[Saloon-lovers] 3.8S heater vacuum control

For years my cowl vent would slowly shut down about 3
minutes after engine shut down but all works fine with the
engine running. When the car was new the system would hold
vacuum for a number of hours. I had some time today to
figure out why. Using a ‘‘Mityvac’’ I proved the heater valve
and scuttle vent capsule are holding vacuum. The problem
is the three button control unit in the center console is
leaking by. Is this a common fault in these 40 plus year
old cars and is there anyone out there in the business of
refurbishing and making the control units leak free? Or am
I just being too nit picking and should forget about it?–
John Quilter 1965 3.8S, 1951 MGTD, 1960 Morris Minor,
Brisbane, California, United States
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In reply to a message from John Quilter sent Tue 26 May 2009:

John,

there are three vacuum hoses connected to the three-button Control
Unit in the 3.8 S. One goes to the Heater valve, one to the
Scuttle vent valve, and one to the Vacuum tank located under the
left (driver’s side in LHD cars) wing.

If you have enough length in those hoses at the point where they
connect to the Control Unit under the ashtray, cut about 1’’ inch
from the end of each hose, re-connect them so that they are very
tight, and try measuring again. (the ends of the hose tend to
become stretched and lose in the tubes with age).

I would also measure the vacuum Tank and the plastic check valve
mounted to the tank, where the leakage may also happen.

The hoses from the vacuum tank to the Brake Booster are also
leakers if old and frayed. From The Brake booster to the rear of
the Cylinder Head (intake side) there is a steel and rubber hose
that is also a leaker.

If at the end of the investigation, the Control Unit is still the
problem, it may just be a crack in the welds, and you could fix
that with plumbing solder and a small torch. If you want to get
real picky, you can open and dissect the control unit to look for
any cracks in the welds, it is not a sealed unit.

Otherwise I don’t know of anybody in the business of refurbishing
the control units. It is rare for those to need repair.

Zurdo–
The original message included these comments:

figure out why. Using a ‘‘Mityvac’’ I proved the heater valve
and scuttle vent capsule are holding vacuum. The problem
is the three button control unit in the center console is
leaking by. Is this a common fault in these 40 plus year
old cars and is there anyone out there in the business of
refurbishing and making the control units leak free? Or am
I just being too nit picking and should forget about it?
John Quilter 1965 3.8S, 1951 MGTD, 1960 Morris Minor,


1965 3.8 ‘S’ 1984 XJ-6
Florida, United States
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In reply to a message from zurdo sent Wed 27 May 2009:

In my test of the control unit I blanked off the pipes to
the heater valve and scuttle vent at the control unit and
applied vacuum to the supply pipe (center pipe) and the
control unit would not hold vacuum so I am almost 100% sure
the problem is internal to the control unit. I’m a little
reluctant to pry open the unit so see what makes it work as
I may make the problem worse. Has any one had one of these
apart and could explain how they work and how they leak
internally?–
The original message included these comments:

If at the end of the investigation, the Control Unit is still the
problem, it may just be a crack in the welds, and you could fix
real picky, you can open and dissect the control unit to look for
any cracks in the welds, it is not a sealed unit.


John Quilter 1965 3.8S, 1951 MGTD, 1960 Morris Minor,
Brisbane, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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