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The Gentleman's Express: Tech-Tips from the JIOC

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Heating and Air Conditioning Vacuum System

Author: Walter Okon

The heating and air conditioning system in the Jensen is controlled by engine intake manifold vacuum (12 in as measured in my 1974 Interceptor Mk III). The vacuum line goes to the control valve but the fan will not operate until the blower switch is on positions 1, 2 or 3. It also controls the fresh air valve position.

Vacuum ports K and L (See figure 7, over) are on the blower switch; A through H, and J are on the temperature control. Ports B and C are part of the device that allows a variable vacuum to be passed to port D.

The outputs of the temperature control go to the vacuum actuated heater water valve and three vacuum motors, Jensen P/N CT4045, (metal cylinders about 2" dia and 3" long containing a rubber diaphragm) which control the demister flap (air to the defrost outlets vs air to footwell outlets, fresh air flap (intake air from scuttle grille vs recirculating inside air), and a/c thermostat (connected to a switch which along with the capillary which goes to the evaporator controls the compressor on/off cycling. TABLE 1, p88, shows when 60 vacuum is applied to the motors. If your temperature control does not operate properly, try taking it apart carefully (three bolts) and cleaning both the rubber plate and the plate with the pin holes in it. Use a light oil to lube the rubber plate

The control is accessible through the radio opening or the right hand carpet covered tunnel panel. Use an awl to push in on the button to remove the temperature control knob.

The problem with my system was that the demister and fresh air valves did not operate. The demister vacuum motor is accessible through the right tunnel panel. My motor had a hole in the diaphragm. A replacement was impossible to find (I understand they are $120 if findable) so I, at Norm Fuller’s suggestion, adapted a Ford part, #D3TZ-18A318- B ($14 - $20 depending on dealer) and it works well. But it needs modification (See Figure 8). The unit is used on several Ford models of the ’74-80 period. I understand that Ford #COAF-18A318-B could also be used but is more expensive ($30) and would require different modifications.

The D3TZ could also be used for the a/c thermostat and probably the flap valve if one could reach it. It is tucked away under the dash and requires about 14 hours of shop time to get the dash in and out. As noted elsewhere, a suitable replacement heater valve is the EVERCO H1989, available from most autopart stores.

E121212,0000Not used (plugged)
F000**1212Heater water Valve
G12121212120Demister flap
H01212121212Fresh air flap
J0**12,0000A/C thermostat
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