MACS Monthly Newsletter - 2021 - JAN6

Mitsubishi Heavy

Quarto Technical Services, LLC

Figure 13: Graphic of a PTC heater design indicating 4
PTC elements.
Figure 12: Picture of a DC Current Clamp being used to
test cabin heater load current.

BMW

Cabin Heaters can have one or more PTC resistive elements.
Most of the heaters will contain either three or four PTC elements. The F/W can determine how many PTC elements are
powered ON to increase liquid coolant temperature. The higher
the temperature requested, and the temperature of the liquid at
the time of the command, will determine how many PTC elements are needed for liquid heating. For example, the control
strategy may have a total of 6 heating stages for a Cabin Heater
that is designed with three or four PTC elements (Figure 13).
As previously stated, these stages can be PWM and linear
power controlled. For the following example (Figure 14), it will
be assumed that the Cabin Heater is using both PWM control
and linear power control strategies, with the F/W determining
the PWM% for each PTC element being used at any of the three
stages.
A final note on the PTC resistive heaters. Although a Cabin
Heater system may indicate three PTC heaters on a schematic,
there are likely a significant number of PTC resistors that comprise an entire network of resistors. As an example, a Cabin
Heater may contain up to 16 different sections within the heater
that are comprised of four resistors per section (total of 64 PTC
resistors). The point here being the Cabin Heater could easily
have one or two sections that are not performing, but the others
are operating properly. Depending on how the F/W is architected, a diagnostic fault may or may not be logged with a diagnostic message reported over CAN or LIN to the controller. This
means there are temperature bands (narrow or wide) that will
plateau or be lower heat output that could result in a customer
heating complaint. This is an important concept to remember
when considering a diagnostic process that would augment
OEM service information.

Figure 14: Block diagram of a PTC power control table.
that Scan Tool PIDs and Output Controls can significantly vary
between OEMs and Scan Tool manufacturers. As systems mature, technicians will provide feedback to Scan Tool manufacturers or OEMs to request additional or targeted PIDs and Output Controls to assist in diagnostics (if necessary). An example
of a more mature vehicle model, the Generation 1 Chevrolet
Volt. The service information has few PIDs or Output Controls
(Figure 15) associated with the Cabin Heater (information provided by James Avery - Chevrolet Volt Technician, Thank You,
James!). This would compel a technician to use alternative diagnostic processes (i.e., off-board tools) to test and confirm heating
circuit performance.
Although not shown in the aforementioned Chevrolet Volt
PID and Output Control Scan Tool data, the Generation 2 Chevrolet Volt provided approximately the same (limited) PID data
as the Generation 1 Volt and significantly less in the Output
Controls data list. Also, the DTCs listed for the Cabin Heater are
limited to Open, Shorted, and Grounded circuit variety with no
specific assistance with heater performance codes (Figure 16).
Although the Generation 1 Chevrolet Volt is only one example (with limited PID data), there are/will be vehicles with more
PID data and Output Controls that would further aid a technician in testing and verification of Cabin Heater performance.
However, with limited Scan Tool information, this will be of
minimal assistance to a technician that is either testing or verifying the performance of an HV Cabin Heater. However, armed
with specific technical information, a few common off-board
tools and a Scan Tool (PIDs and Output Controls), determining
Cabin Heater performance can be a simple process.

Diagnostics

From the perspective of Cabin Heater diagnostics, as with
most other diagnostics, the tool utilization would include: 1)
Scan Tool PIDs and Output Controls/Special Functions, 2) Offboard tools for dynamic testing (i.e., current clamps), and 3) Offboard tools for static testing. Most technicians have experienced

January 2021

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MACS Monthly Newsletter - 2021

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