MACS Monthly Newsletter - 2021 - JUN6

Steve Schaeber

For over a decade, Toyota and Lexus vehicles' DENSO piston compressors on models with the 2.5-liter
four-cylinder and 3.5-liter V-6 have used a refrigerant
flow sensor. This sensor (Figure 10) measures refrigerant flow, which it does this with a spool valve that incorporates a magnet. The spool is spring-loaded on the
top and bottom, balanced in a cylinder, with refrigerant
flow to top and bottom. If compressor displacement
causes refrigerant flow to drop, there is a lower pressure differential in the assembly, and the lower spring
pushes the spool upward, where the magnetic flux is
measured in the electronics of the sensor connector
to produce a voltage signal to the A/C amplifier. And
the A/C amplifier translates this into a refrigerant flow
signal to control the compressor. If the pressure differential increases, the upper spring pushes the spool
down and the change in flux uses this signal to change
the flow measurement. The voltage signals to the compressor are key inputs to amplifier outputs.
If the amplifier logs a B1479 trouble code, a high percentage of technicians will conclude that the sensor is
bad, and since they have heard (correctly) that Toyota
does not supply a replacement sensor, will tell the customer that, " it needs a new compressor. "
Inasmuch as a new DENSO compressor isn't pocket
change, even though the refrigerant flow sensor qualifies as a " most likely, " the customer is likely to want the
technician to be absolutely sure. Well, confirming that
the sensor is bad goes beyond the trouble code, because the code could be logged by a loss of refrigerant.
So, the first diagnostic step is to recover and recharge
the refrigerant to specifications. Clear the code, turn
on the A/C and see if it resets. If the code comes back,
low refrigerant charge is not the issue.
However, the sensor still is not necessarily the cause,
just what we call a high probability. There could be a
fault in the wiring harness or even (remotely) the A/C
amplifier, and Toyota has test sequences in its service
information. The harness and amplifier are not the
problem? At this point, a technician may think, " Well, if
Toyota doesn't offer the sensor, why? "
Experienced technicians have encountered the " detail parts not available " factor in enough cases over the
years. But in this one, you can search the internet and
find listings for replacement refrigerant flow sensors,
for a small fraction of the cost of a new compressor.

Figure 11: This replacement sensor (note its size compared
to a typical AA battery) is available as something a technician might want to try if the Toyota/Lexus vehicle logs
a B1479 code. But the Toyota recommended procedure,
if there are no refrigerant charge or wiring issues, and
the amplifier checks out, is to install a new compressor.
But these " sensors " (Figure 11) are not OE parts. However, they're a modest price (perhaps $20-40) and if
you want to try one, we can't blame you. After all, the
other choice is to replace the complete compressor, so
why not.
But you're likely not getting a complete sensor assembly, just the electronics built into the connector.
The refrigerant flow sensor, as we've noted, includes
a housing with a magnet sliding in a cylinder, balancing springs and a flow throttle. Which is one possible
reason Toyota and DENSO choose not to supply just
that connector with built-in electronics-it may not be
the problem. There's no practical way to validate everything in that refrigerant flow system (say a weak spring
or sticking magnet) so to ensure customer satisfaction,
even if it costs more, the Toyota decision to recommend
a new compressor is one we can understand from an
OE or OE supplying manufacturer's standpoint. v

Things That Happen To Old Cars
When independent garage technicians talk about the
hardest-to-find failures that created the biggest headaches they had ever encountered, it's usually on " older "
cars. Well, with the average age of cars on the road now
past 11 years, that translates to lots of what we used to call
" older. " There are several reasons why cars on the road are
getting older. One is the obvious improvement in overall

June 2021

quality. Unless a car is left outside in areas where snow or
rain is heavy, the body, chassis and suspension can look
pretty good even on a car 15-20 years old. Powertrains
that used to be close to the end of the line at 100,000
miles now may go 200,000 miles, even longer. With the
prospect of major repairs becoming cost-effective, they
are becoming more common. Individual components that

MACS Service Reports

MACS Monthly Newsletter - 2021

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