March 2022 - 28

Direct versus indirect TPMS:
How they work and how to
diagnose them
No matter the system, ensure you follow service information closely.
By Tanner Brandt, Contributing Editor
V
ehicles on the road today
could have two possible
types of tire pressure
monitoring systems (TPMS).
Either indirect meaning the
vehicle has no pressure sensors
in the tires or direct where the
vehicle has individual sensors in
each tire. Let's take a deep dive
into each of these systems and
learn how they work and how to
diagnose them.
DIRECT TPMS
Direct TPMS systems use individual pressure
sensors in each tire mounted to the
rim. Most of these sensors will be part of
the valve stem although some early systems
used a sensor mounted to the rim
with a large band clamp. The sensors send
out either a 315 MHz or 434 MHz signal.
The signal is then received by the TPMS
module which could be standalone or be
built into another module - sometimes
it may be built into the door lock receiver
module or the radio transceiver module.
The tire pressure data is then sent to the
various modules that request it.
When servicing vehicles with direct
TPMS, it is critical that service information
be followed. On some vehicles the
system may need to be calibrated when
the tires are rotated, and others may ask
that the system be reset even after tire
pressures are changed. During this service
I recommend using a tool that shows the
battery state of health of the sensors like
in Figure 1. Have you ever had a customer
that came back after a tire rotation with a
low tire light and now the car has a dead
Figure 1: A TPMS report is displayed on
the face of a tool being used to capture
the sensor's battery state of health
(SOH).
sensor? Once that happens, it becomes an
" ever since you " scenario. If you look at the
TPMS sensor battery level during routine
service, you can catch the sensors before
they die and let your customer know they
need to be replaced.
In my opinion, it's not a bad idea to scan
the sensors anytime the vehicle is in for
an oil change just to check the batteries.
When a direct TPMS has a fault it's
Figure 2: The tool's display indicates
a mismatch of the sensor's IDs and
location, as compared to where they
are located on the vehicle.
easy to diagnose. If none of the sensors are
being seen by the vehicle then we need to
use a TPMS tool to check them and verify
they are the correct sensor type/frequency
and that they have a programmed ID
number. Once that is done, if they are not
correct or are not reading at all then they
need to be replaced. If they are correct but
the vehicle is not able to see them, then
it could be a problem with the module or
its input circuits. The module houses the
antenna for the system. If the system is
built into the remote door lock module, a
simple test with the door lock remote can
tell us if that module is functioning. If the
door locks don't work either, then we can
head directly to the module and check all
the inputs. If just one sensor is dead, then
it's easy to find with a tool that reads all
the sensor information.
28 PTEN MARCH 2022 www.VehicleServicePros.com
http://www.VehicleServicePros.com
March 2022

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