Seems to be that as the weather gets colder I get more and more requests for heater diagnostics, so here it is.
The heat in all automobiles today is supplied by the hot coolant circulating through the engine. A small amount of that hot coolant is diverted into a small hose that goes into the firewall and then into a small heat exchanger that looks like a miniature radiator. Cold air from the outside, or recirculated air from the inside, is directed either completely or partially through that "heater core", as the industry calls it.
In some vehicles the hot coolant flow is modulated by a valve which is controlled by the user via a sliding lever on the heater control panel labeled "hot" on one end and "cold" on the other. This method is not used by all automobile manufacturers. Others modify the percentage of incoming air that flows through the heater core, bypassing the core to reduce the temperature. This is done by a moveable baffle that is controlled by the "hot - cold" lever.
So, it's cold and you fire it up and slide the control to "hot" and cold air blows out. Now what??
First thing to do is to make sure that the coolant temperature is up to operating temperature, about 190 degrees F. That is controlled by the engine thermostat. A quick check by feeling the radiator hoses would tell you if it is hot. Using a thermometer taped to the upper hose of the radiator will tell you exactly where you are. If it isn't hot enough, change the thermostat. While you are in the vicinity of the radiator, make sure the coolant level is correct. A low coolant level will reduce the flow to the heater core.
Next, find the two hoses that go into the firewall. Feel them. Are they hot? If not, check for a control device in one of the lines. It may have a vacuum hose attached or a pull cable hooked onto a lever. Work the control inside the car and see if there is any response. If there is a vacuum line on it, pull it off and see if there is vacuum there. If not find the vacuum source and fix it. The problem is normally just a hose pulled off a small fitting. When in doubt, remove the device and replace it with a piece of pipe. Then see if the hoses get hot. If they do, replace or repair the control.
Another reason for the hoses to not be hot is that the heater core is plugged. Remove both hoses and flush the core with a garden hose. Careful because the garden hose can supply up to 100 PSI which can rupture a heater core. Just supply sufficient water to flush out any junk in the core. If the core is plugged too solidly, replace it.
Next, assuming the hoses are hot enough check to see if there is an inside control of the air flow across the core. This is not always easy, however you can get under the dash and move the "hot - cold" control lever and see what moves. A lot of times it is simply a broken or disconnected control cable. Also check to see if there are vacuum hoses on the heater control. If there are, then check under the hood for a ball the size of a softball. It is the vacuum accumulator, a small tank which is used to supply constant vacuum to the heater controls. Look for disconnected or cracked hoses in the vicinity of the accumulator and repair as needed.
One final note, if you have heat most of the time but it disappears when you are going up a long hill you probably have a vacuum controlled system. Going up hill normally reduces engine vacuum. If there is a vacuum leak in the engine, or the engine is old and tired, then the vacuum will be reduced to the point where the controls will go back to the default condition of "off". Check engine vacuum to see if that is your problem and repair where necessary.
If there is no air flowing when you turn on the blower then you have the task of checking the electrical system of the blower. Check for a fuse first. Then using a voltmeter, follow the wires from the fuse block to the heater switch, and then on to the blower motor. If there is voltage all the way to the motor then check the ground wire from the motor. If it's good then the blower motor is bad.
Sometimes your blower motor will only work on high speed. In that case here is a paragraph or two about what may be wrong.
Heater blower motors come in two or three varieties. Older models had several windings inside for the various fan speeds. It was expensive and is no longer used in most modern cars. Newer blowers have only one winding and therefore only one wire going into the motor (plus, of course, a ground wire). The speed is controlled via a series of resistors which are switched in as needed to lower the fan speed. These resistors are wire coils and get very hot. Therefore, they are normally mounted inside the heater plenum chamber where air from the blower flows over them to keep them from burning up. Sometimes the burn up anyway.
The blower switch in 80% of today's cars have different positions which control the flow of current to different resistors thus changing the speed of the blower. Therefore you have two places to look - a faulty switch or a faulty resistor bank. Since the blower runs on high it should not be a problem.
If you have a system similar to the one in my Buick Park Ave. then you are in for big money since the blower speed is controlled by a solid state device which pulses the current on and off in a square wave sequence varying the duty cycle to vary the speed. Big bucks and expensive diagnostic equipment there (oscilloscope).
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