The car is in very good shape and has had the coolant changed on
regular intervals with Dexcool so I am not interested in dumping known
good antifreeze.  When I said bursting out I was trying to stimulate
the picture of  the fact that the heat suddenly not slowly comes back
on.  There is nothing wrong with the air flow or flaps.  It is obvious
to me that the core is suddenly cooling off  and heating back up and it
is connected to the speed of the car or the engine.  It is hard to do
diagnostics at 60 miles an hour LOL.  The heat goes away at 45 and
returns at  30 it is predictable as the transmission shifting.  I was
wondering if there is some way the mono valve can screw up with higher
water pressure and start flowing with less.  I doubt  that the core
could intermittently block off because of the way it is built it would
take many obstructions to completely stop the flow.  When there is heat
it cooks you the heat comes out real good.
Thanks, Bruce

Good thought! I like this theory.

Group,

Thanks to all of you for the thoughts and info. That was returned to my
recent posting.  I am embarrassed to report that I gave misleading info
in my posting about recent heating trouble with my 300sd.  I must
explain that I only work on my heat when it is cold, I am senile, and
it has been a year since I last had worked on this heating problem.
Last week I posted that I had tried removing the wires to my monovalve
and that I was sure that my problem  was not electrically related.  Now
today I am quite positive that it is electrically related.

I just returned from a trip to North Kansas and it is 30 degrees cold
there.  I live in South Texas and it is hot here.  So I only have
heating trouble when I take my annual northern trip.  The last three
years I have fought this damn heater on this trip.  The heater works
fine and will burn you out of the cabin if you don't  turn it down.
Until you get over about 45 mph then all heat is gone from the air (the
core is cold).  The climate control module has been changed and I
thought the problem was fixed.

Yesterday as I was coming home and still in the cold north my heat quit
as usual  so I started to play with things and when I pulled the wires
on the monovalve I got heat galore.  Had to use the sun roof  to
regulate the heat.  I stopped at a Radio Shack, got wire and a cheap
volt meter, attached the wire to the reconnected  monovalve ran it into
the cabin and hooked it to the volt meter.  As I drove the car in 40
degree weather, at 30 mph, with the control set at max the meter showed
no voltage and we had loads of heat.  As I approached 45 or so the
voltage goes to 13 and the heat stops.  Yes and when I slow down to
about 30 I get no volts again  and lots of heat.  Well with this
revelation I settled down to finishing my 800 mile drive home and had
bountiful loads of heat with the monovale disconnected.  Today I am at
home and can now focus on the car.  I hooked my voltage meter back up
so I could play around with this problem some more, took the care for a
drive and much to my frustration the darn car blows non_ stop heat hear
in South Texas.  When I move the wheel up and down the voltage comes
and goes and the valve pulses like it should.  The wheel turned to max
and the voltage stays away full time with full heat.  I am dumbfounded.
 

Bruce Buchanan
81 300sd 165K