MACS Monthly Newsletter - 2017 - FEB3

and a standard HVAC case section and rely on shaped
tubing from the core to the underhood heater hose connections. If you encounter an O-ring issue, the possibility
of a pressure buildup in the core, causing the O-ring to be
dislodged, should be considered.
The 1997 Buick Skylark leakage was primarily from underhood, also into the heater housing, but there seemed
to be no obvious issue. The hoses, heater core necks and
connections all looked good. However, once the hoses
were removed, the issue was obvious: the inlet neck had
fractured, and although a jury-rig repair might have been
possible, the shop replaced the core. FIG. 2. ■

Paul Weissler

techinfo.toyota.com

the leak is into the heater housing and it shows up nicely
as a drip-drip on the driver's feet. But even a case like
that doesn't necessarily mean that the leak is from the
heater core, because the forward movement of the car often forces the coolant flow back and that disguises the
leak source. A couple of examples:
The 2007 BMW 7-Series with the 4.8-l V8 had an external coolant leak from what appeared to be a hose under
the brake master cylinder. Was it the heater hose, the heater valve? Neither. It was the O-ring seal for the heater
hose tubing to the core. We're seeing this leak more and
more often as manufacturers use the same heater core

Figure 2: Skylark heater core leak was not from the core itself, but from
fractured plastic tank neck, as shown.

Figure 3: Toyota Tacoma diagram shows A/C control assembly, resistor,
blower motor and two of the damaged circuit grounds at lower left.

AS WE SAY, LOOK AT THE WIRING DIAGRAM
The 2009 Toyota Tacoma problem was low airflow and
the shop made an "instant" quote for a new cabin air filter. Sure enough, the filter was plugged and the shop
replaced it, but there was no improvement in airflow. It
is one of those cases where a little diagnosis would have
made for a happier customer, but at least the shop had
the cabin filter it had changed, to show to the customer
that the new filter was not a waste of money.
But it still meant the shop had to find out what was
wrong, so the technician started operating the A/C control and blower from one speed to the next. There was
no difference in airflow, so the technician made the clas-

February 2017

sic mistake: he replaced the control head and of course,
that had no effect. At this point, he was deep into parts
replacement, so a new resistor was next, followed by a
blower motor. Had he at least pulled out some jumper
wires, he'd have determined by hot-wiring that the blower motor was good. But no, and so he finally pulled out
the wiring diagram (FIG. 3), at which time he concluded
that a bad ground was worth considering. Checking
under the dash right side panel showed that a couple of
ground wires were damaged, and that simple repair restored blower operation. ■

MACS Service Reports

http://www.techinfo.toyota.com

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