Noticed my electric fans cycling on and off every 15 seconds. The coolant temp is between 190 - 173. I’m at 490K now and installed a replacement transaxle two week ago. I do not recall this before the swap and I wondered if the fan was two speeds and has now become just high speed?? iPhone ?
At 190F, it shouldn't be cycling on at all, at least on my Gen3 Prius and two (present and past) Subarus.
Engine coolant temperature and the status of air conditioning both figure in to the control of the fans. And yes, there are (originally) two speeds available for the control (three, counting "off"). -Chap
Yes, it is related to the AC ( I’ve always set it on auto 71 F) and yes when I turn it off the fan stops cycling. I don’t recall it doing this before the repair and thought I must have forgotten to attach a sensor. iPhone ?
inspired to figure it out I went under the hood for a closer Look. Only the driver / Transaxel side fan was cycling on high every 15 seconds. Checked the connection and the upper fan was not connected (note to self, amend radiator install instructions!) Great way for the engineers to get me to notice! Seems the second fan runs on high when the first one isn’t doing the job? When plugged in, it started running at a quiet speed and the high speed fan shut down. iPhone ?
One clever way to get two fans to run at two speeds without resistors, fancy speed controllers, etc. (and the way Toyota does it) is to have relays configured in a way that either connects the two fans in series (12 volts has to travel in and out of one fan, then in and out of the other, each fan seeing about 6 of that), or connects them in parallel (each fan seeing the full 12 volts). By having one fan disconnected, you broke the series circuit, so no "low speed" was possible. For high speed, in the parallel circuit, the one fan disconnected was not a problem for the other one. Because nothing happened when the car wanted low speed, temperatures/pressures continued rising enough to trigger high speed, which worked (for one fan, anyway). -Chap
Glad you got it figured out. Had similar issues on Mercedes with A/C, it has to do with pressures in an system. Hot A/C, increases pressure, kicks on the fans... Z981 ?
On my old Honda, which sometimes suffered a rising temperature gauge in slow and stop-and-go traffic as it aged, I deliberately used this to cool of the engine. The engine normally used a single fan above some threshold, but the AC forced both fans to run, and I could see the temperature needle start falling very quickly. But in hindsight, I don't know what actual coolant temperatures were involved. It preceded the OBDII port era, so I couldn't plug in my ScanGauge to get actual temperatures. My first car with OBDII, a Subaru, proved to have 'customer expectation management' designed into the temperature gauge. It appeared to have rock solid temperature stability, but in reality had the entire span of 145-210F collapsed to a single point on the gauge. It didn't start climbing until well after its fan turned on, unlike that old Honda that showed a rising needle well before the fan turned on automatically.
The Prius, as already covered here, always uses both fans (provided you've plugged them both in), but can run them at two different speeds. -Chap
