A major problem with the Bmw E34 and BMW E32 is the heating
How I think the Heating System works
Please be warned that I don't know
exactly how this system works but as there is no
other information available I would like to provide my best
opinion as it may help others troubleshoot the system. There
is some doubt in my mind as to whether Bentley's electrical
diagrams document the standard system , or possibly an optional
full climate control system. The following applies to the
standard system which automatically regulates heat only.
Refer to Bentley if you have one, pages 131-EWD to 133-EWD.
The E34 (non-climate control) heater system consists of
a) Solenoid valves and auxiliary water pump at the
left-rear of the engine bay.
Referenced respectively on EWD-131 at the upper-right, and
EWD-133 at the lower-right. See my "Brake Fluid"
procedure for a photo.
b) Heater cores (little radiators) with temperature
sensors (installed behind the dashboard) one each for the
left and right sides of the cabin. Referenced on EWD-131
at the lower-right.
c) The HVAC blower and speed control sub-system, referenced
on EWD-132 at the top.
d) Control Panel, see the bottom of EWD-131 and 132.
e) Two stepper motors controlling fresh air and recirculation
flaps, see the bottom of EWD-131 and 132. There may
be more if the Bentley diagram is in error.
f) Integrated Climate Regulation Control Module, referenced
on EWD-131 and 133.
The engine coolant flow is typically minimal during the warm-up
phase, flowing just enough to ensure the thermostat is always
wetted with freshly heated coolant. To provide rapid
cabin heating on startup, coolant for the heater cores is
drawn from the cylinder head as it is the first area to get
hot. As there is very little pressure drop between the
supply and return points at idle, an auxillary electric pump
provides additional pressure to ensure adequate coolant flow
through the cores.The two electric solenoid valves control
flow to the left and right cores. The valve are either
fully "on" or fully "off" and are therefore
operated in PWM (pulse width modulation) mode, where they
are switched on and off at some fixed frequency, during which
the "on" time is set between 0 and 100% of the total
cycle time to provide the desired average flow. (Fuel
injectors work in a similar way) They should click audibly
and continuously during operation. It has been brought
to my attention that the valves are open when de-energized
therefore the duty cycle actually would be the "off"
duration. Also when the heater controls are "off"
there should be a continuous 12V present at the coils. If
you disconnect the coil(s) you should get full coolant flow
through the cores.The "squirrel cage" HVAC system
fan has four speeds. The method of providing a voltage
(or power) drop to obtain the lower speeds is typically a
set of resistors placed in the air flow path for cooling.
Bentley shows this at the top of EWD-132 but I am unsure if
this is actually correct for the later E34's. Not having
verified this myself, I was under the impression that the
later cars used a more advanced and costly "sword"
or "final stage unit" consisting of various transistors
and resistors. Bentley show this design on page EWD-122
for the earlier E34's. I would appreciate an email from
someone who knows which system applies to the '94-95 530i.
Neither system is noted for reliability, however.
The left and right knobs on the panel appear to be calibrated
in degrees F (US spec cars.) These are actually potentiometers
(variable resistors,) not mechanical valve actuators as in
most cars with manual controls.Aside from the obvious fan
speed control and A/C buttons, there is a small grill opening.
A continous sample of cabin air is drawn in through this grill
past a temperature sensor.Note that the driver's side temperature
control has a detent at both ends of travel. The manual
explains that the detents override automatic temperature control
in case of system failure. This should be your first
clue as to their confidence in the design.
"Inner Loop" Temperature Regulation
The first electronic control loop to learn (speculate?) about
is the temperature control of the heater cores. As I
mentioned, each has a temperature sensor. Based on
some "demand" temperatures (to be defined later,
let's say for now "T-left" and "T-right")
the control computer will vary the duty cycle of the pulses
supplied to each solenoid valve with the goal of getting the
cores to reach those demand temperatures as fast as possible
and hold them there. With a warm engine I would guess
that they would reach temperature within one minute and hold
within +/- 2 degrees.This inner loop is important to filter
out fast moving disturbances which might affect coolant temperature
and pressure available to the cores. Examples are engine
operating speed and temperature. In control system parlance,
it is a "PID" loop (proportional-integal-derivative)
which is tuned to compensate for the dynamics expected in
the application.The inner loop provides a temperature-regulated
"hot plate" for the outer loop to work with as it
determines the best way to provide the temperature the driver
and passenger have requested on the controls.
"Outer Loop" Temperature Regulation
The outer loop has no knowledge of the inner loop.
As far as it knows it has a pair of radiators that provide
any two temperatures it wishes, instantly on demand.
The outer loop reads the heater controls and the cabin temperature
sensor and understands that its goal is to make them match
and stay matched. The outer loop also regulates much
slower than the inner loop.If, for example, you think of the
cores being at 72º F, what would the cabin temperature be?
We'll not likely 72º F, that's for sure! There are many
things that will prevent that from happening, for one outside
cold air entering the cabin via a cold-air vent or an open
window. Heat losses via conduction through the body,
the number of people in the car, to say nothing of the fact
that air flowing across the radiator will never actually reach
the radiator's peak temperature. (Heat flow can only
ocurr across a temperature difference)In order to do its job
the outer loop needs to predict the additional temperature
needed at the radiators to bring the cabin to the desired
temperature. Note that it no longer cares about left
and right. There is only one cabin temperature sensor
- once it understands the delta-T (difference) required, it
applies equally to both sides. Keep in mind
that the influences we talked about are also mostly non-biased
per side, therefore a single correction is sufficient.So,
with information from the outside and inside (cabin) temperature
sensors, the fan speed setting, and possibly some vent settings,
the outer loop determines the inner loop demand tempertures
T-left and T-right by applying a single correction
temperature to the left and right control knob settings.
What is the net result of all this? We'll first of all
the system doesn't "look" like the regulated heater
control that it is. In fact it looks just like the heater
controls on the 70's and 80's Mercedes that were just simple
mechanical coolant valve knobs, also one for each side.
Without the looks of climate control system, people expect
to have to play with the knobs continuously to be comfortable.The
only feature that prevents it from being a full "climate
control system" is that it doesn't operate the A/C.The
fact is that most people are so accustomed to regulating their
own heater controls that they would be unlikely to have the
patience to allow the automatic system to do its job.
Sadly, the lower reliability of such a complicated system doesn't
really justify the results.