HVAC tutorial, HVAC tutorial by Joe Grubbs Options

[webmaster]

You guys should probably tear into the system and check everything--
here are some things to remember...

The HVAC ECU uses input from:
-----------------------------
Heater core temp sensor
Outside air temp sensor
Cabin temp sensor
Evaporator temp sensor
Air flow pressure sensor
Selected temperature
Selected "vents"
Position of the "ECO" button

The HVAC controls:
------------------
Coolant valve position
Operation of the A/C compressor clutch


The 505's automatic climate control system is a very nice thing when
it works correctly. Since there are many aspects to its operation, it
is not surprising that many things are overlooked when a repair is
attempted.

The ECU has 2 modes regarding "vent" selection--If defrost,
floor/defrost, or floor are selected, the ECU puts the A/C logic
in "defog" mode--where it uses the A/C compressor primarily to lower
humidity instead of temperature (like it does when in face or
face/floor mode).

When "ECO" is depressed, the ECU controls the environment using
everything except the A/C. The default mode is A/C on ("ECO" button
released) because the operation of the A/C is meant to be seamless--
integrated into the climate control regardless of season.

In the end, it is the ECU's goal to use the coolant and the A/C as
little as possible. It relies a lot on its perception of the outside
air temperature.

-----
Assuming your coolant level is full and your thermostat is operating
(engine reaches operating temp)...

The coolant valve is a spring loaded "normally closed" contraption.
It always recieves a +12V supply, the ECU controls the ground circuit.

If no current is applied to it, sufficient coolant pressure acts on
the diaphragm and coolant begins to flow through the heater core.

If the ECU grounds the valve, a solenoid more-or-less reinforces the
spring and coolant pressure will not be able to open the diaphragm--
no coolant flows.

You might also want to check the hose orientation. The valve's bottom
connection is the inlet, the upper connection is the outlet.

-----
To control the operation of the heater, the ECU probably relies most
on the selected temp, heater core temp sensor, the outside temp
sensor, and the cabin temp sensor. If any one of these is not mounted
correctly -or- their wiring is screwy, the heater will likely
malfunction.

The temperature selector is a collection of 28 resistors (331 ohms
each) and is best described as a "stepped attenuator".

The bottom most temp selection will effectively disable the ECU's
control over the coolant valve--the ECU will provide a steady ground
for the valve (no coolant flow).

The top most temp selection will effectively disable the ECU's
control over the A/C compressor clutch--under no circumstances will
the A/C clutch be engaged (similar to depressing the "ECO" button).

First I would check to make sure the temp selector is working
correctly. There are 2 main connectors on the HVAC "head" (the part
on the dash). We'll be checking pins 4, 5, and 6 on the 6-pin
connector.

- Resistance at pins 4 and 6 should be around ~9.6 kohms.
- Resistance at pins 4 and 5 should be "open" or atleast very high
when the temp slider is all the way down.
- The resistance should be very low--only a few ohms (or ~331 at the
most) when the slider is all the way up.
- Check the intermediate settings--you should see the resistance vary
in a nice stead manner (in ~331 ohm steps).

-----
All the temp sensors for the HVAC system should be ~10 kohms at ~75-
80 degrees F. And they should be mounted securely--with wiring in
tact.

The heater core sensor is clipped to the inside of the air
distribution housing near the heater core--it is referred to as a
fast-acting sensor. If the sensor falls in or is contacting the
heater core, it might explain a few things.

The outside air temp sensor can be seen if you remove the cowl and
look down at the air intake. If it is contacting something hot
(somehow) the ECU would likely reduce coolant flow through the heater
core and try to rely more on the outside air...

The cabin temp sensor's intake is located on the dash just above the
glove box. This little vent leads down to a small air horn/venturi. A
pressure drop is created and cabin air is drawn in. This is referred
to as a slow-acting sensor.

The evaporator sensor is mounted in the evaporator's fins. This
sensor's output would be used primarily during heavy A/C usage.

-----
Hypothetical situation--

You select a warm temperature, press the button with the big RED
arrow (floor vent), and turn the blower up. Why? Because you're
freakin cold!

The engine is at operating temp. The HVAC ECU sees the cabin temp
is "freakin cold" and so is the outside air temp. The ECU says "Looks
like I'll have to use the heat from the coolant to warm things up".
It opens the ground to the coolant valve, and coolant flows through
the heater core.

Your heater core temp sensor's output is unusually high because it's
making direct contact with the heater core or it's getting sprayed
with hot coolant... In response, the HVAC ECU says "WOW that heater
core sure is a whole lot hotter than it should be... I think I'll
reduce coolant flow to it to better regulate the temp in the cabin."

The air from the vents starts to cool off, but the ECU is not aware
of the air temp at the vents. To determine cabin temp, it uses the
cabin temp air sensor--which is a slow reacting sensor.

At first the cold air from the vents, while uncomfortable, is not
really affecting the actual cabin temp too much. Finally however, the
cabin temp begins to drop and the slow reacting cabin temp sensor
finally catches up.

The ECU says "Holy S@$#, it cooled off in there a whole lot! And the
heater core temp is a little low now--let me open up that coolant
valve again to warm things up"... Suddenly the air temp from the
vents shoots up again.

The cycle repeats itself--heat, no heat, heat, no heat...


-----
I'm not saying this is happening on your car exactly--I'm just trying
to exemplify how the system might react to such a fault.


-Joe G