This morning I had no heat in 1992 940T. I think heater core control valve is not opening to circulate coolant. Where is it located, and how do I troubleshoot it?
There is enough coolant in the engine, thermostat is fine too. No leaks inside or out. Water pump is fine as well. So is the blower motor. A/C is fine too.

I fixed it. Problem was in the electrical motor that operates a gate (flap) inside blower unit. I removed the glove box and could easily see it. I took this motor out, and took it apart. Cleaned the interior, and used new grease. Next i plugged in the connector and tested it before full install. With heater fan turned on the motor worked. Red rotary knob regulated the distance of the movement.

Dear Moe,

Hope you're well. It would be great if you could do a write-up, describing in detail precisely how you opened the motor casing (for example which tools used), where you applied grease, etc.

I'd guess this part can't be ordered from a Volvo dealer, so your insights - into how to extend the service life of this motor - will be very valuable. They should be added to the site's FAQs.

See at the upper left corner of this page, for how to submit additions to the FAQs.

Thanks again for your contribution.

Yours faithfully,

Spook

The heater valve is engine vacuum operated thru the AC console. Find the heater valve make sure its vacuum line intact. Or start the engine, pull the valve's vacuum line off to check for any suction. No suction could mean vacuum leak - trace the leak. If there is suction then the valve itself may be faulty.

Regards,
Amarin

Thanks for great suggestions. I'll do it. Just curious...when I turn the heater knob clockwise to increase / decrease the temperature, what is actually happening? Is the knob connected to some cable, some electronic switch, some gear...In engine compartment I can see the vacuum line on top of the heater valve near 2 hoses going to the heater core. Where does the other end of this vacuum line go? I'm guessing to the MCC. Maybe it got disconnected there, or pinched.

The MACC heater knob controls 2 things - the engine vacuum going to heater valve AND the flap door inside HVAC box. The flap door is controlled via a motor to vary the degree of opening either from AC evaporator or heater matrix for temp control. The heater valve opens fully (I think) once engine vacuum is routed there.

Based on your recent post I guess you've figured it out already.

Amarin.

Did you work on the coolant system?/Maybe you need to raise the front end to let air out,,ormaybe the thermostat is in the closed position
condition? usually if the valve fails you have heat not cold,,

I did not work on the cooling system recently. I'm pretty sure thermostat is fine as both upper and lower hoses feel hot. Also radiator fan isn't kicking on and off any more frequently than normal. With closed thermostat I could not do the kind of driving (city) I do every day, without serious overheating. Temp gauge is in the normal range as well.