MACS Monthly Newsletter - 2017 - APR3

AND NOW FOR AN EVAPORATOR LEAK WE'D LIKE TO TALK ABOUT
The evaporator is an object of high suspicion when there's
a small but still needs-attention refrigeration system leak, and
the shop can't find it at an O-ring joint or compressor (shaft seal
or head cover seal). Of course, we've covered many causes of
hard-to-find leaks of this type, which often turn out to be rubthroughs on refrigerant lines buried out of sight. And on systems with rear evaporators, the underbody refrigerant lines frequently hold the cause.
With today's sensitive-but-not-too-sensitive electronic leak
detectors, going through the A/C registers for evaporator access
is a common procedure, and although it seems to work on some
cars, it's hardly universal on all. At one time a leading maker of
HVAC cases was considering marking each case with a place to
make an access hole, using a hole saw, that would permit use of
borescope and leak detector probes. Many technicians remove
the evaporator temperature sensor to gain access to the evaporator area, and just probe the evaporator core face with an electronic leak detector.
Nothing wrong with that, of course, provided it's an SAE certified J2791 detector. But combining with borescope inspection
is even better, when it confirms the leak is actually from a localized area on the evaporator core face. We say this because if the
probe tip lands on case joints or the gasket surfaces, the detector
may be responding to emissions from the elastomer used. That
used to be more of an occurrence than we hear about today, but
the borescope confirmation can't hurt.
On vehicles with rear evaporators, we just heard of another
leak source, at the rear evaporator. But although it was anything
but conventional, it actually was a rear evaporator version of a
leak we had encountered several years ago on the front evaporator of a Volvo.
In this case, the leak affects 2013-on General Motors full-size
crossover SUVs, specifically the Chevy Traverse, Buick Enclave
and GMC Acadia. The symptom is a low refrigerant charge that
the technician traces to the rear HVAC system. Although these
rear systems don't exactly fly out of the rear quarter panel, they
are a lot easier to service, and some partial disassembly should
provide good access. In the case of these GM SUVs, the access

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to the evaporator is by removing the rear blower control module
(accessible through the rear console) and inserting the probe of
an SAE J2791 leak detector.
What could cause the rear system evaporator core to develop
a leak? Well, it's surely corrosion, but why and how at the rear
system. Turns out that the rear blower motor has brushes (there
are brushless motors, but not this one). As the brush wears in
against the commutator, it picks up copper particles from it, and
those copper particles blow off the brushes and onto the evaporator-aluminum and copper react to cause corrosion and a
leak results.
Interestingly, there's nothing functionally wrong with the
blower motor after it wears in, so don't replace it. But that leaking rear evaporator has to go. And of course, remove the blower
motor (FIG.3 ) and with a shop air gun, thoroughly clean carbon
dust and copper particles from the blower motor assembly and
the blower case, then reinstall. ■

Figure 3: Blower motor for rear system on GM full-size SUVs sheds copper particles as it wears in. Particles are picked up by the motor brushes
and blown onto the aluminum evaporator, causing corrosion and leakage.
Evaporator must be replaced, but motor and brushes may be cleaned with
shop air and kept in service

WHAT? ANOTHER GENERAL MOTORS ANTI-FREEZE
Every engineering group of every car company wants
exclusivity, and that often extends down to the smallest things. Then the corporate purchasing teams for the
parts departments look them over and decide there are
too many parts in the system that seem to be duplicative,
and they start eliminating them.
That's why the car companies have multiple antifreezes at the start of new model families, and some time
thereafter they're down to a single antifreeze. GM has
been using "DexCool," a mixture of two organic acid
inhibitors (2-ethylhexanoic acid and sebacic acid, plus a
copper-brass inhibitor called TT (tolythriazole). This is

April 2017

formulated with an orange dye.
GM now has a new line of Chevrolet low cab forward
medium and heavy-duty trucks and one of the engines is
a 5.2-liter diesel four-cylinder. But it's supplied by Isuzu
and that pretty much explains why there's specific anti-freeze. Isuzu has certified its engines with a formula
that it tested and supplies, and describes as a "long life"
formula. So GM has to use that anti-freeze, for openers
anyway, until it can validate an alternative.
That anti-freeze is green as supplied, and GM warns
not to use conventional American green, but a yellow
dye antifreeze that is available from dealers (FIG. 4.

MACS Service Reports

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