MACS Service Reports - 2016 - AUG8

TECH TIP: Paul Danner a.k.a. "Scanner Danner"
with Rosedale college has a very in-depth YouTube
2-part series on testing electric thermostats on a
GM vehicle. Simply use Google to find these great
video clips and see for yourself what a flaky electric thermostat heater grid looks like.

where a bulb that has an internal resistance of 4
ohms should draw 3 amps at 12 volts but instead
only draws 400 mA (0.4 amps) when it is powered
up due to heating.

Default Modes
Remember two main things; Too Hot or Too Cold
1. If there is a faulty mapped thermostat
condition such as no power to the electric
thermostat the coolant temperature will be
higher than normal under heavy throttle conditions.
2. A mapped stat with an electric control
circuit shorted to ground will create lower than normal coolant temps earlier in the
warm up process from a thermostat opening
too soon creating potential heater performance and cold drivability issues.

3. Using an inductive amp clamp that can
connect into your scope, place the jaws of the
probe around either wire going to the stat.
As the engine runs and the ECM (naturally
or bi-directionally commanded) increases
the PWM duty cycle to the electric thermostat, you'll see the pattern of voltage shift (12
volts down to ground) and current increase
(0 amps up to 800 mA or higher).
TECH TIP - HEATER GRID OHMS SPEC: VW
lists a specification in their diagnostic charts for
the resistance reading of their thermostat heater
circuits to be between 14 -16 ohms at 77 deg. F (25
deg. C). Using Ohms Law you can divide the heater 's supplied voltage (12.5 - 14.8 volts) by the resistance internal to the heater and come up with an
expected current draw between 700 mA and 1000 mA
(0.7 amps and 1.0 amp). Always keep in mind that
when circuits heat up, that circuit's internal resistance goes up with temperature resulting in slightly lower current draw figures. This is especially
evident with a circuit such as light bulb filaments

Default modes occur when the ECM senses a problem with the electric thermostat, either ECT sensor
or the electric cooling fan. Fails with the ECTs will
result in cooling fan activations and default values
appearing on your scan tool in enhanced powertrain
data viewing mode (Generic OBD II should show
the actual failed temp value). If both ECTs fail, or
the fan fails or the engine's coolant temp raises to a
select (high) value, the PWM output from the ECM
to the mapped thermostat will go full on in order to
open the thermostat all the way. ■

MACS 2017
MACS has big plans for 2017
In 2017, the MACS Training
Event and Trade Show will take place
at the Marriott hotel in Anaheim, CA,
February 15 to 18.

MACS Service Reports is published monthly by the Mobile Air Conditioning Society Worldwide. It is distributed to members of MACS Worldwide
and is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will be
considered for publication, however, MACS Worldwide reserves the right
to choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Forcey
Manager of Service Training:
Steve Schaeber
Mobile Air Conditioning Society Worldwide
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsw.org * Website: www.macsw.org

Unless otherwise noted, all photos/art by author.

August 2016

MACS Service Reports

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Table of Contents for the Digital Edition of MACS Service Reports - 2016