When Vacuum Motors Fail
The real workhorses of our Mustangs' factory climate-control systems arethe four vacuum motors that move the air doors and switches throughoutthe system. They have a rubber diaphragm tied to the control rod thatmoves the door and/or switch. When the diaphragm develops a leak, thevacuum motor doesn't work as well. When a tear becomes significantenough, they don't work at all. This may also adversely affect engineperformance because it creates an intake-manifold vacuum leak.
Genuine Motorcraft vacuum motors are available from Mustangs Etc. forfactory Ford climate-control systems. You can replace the affectedvacuum motor. Or, in the interest of reliable system performance,replace all four motors.
More Heat In Here
Another workhorse is the hot-water control valve. This component usuallyfails because of a stuck valve piston or a torn diaphragm. This means noheat and cold feet. The aftermarket offers a generic- looking heatercontrol valve that replaces the factory component. If you want a genuinefactory-original appearance, Classic Auto Air has the original-equipmentheater-control valve for '67-'73 Mustangs and Cougars.
Low or No Vacuum
Likely the greatest source of climate-control-system malfunction is themassive bundle of vacuum hoses, which is like an interstate highwaysystem behind the dashboard to route vacuum to all of the servo-motorsand the heater control valve. Time and ozone take a toll on rubbervacuum hoses, as do sloppy mechanics. Hoses dry-rot and split over timeand are frequently reinstalled incorrectly. Classic Auto Air can helpwith the complete vacuum-hose system for '67-'73 Mustangs and Cougars.It's color-coded for easy installation.
Vacuum Actuated
This is the '67-'73 basic vacuum schematic. Although it may not beexactly like your '69-'73 Mustang climate-control system, the basics arethe same. The heating and air-conditioning control gets its vacuumsignal from a main hose from the vacuum reservoir in the enginecompartment. The vacuum reservoir gets its vacuum from the engine'sintake manifold. A check valve between the intake manifold and reservoirenables the vacuum reservoir to maintain a constant vacuum supply at alltimes. Shut the engine off and the check valve (which allows airflowonly in one direction) seats, keeping a vacuum supply in the reservoir.It also prevents engine backfiring in the intake manifold from damagingthe vacuum system from a sudden overpressure.
Vacuum does all of the work in your Mustang's heating andair-conditioning system except the temperature control, which is amanually operated cable that opens the hot-water-valve vacuum switch andheater-core air door. If you move this control off COOL, vacuum goes tothe hot-water valve underhood. Hot coolant flows through the heatercore, which is a small radiator inside the climate-control system. Movethe TEMP control further and you allow hot air from the heater core towarm the cabin. Move the TEMP control back toward COOL and you limit theflow of hot air.
When you move the climate control to MAX air conditioning, a vacuummotor moves the A/C-DEFROST door to all of the dashboard outlets. At thesame time, the vacuum motor depresses the air-conditioning-compressorswitch. This sends electricity to the thermostatic switch, which sensesclimate-control-box temperature. If it is warm, power from thecompressor switch flows through the thermostatic switch to thecompressor clutch. When the box temperature reaches a given point, thethermostatic switch opens and disconnects the compressor clutch. As thebox warms, the thermostatic switch closes, engaging the compressoragain.
When you slide the selector to FRESH or VENT, the same sequence ofevents happens except for one thing. The outside/recirculation air dooropens to outside air. In MAX mode, cabin air is recirculated. In FRESHor VENT mode, outside air is used.
In DEF mode, the climate control system does all of the same things itdoes in FRESH/VENT mode, including A/C-compressor-clutch engagement.However, the A/C-DEFROST door moves to direct air to the defrosteroutlets. The air-conditioning-compressor clutch is engaged to dry thecabin air for improved defrosting. Sliding the TEMP control toward WARMputs heat and drier air on the windshield.
Sliding the control to HEAT takes the A/C-compressor clutch out of theequation and we get hot air only on our feet. Sliding the TEMP controlto COOL gets cool air to our feet because we stop the flow of hotcoolant to the heater core. We also close off the flow of warm air fromthe heater core.
Still No Heat?
This is the water-valve vacuum...
This is the water-valve vacuum switch, which is behind the glovecompartment on the climate-control-system box.
Here are two things to look for when you've checked everything else. Thewater-valve vacuum switch can stick closed, which keeps the heater valvefrom opening. Check the temperature cable adjustment and ascertainproper operation of the water-valve vacuum switch. Also make sure theheater core has proper coolant circulation. A clogged heater core willgive you chills more quickly than anything else.
Finally, what's your cooling-system thermostat's temperaturespecification? If you have a 160-degree thermostat, forget having a warmcabin. A thermostat stuck in the open position will have the sameeffect: a cool cabin. That large radiator you installed to cope withhot-weather driving can be your undoing in the wintertime because itwill keep engine and cabin temperatures cooler.
Don't Forget The Electrics
Electricity from the blower...
Electricity from the blower motor to ground is routed through thisvariable resistor in the fan case. When the fan switch is set on LOW,resistance is high to ground. When the fan switch is moved to MEDIUM,resistance to ground becomes less, allowing more current to flow toground. And when the switch is moved to HIGH, there is virtually noresistance, which allows a high flow of electricity to ground, makingthe blower run at full speed. The variable resistor is located there tohelp dissipate heat from the resistance to electricity flow.
Proper vacuum-system function is certainly paramount to your comfort.However, don't forget the electrical end of your Mustang'sclimate-control system. A faulty A/C-compressor switch or thermostaticswitch can stop your air conditioning and defrost functions cold. Propercompressor function is dependant on continuity through the two switcheswe just mentioned. The most common switch failure is the thermostaticswitch that senses evaporator temperature inside the heating andair-conditioning box. The thermostatic switch consists of a diaphragm, aspring, contacts, and a wet bulb (also called a thermocouple). Study thethermostatic switch and you'll see a long capillary tube filled withrefrigerant. This is the thermocouple or wet bulb.
The tube is inserted into the air conditioning's evaporator where thesystem is at its coldest. The refrigerant inside, a gas, is affected bytemperature. When things get warm, it expands. When they cool, itcontracts. As the gas inside the sealed tube expands, it presses againstthe diaphragm, which closes the switch contacts to complete the circuit.Preset spring pressure works against the diaphragm. Because thethermostatic switch is adjustable, we can control how cool the cabinbecomes.
Thermostatic switches fail for three reasons: burned contacts, diaphragmleakage, or capillary tube leakage. When the refrigerant gas inside thecapillary tube or diaphragm leaks out, the thermostatic switch stopsworking. The contacts don't touch and power never gets to the compressorclutch. Because this is a high-amp switch, meaning it has to carry a lotof current, contacts burn and fail.
 This is the air conditioning's...  This is the air conditioning's thermostatic switch. Note the switchbody, where the diaphragm, spring, and contacts are. The capillary tubeis inserted into the evaporator to "sense" box temperature. |  Here's the A/C compressor...  Here's the A/C compressor switch, which closes when the vacuum motoropens the air door. This switch has to close, along with thethermostatic switch, in order for the compressor clutch to engage. | |