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On the very few occasions my radiator fan came on, I noticed that my temp gauge in the gauge cluster was not showing any higher temperature than during normal driving conditions. For me, this is kind of scary. As we know, the RX400h does not have a warning light for low coolant levels. If the temp gauge doesn't show high coolant temps either, then it has very little use, other than showing when not to stress the engine, because it hasn't yet warmed up properly. Did anyone else notice this?

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Hi John, I'm not a hybrid driver but..  The radiator fan is there to ensure the vehicle doesn't overheat.  

You should hear it after extended periods in slow traffic, hot days, etc.  Certainly if the system is working harder to maintain it's safe operating temperature it could point to an issue so it could be worth investigating.  

Are you losing any coolant?

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I believe the rad fan also comes on to cool the a/c intercooler rad mounted in front of the engine rad. At least on other cars I've owned...

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5 hours ago, BROUWERS said:

On the very few occasions my radiator fan came on, I noticed that my temp gauge in the gauge cluster was not showing any higher temperature than during normal driving conditions

That's exactly how it's meant to work. I've never had a car who's temp gauge rises and falls with the operation of the rad fan. Every temperature gauge I've ever seen in any car I've ever been in starts off at cold, climbs to normal and stays there rock steady, while the fan goes on and off as it pleases.

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On my other car, a classic Citroën, I do see the needle pointing at a higher temperature before and while the fan is kicking in. Surely it starts operating only from a certain temperature, let's say 95 degrees. It then quickly lowers while the fan is doing its work.

My point is, how will I ever know my engine is overheating when a temperature increase enough to let the fan run is not registered by the temp gauge.

@Jaystar77@NemesisUK

I let the engine run until the fan kicked in as part of the coolant refill/bleeding procedure after changing the timing belt and water pump. A/C was off.

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All in the design.

Yes, the instrument cluster needle sits at the mid point (usually regarded as 90 degrees in most vehicles) but in order to save any alarm it does this between say 85 and 114 degrees.  

The system brings the fan on well below the point at which the needle would move so you don't see any movement when things are operating normally.  

If it was overheating you will see it rise above the midway point pretty quickly - I had a fanbelt break on my Starlet and as it turns the waterpump I got to experience an overheat firsthand!

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41 minutes ago, BROUWERS said:

On my other car, a classic Citroën,

Well, that says it all doesn't it  😁

Sorry John, only joking. Seriously though, modern cars are designed to act in the way that Jay describes above. There often are no markings on the temperature gauge other than cold, hot and maybe something midway between the two, which is considered to be 'normal' operating temperature, and it will sit there rock solid, never moving, while the fans come on and go off as they are required.

Rest assured though, if anything does go wrong, the needle will make its way upwards and you'll definitely notice it.

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Thanks for clearing this up guys. I guess only tech freaks like us want accurate information from our cars 🙂

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On 12/2/2018 at 3:46 PM, BROUWERS said:

Thanks for clearing this up guys. I guess only tech freaks like us want accurate information from our cars 🙂

The information it displays is accurate.  I disagree with the claim that it "displays" a constant temperature despite a variation.

 

The thermostat with thermostatically open at a fixed temperature - usually around 90 degrees (it will be fixed, but they vary from stat to stat and car to car).  The thermostat will allow coolant to flow to the radiator when the coolant from the engine reaches this temp, in order to keep it regulated at a constant temperature.  If the temperature rises above the threshold, due to lack of airflow onto the radiator that you'd get from moving (so when stationary, basically), it will activate the fan to substitute this.  The fluctuation in temperature in the system is minimal, and certainly not across a 30 degree variation.

The coolant temperature sensor is very accurate, and you can read the exact temperature via the OBD port using an appropriate dongle/software.

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2 hours ago, engine said:

The information it displays is accurate.  I disagree with the claim that it "displays" a constant temperature despite a variation. [...]  

The fluctuation in temperature in the system is minimal, and certainly not across a 30 degree variation.

The coolant temperature sensor is very accurate, and you can read the exact temperature via the OBD port using an appropriate dongle/software.

You're sort of right but not completely.

Yes, the temperature sensor is very accurate and yes, it can be read with decimal point accuracy - but the temperature gauge itself doesn't indicate those fluctuations.

Think of early petrol gauges. The float in the tank basically sent a voltage that was read directly by the gauge but because it was a 'raw' voltage the needle used to fly about all over the place as the car went round corners, uphill or downhill and the petrol sloshed around in the tank. To combat this they introduced voltage stabilisers and/or some sort of timing circuit 'damper' so that the gauge reacted more slowly and less erratically to give a smoother reading. The same principle is applied to the instrument panel temperature gauge.

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11 hours ago, engine said:

The information it displays is accurate.  I disagree with the claim that it "displays" a constant temperature despite a variation.

I'm only going off what I've seen on other Toyota's in the past.  The temps I mention aren't accurate, merely given to put the point across.

Fit an aftermarket temperature sensor and take it to a track day and you will find there is a variation.  

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The air con can kick cooling fans in even on a cold engine.  As someone else said above, it is to to cool the condenser which usually sits in front of the radiator.  Easy test - turn off air con!

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20 hours ago, Herbie said:

You're sort of right but not completely.

Yes, the temperature sensor is very accurate and yes, it can be read with decimal point accuracy - but the temperature gauge itself doesn't indicate those fluctuations.

Think of early petrol gauges. The float in the tank basically sent a voltage that was read directly by the gauge but because it was a 'raw' voltage the needle used to fly about all over the place as the car went round corners, uphill or downhill and the petrol sloshed around in the tank. To combat this they introduced voltage stabilisers and/or some sort of timing circuit 'damper' so that the gauge reacted more slowly and less erratically to give a smoother reading. The same principle is applied to the instrument panel temperature gauge.

Yes, but not +-40 degrees as was referenced.. A few degrees either way, but nothing that would make a noticeable movement.

The difference in a fuel gauge is that there is an excusable reason why the reading would fluctuate - a cooling system tends to be designed to read at the thermostat (location of the sensor), which is also mechanically designed to open at a set temperature.  The system would have to be either poorly designed, or incredibly faulty to read any significant fluctuation.

The directly read temperature (i.e. the raw reading) can be observed via OBD2, the PID for which is independent of the instrument panel.  

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Okay lets narrow the band a bit, say 90-104 degrees, any happier?

The question wasn't when will the fans kick in, it was when does the temp needle show an overheat situation.

Sitting on a driveway with an OBD reader isn't going to answer this unless you disconnect the fans and let it cook a little.

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On 12/5/2018 at 6:15 PM, Jaystar77 said:

Okay lets narrow the band a bit, say 90-104 degrees, any happier?

The question wasn't when will the fans kick in, it was when does the temp needle show an overheat situation.

Not exactly - the question related more to "when does the radiator fan come on?" (with respect to the needle position).  As per my first post on the subject:

On 12/3/2018 at 9:59 PM, engine said:

If the temperature rises above the threshold, due to lack of airflow onto the radiator that you'd get from moving (so when stationary, basically), it will activate the fan to substitute this. 

The radiator fan doesn't come in any "overheat" situation - it comes on to regulate the coolant temperature, so can be expected to come on at a small amount above normal operating temperature.  It simply simulates the airflow that normally cools the coolant when moving.

The needle will move as appropriate.  There isn't a magical temperature range where the needle jumps to half way and/or stays at half way - it is an incremental movement.

 

On 12/5/2018 at 6:15 PM, Jaystar77 said:

Sitting on a driveway with an OBD reader isn't going to answer this unless you disconnect the fans and let it cook a little.

It will.   You can do it with a multimeter directly from the coolant temperature sensor itself, if you wanted to compare the voltage output with a temperature reading.  Disconnecting the fans is redundant, as the test scenario in this case is "at what temperature do the fans come on".

 

The coolant temperature gauge would be useless if it switched from "cold" to "normal" to "overheating", as you appear to be describing.   I previously worked for a motor manufacturer and cooling systems was my area of testing; some days I had to spent the majority of my day observing sensor outputs and testing functions in software that relied on the values (waiting for the damn things to warm up, then cool down, then repeat), so I have previous in observing the indicative accuracy of temperature readings.  Ideal area to work in for people who like sitting around, but I didn't.

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Good to hear from someone with clinical experience.

That being said the accuracy of Toyota coolant temp sensors has always been more accurate in the lower half of the gauge, but once they reach operating temp it will only raise the needle if it sees a voltage reading that indicates the system is overheating.  

Just out of interest, which manufacturer was it?  My head is tortured daily with VW testplans so you can understand why I drive a Lexus. 

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