater vents in the foot wells. and 3) closes the outside air so the car recirculates inside air. if both 1 and 2 are 'off', then the air comes out of the aimable dashboard vents (which have their own manual control to open/close). flap 3 is controlled by the recirculate button, and selects where the fan sucks the intake air from.

the fan blows the air past the AC condensor, then past the heater core, then up to the manifold where these various flaps are.

if your fan is working, the air has to be going SOMEwhere. AFAIK, no 740/940 has a cabin air filter.

So I'm back at it again today. I have been doing some poking around and talking to some people and want to bounce something off you guys. Someone said that I may have switched polarity on the blower motor causing the blower to spin in the incorrect direction. I grabbed some toilet paper and directed it to the intake to see if it would suck in. I have to be honest that it was difficult to tell whether it was blowing or sucking. Also I looked at some of the wiring to the motor it self and found two butt splices connecting the wires from the harness to the motor. I doubt that this is factory but then again it could be. My question based on the fact that I haven't taken the time to figure out the exact wire routing for the ballast resistor, is if I went ahead and switched polarity to the motor would I cause damage to the system if this guess is incorrect.

Any help or previous knowledge of this would come in handy.
Thanks in advance.