the switched ignition source and which one is the ECM supplied PWM ground. (Fig. 11) TECH TIP: When an accurate schematic just can't seem to be found, an easy way to do that is simply turn the key with the heater connector disconnected and find which terminal has power. The other terminal is the ECM supplied PWM ground. 2. With the ignition on / engine off connect a voltmeter between chassis / engine ground and a back probed terminal in pin 1 (figure 11) of the electric thermostat with it plugged in. You should read battery voltage. If you do, you've quickly established: a. The 10 amp fuse is NOT blown and powertrain control relay (if equipped) is functional. b. The power feed to the actual electric stat heater element is not open. c. The heater element does NOT have an open circuit in it. d. The ECM's ground side control of the heater is NOT stuck shorted to ground. That's quite a bit of information with a single Figure 10: This Audi has set an electric assisted thermostat DTC for an open circuit. Like any other actuator with an open circuit code, the problem could be an open or high resistance connection / wire between the stat and the ECM or the stat's internal heater coil winding. A generic DTC for an electric coolant thermostat heater code indicating an electrical circuit problem could also be P0597. fault with the system that could cause an overheating condition. Knowing these simple facts we can perform a series of passive tests to check out the basic circuitry of the system: 1. You'll need to determine which wire is Figure 11: This Chevy Cruze wiring schematic shows the clue you need to determine the electrical connection on / near the thermostat is indeed an electric stat heater element and not an ECT sensor. Once you've determined what it is, there is a way to check it. August 2016 6 MACS Service Reports