MACS Monthly Newsletter - 2017 - JUN3

Photo Courtesy Eberspaecher LLC
Drawing Courtesy Ford Motor
Company

Photo Dave Hobbs
Figures 7 and 8: Another heated device we see as a comparison is the
common diesel engine glow plug. The glow plug's heating element is
1.1 ohms when cold (Fig. 7, top) which should, therefore, draw around
12 amps of current. As the glow plug gets hot (tip glowing Fig. 8 on
bottom) we see its heating element actually drawing 11.39 amps. Ohm's
Law calculations have it close at 10.9 amps meaning this glow plug's
metal doesn't change its resistance very much when heated. Why did the
glow plug's resistance change very little while the halogen headlamp bulb
increased a lot? Typical halogen bulb filaments are much hotter (around
5,000 deg. F) compared to diesel glow plugs (around 1,000 deg. F) therefore have a greater increase in resistance when lit up compared to a glow
plug.

Figures 9 and 10: PTC heater controls can be quite sophisticated in order
to be controllable and prevent premature burn out. Solid state circuitry
at the PTC assembly (Fig. 9, top) along with computerized controls (Fig.
10, bottom) work together to accomplish these goals.

The BCM activates the electric booster heater depending
on the following CAN bus provided values:

it a fair degree of protection from overheating damage and
it does not consume unnecessarily large amounts of current
once it gets warm.

* Engine coolant temperature is < 140°F
* Ambient air temperature is < 50°F
* Charging system operational
The PTC / booster heater electronics activate 3 output stages
depending on a PWM signal generated by the BCM. The output
stages switch the 3 heating elements of the electric booster heater ON or OFF individually, allowing the heating periods of the
individual elements to overlap. Due to the variable switch-on
duration, continuously variable temperature control is possible.
The overall heating power of the 3 heating elements is linearly
proportional to the PWM signal. If the PWM signal is < 10% or
> 95%, the PTC heater defaults to off. The electric booster heater also shuts off when ECT temps are > 158°F or OAT / AAT
temps are > 68°F.

PTC / Booster Heater System Hardware
* 3 separate PTC elements
* Built in micro-controller. The micro-controller allows
the 3 PTC elements to be activated or deactivated individually or together depending on the PWM input from
the BCM. (Fig. 9)
* BCM - Determines the operational request for the
unit to turn on. (Fig. 10)
* PCM - Passes along ECT sensor and OAT / AAT sensor values on the CAN bus to the BCM.

When PTCs / Electric Booster Heaters Work

Typical PTC Heater System Scan Tool PIDs:

If the interior temperature has been set to HI, or if the heater controls have been switched to the highest setting, the air
conditioning system sends an "electric booster heater ON"
request signal to the BCM via the MS-CAN. The BCM controls the electric booster heater with a PWM signal.

June 2017

* HVAC Control Module
a. Request activation of the electric booster heater.
* PCM
a. Engine coolant temperature
b. OAT / AAT (raw value from the ambient air temperature sensor) ■

MACS Service Reports

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