MACS Monthly Newsletter - 2017 - FEB6

temperature unexpectedly low, the computer concludes that
there's a coolant flow problem and it logs a specific trouble
code. On engines with electric water pumps as the primary
pump (vs. auxiliary-only pumps), of course, the computer
control system can monitor the pump operation and develop the same data.
We've also noted issues that arose with a couple of other
parts, worth discussing.

TWO ENGINE THERMOSTATS

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A number of heavy-duty diesels have a second thermostat.
But there's a high-volume gasoline engine of current
vintage with a second thermostat: the Chrysler 2.4-liter
four-cylinder engine without a turbocharger. Basically
the second stat serves as a bypass control. The primary
stat is in a thermostat housing at the radiator coolant inlet. The second stat is in the cylinder head coolant plenum (FIGs. 7, 8).
When the engine is cold and started, both thermostats
are closed. Coolant flows through the engine, heater
and bypass - there's none through to the radiator. When
engine coolant temperature hits 170-180 degrees F, the
primary stat begins to open and coolant starts to flow
through the radiator (and also the internal automatic
transmission cooler. By 203 degrees F., the primary stat
should be fully open. At that point (fully-open primary
stat), the secondary stat starts to open, which increases
coolant flow through the block, head and radiator. It
should be fully open when coolant temperature reaches
220-230 degrees F. If the primary stat is stuck open, the
computer control system typically will decide that emissions and fuel efficiency will be affected adversely, log a
trouble code and turn on the MIL (Check Engine light). ■

Figure 8: Secondary thermostat on Fiat Chrysler 2.4 four-cylinder.

or cover transient situations. For many years Ford premium models had a sensor in the heater coolant flow circuit,
wired to the blower electrical circuit, to keep the blower off
in cold weather until the coolant was warm enough, so cold
air didn't blow out of the registers prematurely.
Today we see use of a second coolant temperature sensor
as a diagnostic aid for a poor coolant flow problem - possibly a failed water pump, stuck thermostat, or restricted
cooling system. General Motors small four-cylinder engines
with mechanical-only water pumps are an example. If they
have the second sensor (in the radiator), the powertrain control system can check and compare coolant temperatures in
the engine and radiator, and see if they make sense, factoring in the engine run time. If they don't, with the radiator

DEAD BATTERY OR VERY-LOW VOLTAGE
The 2002 Chevy Tahoe with Automatic Temperature Control came into the shop with the air conditioning inoperative. It was a dealer shop and one
of the first things the technician did was check the
refrigerant charge. Right on the money. Next, he
jumped the pressure cycling switch, with no results. So he connected a scan tool and saw no A/C
request. Inasmuch as there is an A/C relay control
terminal on the Powertrain Control Module, this
clearly indicates a circuit/module/sensor issue
(FIG. 9). With the scan tool, he found no diagnostic
trouble codes, so there was no indicated pathway
for the next step in troubleshooting. He was able
to command the compressor on and that the system
cooled. Next step: using a relay-tester tool with a
button, he engaged the compressor and let the system run for a half-hour to be sure the clutch was

February 2017

staying engaged. It was.
However, he did see that the cabin temperature
reading was minus 40 degrees F. and the outside
temperature reading was plus 39 degrees F., neither
close to the warm (over 90 degrees F.) ambient of
the day. So first he pulled down the headliner and
saw the interior sensor was missing. Was this it?
He installed it and the system cooled normally -
for about a half hour. That cabin sensor still read
normal, but the outside sensor still read just 39 degrees F. He found an outside sensor at the left front
on the radiator support bar near the grille, cut the
wires and ran a temporary harness to the control
head. At first the cooling continued, and although
the sensor read in the 80's at first, it soon dropped
to just over 40 degrees F. The technician suspected
high resistance somewhere in the circuit-not an

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Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017