re: heater hot or cold

["Ray Kaupp" <rkaupp@xxxxxxxxxxxxxxxxxx>]

John, yeah, that's the cabin temp sensor, over in the left
kick panel near the hood release. And the heater valve is an
on/off deal, no partial opening happens (unless its seal is
torn, in which case you'd have some heat all the time.) The
valve is opened and closed by three parts: (1) The
temperature sensor in the cabin you found; (2) a temperature
sensor in the heater core (accesible from the left side of
the console, if you take some stuff off); and (3) the
control knob, which compares voltages from the two
temperature sensors and opens and closes the control valve
as needed.

The order to tackle this is in the most likely/easiest to
fix or get at/cheapest matrix, which I think looks like
this:

1. Check the valve, and replace the guts if needed (you did
this already)
2. make sure the temperature sensors are clear and
positioned where they belong. (My heater core sensor was
pulled out of the core somehow, so always showed a colder
temp that reality, which caused the control valve to be open
when it shouldn't, which caused the car to be hotter than
the dial on the knob.)
3. Pull the cabin temp sensor (its the easiest one to get
to) and check the resistence with an ohmeter, at different
temperatures (there's a chart in the Bently, if you need the
values, email me off-digest)
4. Ditto, but for the heater core temp sender.
5. If all the stuff above works and tests fine, its the
control knob, which should be confirmed by the air
conditioning being off in the same direction as the heating,
like hotter that the dial. The danger of jumping to this
conclusion without steps 1-4 is that the air conditioning
has so many ways to be screwed up that the odds are it is
something other than the control knob.

FYI, I fixed mine once with step 1 above, and again 50K
miles later step 2. Its now running a little hot to the
dial, so I'm visiting it again, maybe next weekend.

Good luck, and sorry if this was a little long or redundant
to the archives stuff.

Ray Kaupp
'85 535i 214K miles

Ray Kaupp
VP Marketing
Digital Impact
650-286-7313
www.digital-impact.com

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