Re: <E28> heater HOT or COLD, no warm, need advice

[John G.Burns <john@xxxxxxxxxxxxxxxxxxxx>]

> As soon as one turns the thermostat up off the peg, the heater comes
>on full blast.  A VOM on the heater control valve reveals a full 12V
>keeping the valve shut all the time, except when one dials up any
>temperature greater than full cold, at which time it opens up fully.

>This happens regardless of actual cabin temperature, as far as we can
>tell.  Is it possible that the cabin temp sensor has gone south?  It
>should be much easier to replace that than the rheostat assembly.
>Also, can somebody verify that the temp sensor is the gizmo stuck
>inside the underdash trim piece with the 3-wire connector and the
>vacuum hose?

There are two sensors. One above the driver's foot and one
in the heater matrix behind the centre console. The control
box is in the centre dash behind the control dial. It
compares the two temperatues and decides how to cycle the
control valve.

The control box has three modes of operation. Variable heat,
no heat and full heat. The latter are controlled vi small
switches and override variable control. This is why you have
on/off control at all, you are using the backup mode!

Either your heater sensors are fauly, badly connected or the
control module is kaput. I'd unplug the control module and
test the resistances of the sensors from the plug, this is
easier and also ensures the wiring is OK.

The interior temp sensor has a vacuum line to draw air
through it which is connected to the intake manifold! There
is a small sponge filter to stop dirt getting in, those
Germans think of everything son't they.

The factory ETM for these cars is an excellent document,
several places sell them on the web. Worth the money :-)

- --
Who needs a life when you've got Unix? :-)   
Email: john@domain.elided, John G.Burns B.Eng, Aberdeen, Bonny Scotland
Web  : http://www.unixnerd.demon.co.uk - The Ultimate BMW Homepage!

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