MACS Service Reports - 2016 - MAY5

HVAC ODOR - ANOTHER WRINKLE ON THE PROBLEM
Every time we think we've covered every possible aspect
of HVAC odor, something new arises. We've gone through
microbe growth, field mouse droppings, proper condensate drainage, special filters, manufacturers' special coatings on the evaporator core, disinfectant treatments for the
evaporator - have we forgotten something? Oh yes, there
are contributions from some cabin plastic trims and fuel
and coolant fumes.
Well, here's another: the engine idle stop-restart system
that's coming in on more and more cars. And because it
provides a credit toward CAFE (Corporate Average Fuel
Economy), idle stop is something we'll be seeing more often.
The odor generally is described as a "sour smell," and
it seems to occur at varying times after the engine stops
on cars with a belt-driven compressor. That is, you don't
get it on a car with an electric-drive compressor, basically
the full-feature hybrid. However, most cars with idle stopstart have that belt drive, and the odor is gone by the time
the engine and compressor restart. Yes, there are control
algorithms that disable the idle stop under extreme temperature conditions, typically in the 85-90 degrees F. range
and above. With more cars also being equipped with humidity sensors, the relative humidity is likely to be included in the newest algorithms, but right now it's ambient
temperature. So what happens in that compressor-off period is a subject of intense research, with Japanese HVAC
supplier Calsonic-Kansei telling us what it found.
The researchers measured the time from the engine and
compressor stop to the appearance of the sour smell. No,
it was never the same - that would be too easy. Rather,
they started to look at the period when the engine was
just going into an idle stop and then coming out of it with
the engine and compressor restart. When the compressor
stops, the temperature of the air from the A/C registers is
stable for a very brief instant, then starts to increase. In the
very brief period before the temperature starts to rise, the

sour smell is generated.
Because the times to that temperature rise varied, the
next step for the researchers was to create a map of when
the odor was being generated. It was apparent that although the time varied, the conditions were the same:
the condensate on the evaporator core face would start to
evaporate as soon as the temperature on the core face just
begins to rise. At this super-short interval, the heat from
the evaporator and the evaporating condensate create a
near-stable temperature and this is when, with the condensate just completely evaporated, the smell occurs. It
is possible to predict the time to which the odor will occur
because the time to evaporate varies according to the following: (1) temperature of the air at the evaporator inlet.
At higher temperature, the time to evaporate drops, (2)
relative humidity. At higher humidity, it takes longer for
the condensate to evaporate, (3) outlet temperature of the
evaporator, and (4) the temperature differential with the
evaporator core inlet. The greater the differential between
inlet and outlet temperatures, the faster evaporation will
occur.
It would seem that simply raising the evaporator outlet
temperature could eliminate the odor under many conditions, but that would require a complex control and some
effect on system performance. So the researchers looked
next at evaporator core face coatings. They found that
even with present coatings, the odor is generated, and it
is from water-soluble airborne particles. This shows that
the research is in an early stage and there are no acceptable solutions. So unlike the odor that persists and can be
traced to fungal deposits on the evaporator, this idle stop
odor apparently will not respond on a durable, long-term
basis to disinfectant treatments you've been using. So our
best advice at this time is to save yourself the time and
the customer the money by just explaining this is a new
phenomenon that will be another addition to the category
of "normal." ■

LEST WE FORGET
It's not your fault that you can't remember every variation on every system. There simply are too many and if
you don't see a particular setup for years, you surely won't
remember how to troubleshoot it, and you might not even
recognize what it is. Here's one that has been used on
just four cars some years ago, including a rather contrasting selection in fact 2001-on Jeep Grand Cherokee with
the 4.7-liter V8 and 2003 Dodge Viper V-10 at FCA (Fiat
Chrysler Automobiles). And going back to the 1990's, add
in the 1993 Lexus SC400 (V8), 1993-94 Toyota Camry and
its luxury twin, the Lexus ES300.
The A/C cooling problem occurred on a 2003 Jeep Grand
Cherokee with the 4.7-liter V8. It had gone through four

May 2016

compressors installed by four different shops. And while
they were at it, they also replaced just about everything
else in the A/C system, including the hoses, on the belief
that there was debris causing a restriction somewhere, because the pressures were building up and the fan seemed
to be spinning right from the start.
However, the particular fan is a hydraulic system that
operates on hydraulic pressure tapped off a section of the
power steering pump. The fan runs very smoothly, and
it's easy to misjudge its operation and rpm. When you
start a cold engine, its normal idle speed is as low as 100
rpm. Yes, you can command higher speed, all the way up
to maximum, but you need a DRB III scan tool. The fan

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016