November 1, 2004

When Vacuum Motors Fail

The real workhorses of our Mustangs' factory climate-control systems are the four vacuum motors that move the air doors and switches throughout the system. They have a rubber diaphragm tied to the control rod that moves the door and/or switch. When the diaphragm develops a leak, the vacuum motor doesn't work as well. When a tear becomes significant enough, they don't work at all. This may also adversely affect engine performance because it creates an intake-manifold vacuum leak.

Genuine Motorcraft vacuum motors are available from Mustangs Etc. for factory Ford climate-control systems. You can replace the affected vacuum 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 usually fails because of a stuck valve piston or a torn diaphragm. This means no heat and cold feet. The aftermarket offers a generic- looking heater control valve that replaces the factory component. If you want a genuine factory-original appearance, Classic Auto Air has the original-equipment heater-control valve for '67-'73 Mustangs and Cougars.

Low or No Vacuum

Likely the greatest source of climate-control-system malfunction is the massive bundle of vacuum hoses, which is like an interstate highway system behind the dashboard to route vacuum to all of the servo-motors and the heater control valve. Time and ozone take a toll on rubber vacuum hoses, as do sloppy mechanics. Hoses dry-rot and split over time and are frequently reinstalled incorrectly. Classic Auto Air can help with 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 be exactly like your '69-'73 Mustang climate-control system, the basics are the same. The heating and air-conditioning control gets its vacuum signal from a main hose from the vacuum reservoir in the engine compartment. The vacuum reservoir gets its vacuum from the engine's intake manifold. A check valve between the intake manifold and reservoir enables the vacuum reservoir to maintain a constant vacuum supply at all times. Shut the engine off and the check valve (which allows airflow only in one direction) seats, keeping a vacuum supply in the reservoir. It also prevents engine backfiring in the intake manifold from damaging the vacuum system from a sudden overpressure.

Vacuum does all of the work in your Mustang's heating and air-conditioning system except the temperature control, which is a manually operated cable that opens the hot-water-valve vacuum switch and heater-core air door. If you move this control off COOL, vacuum goes to the hot-water valve underhood. Hot coolant flows through the heater core, which is a small radiator inside the climate-control system. Move the TEMP control further and you allow hot air from the heater core to warm the cabin. Move the TEMP control back toward COOL and you limit the flow of hot air.

When you move the climate control to MAX air conditioning, a vacuum motor moves the A/C-DEFROST door to all of the dashboard outlets. At the same time, the vacuum motor depresses the air-conditioning-compressor switch. This sends electricity to the thermostatic switch, which senses climate-control-box temperature. If it is warm, power from the compressor switch flows through the thermostatic switch to the compressor clutch. When the box temperature reaches a given point, the thermostatic switch opens and disconnects the compressor clutch. As the box warms, the thermostatic switch closes, engaging the compressor again.

When you slide the selector to FRESH or VENT, the same sequence of events happens except for one thing. The outside/recirculation air door opens to outside air. In MAX mode, cabin air is recirculated. In FRESH or VENT mode, outside air is used.

In DEF mode, the climate control system does all of the same things it does in FRESH/VENT mode, including A/C-compressor-clutch engagement. However, the A/C-DEFROST door moves to direct air to the defroster outlets. The air-conditioning-compressor clutch is engaged to dry the cabin air for improved defrosting. Sliding the TEMP control toward WARM puts heat and drier air on the windshield.

Sliding the control to HEAT takes the A/C-compressor clutch out of the equation and we get hot air only on our feet. Sliding the TEMP control to COOL gets cool air to our feet because we stop the flow of hot coolant to the heater core. We also close off the flow of warm air from the heater core.

Still No Heat?

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. The water-valve vacuum switch can stick closed, which keeps the heater valve from opening. Check the temperature cable adjustment and ascertain proper operation of the water-valve vacuum switch. Also make sure the heater core has proper coolant circulation. A clogged heater core will give you chills more quickly than anything else.

Finally, what's your cooling-system thermostat's temperature specification? If you have a 160-degree thermostat, forget having a warm cabin. A thermostat stuck in the open position will have the same effect: a cool cabin. That large radiator you installed to cope with hot-weather driving can be your undoing in the wintertime because it will keep engine and cabin temperatures cooler.

Don't Forget The Electrics

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's climate-control system. A faulty A/C-compressor switch or thermostatic switch can stop your air conditioning and defrost functions cold. Proper compressor function is dependant on continuity through the two switches we just mentioned. The most common switch failure is the thermostatic switch that senses evaporator temperature inside the heating and air-conditioning box. The thermostatic switch consists of a diaphragm, a spring, contacts, and a wet bulb (also called a thermocouple). Study the thermostatic switch and you'll see a long capillary tube filled with refrigerant. This is the thermocouple or wet bulb.

The tube is inserted into the air conditioning's evaporator where the system is at its coldest. The refrigerant inside, a gas, is affected by temperature. When things get warm, it expands. When they cool, it contracts. As the gas inside the sealed tube expands, it presses against the diaphragm, which closes the switch contacts to complete the circuit. Preset spring pressure works against the diaphragm. Because the thermostatic switch is adjustable, we can control how cool the cabin becomes.

Thermostatic switches fail for three reasons: burned contacts, diaphragm leakage, or capillary tube leakage. When the refrigerant gas inside the capillary tube or diaphragm leaks out, the thermostatic switch stops working. The contacts don't touch and power never gets to the compressor clutch. Because this is a high-amp switch, meaning it has to carry a lot of current, contacts burn and fail.