MACS Monthly Newsletter - 2020 - DEC4

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electric heater, the motorist may not be satisfied, and there could
be the following reason for poor heat:
The Volt is sensitive to low coolant level, particularly if there's
any air in the system. So, if you have to deal with a poor heat
complaint, that's a first thing to check. But you know there's an
auxiliary electric pump (Figure 6), an obvious necessity for a vehicle where the engine is not running much of the time (more on
that subject later in this report).
So, you're probably going to check that first, to be sure
it's running. The pump is noisy? That could seem to be
a " replace the pump " indicator, but wait. More likely, it
indicates air in the system, and so your first step is to
check for a low coolant level and then bleed air out of
the system with a vacuum fill device and top up the system. The system has a coolant control valve (Figure 7),
so bleed with the valve in both positions. You shouldn't
have to drain the system. However, checking the system
for a leak is a logical step if the coolant level was found to
be low. n

Figure 7: Volt has a coolant flow control valve to circulate
(or block) coolant through heater core.

No Thermostat to Replace - The Active Cooling System

December 2020	

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Another reason you're going to see a drop in thermostat
replacements is that some new cooling systems don't have a
thermostat. They're called " active cooling " systems, and the
elimination of the thermostat doesn't mean the cooling system
is simpler as a result. In fact, active cooling systems are pretty
complex.
You may think we're just talking about cooling systems with
multiple flow circuits. And in fact, some of such systems have
coolant flow patterns that are pretty complex, despite the fact
they retain the conventional thermostat.
But the " active cooling " systems have significant differences
beyond elimination of the thermostat. And there's comparable
complexity in the coolant flow patterns with " active cooling. "
Called " Active Thermal Management " by General Motors,
the " ATM " system (which does not dispense money-sic) is
being installed on the 2.0-liter turbo four-cylinder, the 2.7-liter
turbo four (both gasoline engines), and the 3.0-liter in-line diesel six-cylinder, one of the engines branded " Duramax " . And
for the new model year, it's even going into the 1.3-liter threecylinder in the Buick Encore GX, a new crossover model.
At this time, the ATM system is used only on some engines.
And because many engines are from a single " family, " the key
parts of the GM system are all external, the water pump of
course, but all the plumbing as well.
The GM system features an engine computer-operated coolant control valve on one (left) side of the in-line engines (Figure
8). Actually, it's two valves in a single housing. One is a " main "
valve that distributes coolant to the radiator, a bypass circuit,
the transmission and engine heat exchangers. The second is
referred to as the block control valve, and as the name implies,
it controls coolant temperature through the block, and in combination with the main valve, can control combustion chamber
temperature for fast warmup. The electric water pump and the
rest of the cooling system plumbing is primarily on the other
(right) side of the engine (Figure 9).
You could think of these active systems as splitting the ther-

Figure 8: General Motors ATM (active thermal management) system is externally mounted to an engine, in this
case the 2.7-liter four-cylinder. The left side (shown) has the
dual control valve assembly.
mostat function into two parts: 1) The temperature sensing, although in this case it's not a wax pellet but a solid-state sensor
reporting to the computer, and 2) An electric actuator to move
the valve, rather than having the job done by the expansion of
a wax pellet. It might seem that this is over-complicating the
way of getting the operation performed. However, by separating the functions and employing a number of sensors and a pair
of control valves, the active system can produce coolant flows
that are tailored very closely to the combination of customer
settings that ask for heat with the engine's need for a warmup
schedule that minimizes emissions and improves fuel economy.
In addition to the control valves, the ATM's cooling system replaces the conventional engine driven (belt, chain or gear-drive)
water pump with an electric motor-drive design, mounted on
the engine. The motor drive pump is controlled by the engine
computer, which runs it at a variable speed, with the exact rpm

4	

MACS Service Reports


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MACS Monthly Newsletter - 2020

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