As with a conventional thermostat their main purpose is to limit coolant flow to being pumped between the engine block and cylinder head in order to bring coolant temps (and therefore combustion chamber temps prior to combustion) up to operational temperatures. This of course also helps to achieve good heater core performance when the passengers want to be warmed up. VW / Audi may refer to the closed thermostat coolant flow loop as the "small coolant flow circuit" (Fig. 5 & 6) and the condition when the thermostat opens to allow Drawing courtesy VW / Audi Group Drawing courtesy Audi Drawing courtesy GM the engine's coolant when it reaches the set point. For this reason thermostats have traditionally been mounted on the cylinder head. Electric stats, however, may be mounted lower on the engine as with VW/Audi. (Fig. 3) Electrically mapped thermostats Figure 3: Mounting locations may or may not be typical to a conventional thermostat. Due to the nature of these electric stats, they don't have to necessarily be mounted in the head (Audi example on left). The difference is truly the presence of an electrical connector which could be confused as an ECT sensor (Chevy example on right). will open on their own (more on failures and diagnostics later) similar to their simpler mechanical counterparts but in most cases at a slightly different temperature than a conventional thermostat equipped model. A Chevy Cruze with electric stat, for example, may open w/o electric assist at around 225 deg. F. That's 30 deg. F higher than many OEM conventional thermostats! Either type of thermostat may appear to be mounted directly on the head or in a coolant manifold assembly (a.k.a. coolant distributor housing shown in Fig. 4.) Drawing courtesy VW / Audi Group Drawing courtesy VW / Audi Group Figure 5: Small cooling circuit is Audi / VW's way of saying "stat closed - coolant flow goes from engine to block only." Notice the "heat exchanger shut-off valve at top right of drawing. Essentially an electric version of the old cable / vacuum actuated heater valves, the newer electric models keep the passenger compartment from heating up when heat is not wanted. Figure 4: VW / Audi models may utilize a coolant distributor housing to connect the various heat exchangers together with radiator inlet / outlet connections and the electric thermostat. This OEM also uses an ECT on the radiator as well as the cylinder head for making the mapped cooling system more accurate. August 2016 Figure 6: With low coolant temperatures and no PWM signal to the electric thermostat's heater element the "small coolant circuit" is maintained by the larger disc valve of the stat being closed via spring pressure. Note the upper level of the stat (top right) via the open small disc valve allowing the coolant in the upper level of the coolant distributor unit to be moved by the water pump. 3 MACS Service Reports