Peter Frederick wrote:
It has a variable speed fan, speed selected by switching a resistor set
just like a manual HVAC in a Benz. When the temp dial is set to the
minimum, it clicks -- this is an override position, at least on the
later systems, that switches off any reheat via the heater core and
servo or monovalve to provide maximum cooling with no temp control.
Fan speed will drop to minimum and very cold air will be delivered so
long as the temp dial is all the way down. Off the click stop you will
have re-heat and the temp will be controlled at around 60-65F.
Ahh... okay, the Type II (Chrysler) system doesn't have the specific
override, but it will still end up in max cooling as it will try to cool
the car to 65 (same end result, just takes a few more seconds). The
low fan speed just doesn't make sense to me.... I understand that the
low fan speed will result in the lowest temperature outlet air, but I
think that a high blower speed would cool the car off quicker. So lets
say that at a given engine RPM and amount of airflow over the condensor
there will be specific amount of BTUs that can be transferred out of the
air flowing over the evaporator.
At a low blower speed there is less hot air flowing into the system,
which results in less heat (BTUs) being put across the evaporator. When
the system removes the amount of BTUs it is capable of, since there
wasn't a significant amount of BTUs being put in the system, removing
all the BTUs results in very cold air temperatures. Here we run into
the problem of when the temp gets lower than 38 degrees or so, the evap
starts icing up. At this point the compressor will cycle. When the
compressor is off it is not removing BTUs from inside the car.
When the blower speed is on high there is a significant amount of hot
air flowing across the evaporator. This results in a significant amount
of BTUs entering the system. When the evaporator has removed all of the
BTUs it can, the air temperature will not experience as great of a
temperature drop as was experienced when the blower motor was on its
lowest setting; however, the same amount of BTUs were removed. The two
advantages I see about using the high blower setting are that the
compressor will not cycle as the evaporator is not going below 38 deg,
and that higher air volume will allow more air to flow through the car.
IE at a low air volume the back seat might not experience any cold air
flow from the vents, and hot air from the back seat will not flow as
quickly to the air intake (during re-circ mode).
This is the only way I know to look at it.... I am by no means a
mechanical engineer so I'm very likely to be missing something, but as I
see it now the low blower speed just doesn't make sense. Also, I've
always heard of people referring to blower on high for testing max
cooling capability... I believe even the MB manual mentions it, although
I can't find the specific place at the moment.
On the later systems, fan speed is infinitely variable via a transistor
switching system, and the fan speed can get VERY low if the low speed
fan button is pushed.
Type II system has 5 stages.... and the low is pretty low, but still
noticeable. Referring to Bob's original question, "whisper" is kinda
vague as the lowest stage could easily be descibed like that when
compared to defrost! I interpreted it as a the recirculation flap was
still letting fresh air in, and when the blower stopped he was just
feeling the air forcing its way in while he was driving.
Remember, this is an automatic temperature control system, not a
"temperature of delivered air" system. It will maintain cabin temp to
the selected temperature within mechanical limits.
Yes. The Type II system is kinda flakey though... especially when
compared to the Type III system, which is an excellent design.
Defrost and full heat (click stop at the highest tempt setting) do
exactly the same with with heat -- full heater core flow reguardless of
cabin temp.
Yep, and in that case full blower for sure.... the heater core
definitely has plenty of BTUs to play with :)
I hope I don't come across as arrogant or anything... I just want to
make sure of my reasoning behind this... I'm taking senior design at
school and have found some people who liked the idea of designing an
affordable replacement to the Type II ACC system, and as such want to
make sure I'm doing it right!!
Take care,
John
'79 300SD (two of them... each with dead servos)
