MACS Service Reports - 2016 - APR8

VOLKSWAGEN MAPPED COOLING SYSTEM

Volkswagen

Newer Volkswagen and Audi engines have some of the
industry's most complicated cooling systems. All modern cooling systems have two circuits; a main circuit that
includes the water pump and engine castings, an additional circuit that includes the radiator; and a thermostat
that allows coolant to flow through both circuits when it
reaches a pre-set operating temperature. However, VW/
Audi cooling systems have an electrically-operated thermostat that works like a throttle: it can be partially closed
even when the engine is fully warmed up. The Powertrain
Control Module (PCM) operates the thermostat to adjust
coolant temperature according to engine load. Volkswagen calls this "electronically mapped engine cooling," and
they use it to increase coolant temperature during part
load to reduce emissions.
Volkswagen's electric thermostat (F265) has an expanding-wax pellet just like other thermostats, but this one is
electrically heated by pulse-width-modulated voltage. In
some models, the thermostat has two valve discs to control
flow through two different coolant circuits. Some models
with this thermostat also have an auxiliary radiator with
its own electric pump, maybe a check valve or two, and
sometimes a separate circuit with an electric pump for the
car's heater and/or transmission cooler. The V6 TDI diesel
engine actually has separate coolant circuits for the block
and cylinder heads so they can operate at different temperatures.
The thermostat is mounted in the Coolant Distributor
Housing (Figure 14). It bolts directly to the cylinder head
and has upper level (outlet) and lower (inlet) level passages, a dual-circuit coolant temperature sensor (G62) and
up to 7 hose connections. There's a second temperature
sensor (G83) in the radiator, and the PCM (J631) uses both
signals to operate the radiator fan.
Many models have an auxiliary electric coolant pump
(V51) that runs after the engine is shut down. First introduced on the 1985 Audi 5000T, the after-run pump was
originally used to cool the turbocharger housing after the
engine is turned off to prevent oil coking in the bearing.
The after-run pump is still used today, but it may also be

Figure 14: Volkswagen's coolant distributor housing (left).

used to increase coolant flow to the heater core under certain operating conditions, and there may also be a valve
(N279) to control coolant flow through this circuit.
Because the PCM can vary the coolant temperature by
as much as 77˚F (25˚C), manual HVAC systems have an
electronic temperature control knob (G267), and the control unit operates a vacuum-actuated flow control valve
(N147). The same coolant circuit also heats the throttle
body.
The default or failure mode for the moving parts in the
system will provide minimum coolant temperature. While
this will prevent the engine from overheating, it will also
increase emissions, so cooling system failures will illuminate the Malfunction Indicator Lamp (MIL). In addition to
the generic fault code (P0128) associated with cooling system performance, you may also see enhanced codes such
as P2181, P1292 or P1296, or Volkswagen codes 18613 or
008577 that indicate failure of specific components.
As fuel mileage and emissions regulations tighten, other
manufacturers are also developing their own cooling systems that allow coolant temperature to be adjusted rather
than just limited. Troubleshooting these systems may be
easier for techs who know how to use an oscilloscope. ■

MACS Service Reports is published monthly by the Mobile Air Conditioning Society Worldwide. It is distributed to members of MACS Worldwide
and is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will be
considered for publication, however, MACS Worldwide reserves the right
to choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Casey
Manager of Service Training:
Steve Schaeber
Mobile Air Conditioning Society Worldwide
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsw.org * Website: www.macsw.org

Unless otherwise noted, all photos/art by author.

April 2016

MACS Service Reports

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