MACS Service Reports - 2016 - AUG7

Expected Results
You should see at least 700 mA of current draw
and that value should be fairly stable - not dropping even after several minutes of testing. If you
see 0 mA you have a completely inoperative heated
thermostat. If you are seeing less than 700 mA (0.7
amps) or an erratic current draw number that continues to move all over the place, you've just discovered the most common failure on these heated
stats; heater grid breakdown under load.

Figure 12: This Chevy Cruze can have it's electric thermostat opened with
a bi-directional command from a factory scan tool (GM MDI with GDS
2 software) or an OE Level scan tool like this Delphi / BSE PC based
aftermarket tool. Commanding the duty cycle up should bring the temperature down.

Running Dynamic Scan Tool Test
1. Start the engine up and allow it to run

meter connection! But these tests don't check the
circuit out when loaded - real world working conditions. That is when most electric heaters fail
whether in O2 sensor or in electric thermostats.

2. Using a scan tool, monitor the ECT until
the temperature reaches 215 deg. F. (104 deg.
C)

Dynamic Circuit Test

a. Using your scan tool, command
the electric thermostat heater grid
on to as high as possible setting (90%
duty cycle bi-directional request) to
see if the temperature lowers by several degrees. (Fig 12)
b. While driving, monitor ECT and
electric thermostat duty cycle to test
for functionality. ECT coolant temp
should go down as duty cycle ramps
up with heavy throttle.

1. With the ignition on / engine off connect
an inductive amp clamp around the power
feed circuit or ground control circuit. An alternative would be to insert a Fuse Buddy /
Ammeter into the 10 amp fuse spot that powers the electric thermostat.
2. Next provide a ground (fused jumper or
Power Probe) to activate the heater while the
ignition is on to observe the current draw of
the heater element. While this may set a DTC,
it shouldn't damage the ECM. To be better
safe than sorry I like to disconnect the ECM
or open the circuit between the PCM and
its ground feed at the stat's heater connection. The PCM's smart driver circuit knows
when there should be a ground on its output
and when there should be open ignition circuit voltage on its output to the electric stat
heater when that ground is not commanded
on. The PCM is looking for 2 things to set a
DTC; a contradiction with the voltage state
the PCM thinks it should be in and too much
or too little current when commanded on. If
you disconnect the PCM's supplied ground
and provide your own ground to the heater,
you certainly will ensure that damage is not
done with a jumper wire / Power Probe to the
PCM if a mistake is made, such as back probing a ground into the wrong circuit or applying power with the Power Probe instead of
ground. You may also set a DTC but at least
you've not taken the chance of turning your
Power Probe into an expensive "smoke machine for ECMs."

August 2016

Expected Results
A good working electric thermostat should drop
the engine temperature between 18-27 deg. F (10-15
deg. C) shortly after the duty cycle is ramped up to
over 90 percent.

Running Dynamic Scan Tool & Scope / Meter Test
1. Perform the dynamic scan tool test
above.
2. Using a back probing connection at the
electric thermostat's ECM supplied ground,
use a dual trace lab scope to measure the duty
cycle pattern of ground being supplied to
the stat's electric grid. Remember, when the
ECM is not supplying the ground, the voltage level at that circuit is at battery voltage.
When the ECM provides a PWM signal (naturally or with your bi-directional request) you
will see the circuit go to 0 volts. The PWM
signal will result in a square wave on your
scope.

MACS Service Reports

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