This is our first year with the Prius. I have a Scan Gauge connected to our car and one of the displayed parameters is coolant temperature. These observations are probably not interesting to the majority of users here but the operation of this car fascinates me and I want to know exactly what it is doing. I have read and agree with the 5 stages of operation as listed on other threads here as long as the car is driven in warmer weather. The Five Stages of Prius Hybrid Operation | PriusChat This description may not be for a Gen 3 Prius but it is close. As described, there is a transition to Step 2 at 104 degrees F. where the ICE would shut off when pulling up to a stop sign or traffic light. Now that the temperature in Ohio is in the 30's, the car behaves differently. There must be some ambient temperature or cabin temperature component that must figure into the equation that is not described in the 5 stages of operation. With cold weather, when I pull up to a stop sign or traffic light, the ICE keeps running even with the water temperature well above 104 F. At some temperature higher than the 104 F, the system does transition to S2 and the ICE eventually does shut down at a stop sign or traffic light. I have noticed that as the coolant temperature reaches 160 F for example, the ICE shuts down at a traffic light. The water pump must continue to run to provide cabin heat because, the coolant temperature quickly drops about 10 degrees or more and then as soon as the ICE restarts, it begins to increase again. Another unknown is that I don't know when the auxilliary electric cabin heater kicks in and shuts down. Maybe the coolant temperature or cabin temperature has to have reached some target to satisfy that heater. Nothing earth shattering here just some discrepancies with the posted description.
You nailed it with your observation that: "The water pump must continue to run to provide cabin heat because, the coolant temperature quickly drops about 10 degrees or more and then as soon as the ICE restarts, it begins to increase again." If the ICE shut off at 104 degrees in the cold, even with the cabin up to desired temperature, the coolant temperature would quickly drop, the ICE would quickly cycle on again, and you would get into a perpetual short-cycling mode.
And yes, the software does know you are running the heat. It is plotting to keep the engine warm even with the constant drain of passenger cabin heating.
This video of my commute shows that engine on & off cycling for heat... The behavior has been that way for many generations. Here's some old ScanGauge footage..
When partially warmed I can turn the engine off/on, by setting cabin temp lower/higher, and/or by turning the cabin vent system completely off/on. (I think used my slash quota for today...)
This explanation came from another thread. It claimed that it was cut from the repair manual. Not sure which generation. I assumed that it applies to the Prius V as well.
Very interesting. Unsure if that is true (or just for "v"), but I will def be testing on Gen3 liftback and Prius "c" tomorrow.
I did a search of Toyota parts and arrived at this part number for a 2015 Prius V "Quick heater". 87710-12020 So it confirms that a Prius V does have the PTC heater. Note: PTC - positive temperature coefficient means that this is a resistive heating element probably made of ceramic. It would heat up quickly and as it gets hotter, it's resistance would increase which reduces the power and subsequently the heat output. This makes it self regulating. A cold PTC would draw more current and provide more heat so on initial startup in 20 deg F ambient, it could provide an instant boost to the defroster air temperature early in the coolant warm-up cycle. As the coolant temperature increases, the air flowing through the PTC also warms causing the PTC output to decrease. The dropoff of PTC heat to the cabin would be replaced by the warming coolant. So my best guess as to when and how it works: Above 50 deg F ambient or in ECO mode or the blower is not on, the quick heater is disabled. Below 50 deg F ambient, while not in ECO mode, with the blower on and coolant temperature of less than 150 deg F, the quick heater is enabled.
