MACS Service Reports - 2015 - FEB3

ANOTHER EXERCISE IN LOGIC
wiper mounting arm and pivot shaft and you have
access to the vacuum reservoir. Not the toughest job,
but although he can see nothing obvious (typically a
cracked hose neck), he replaces the reservoir anyway.
How often do we hear that? See Figure 4.
It's not a big surprise when a new reservoir does
no good, and then the technician faces the usual dilemma: that new reservoir obviously is staying there,
but what now? He "smokes" the intake and HVAC systems, an increasingly popular step, particularly with
the new smoke machines that also distribute trace dye
in the oily smoke. But once more, nothing shows up.
There's a check valve in this system (very common) so
he replaces that too, and here again, it's not an answer.
Where does he go from here? Fortunately, the need
for a demonstrably good answer took over, and he did
what he should have done at the start: he disconnected
the vacuum hose from the control head and connected the hose to a vacuum gauge. He then proceeded
to drive the car under the conditions that caused the
default to defrost, and the vacuum hardly dips. That
tells him the vacuum supply is not the problem, and
that any loss is through the control head. He installs
a new one and the problem is solved, as it could have
been without digging through the plenum to get to the
unneeded reservoir. ■

Figure 3: Crown Victoria/Mercury Marquis manual A/C is a vacuum operated system: 1) floor/defrost actuator; 2) defrost airflow; 3) floor/defrost door;
4) heater core; 5) evaporator; 6) blower; 7) outside air inlet; 8) air inlet door; 9)
air inlet actuator; 10) recirc air inlet; 11) temperature blend air door; 12) panel
door; 13) panel airflow; 14) panel door actuator; 15) floor airflow; 16) intake
manifold vacuum source; 17) vacuum check valve; 18) vacuum reservoir.

Figure 4: Replacing the vacuum reservoir is not the toughest job on a 2006
Mercury Marquis or its Ford Crown Victoria twin, but although it's often a
leaker, there are also times when the HVAC defaults to defrost when the vacuum leakage is within the control head. It's easy enough to check for a reservoir
leak with a vacuum gauge, so try that first.

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The car is a 2006 Mercury Grand Marquis with the
4.6-litre V8, a vehicle and powertrain that "has been
around" for quite a while, so shops are very familiar
(maybe too familiar) with it. At a time when other premium vehicles were converted from vacuum to electric mode control, this model (and the sister car, Ford
Crown Victoria) still have vacuum-controlled manual
systems as the base (with electronic ATC available).
See Figure 3.
In steady cruising, the A/C blows cold air from the
upper registers, but when the car is accelerating up a
hill, the airflow (remaining cold) switches to the defrost outlets. Nothing hard about the basic diagnosis:
loss of vacuum. So the technician takes the first logical step: checks vacuum and it's a pretty steady 18-19
inches at idle (perfectly normal).
"Everyone knows" that the Ford vacuum reservoirs
eventually spring a leak, so the technician just digs
into the cowl to get to it: remove the cowl grille (lots
of clips) disconnect a washer hose, then remove the

February 2015

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