ECT Sensor on F4i and RR - Do They Work Differently?

[Nishant Jain]

Our team used the F4i engine on Power Commander the first year. This year we're switching to a PE ECU on an RR engine.

Now both these changes find me stuck with a question.

The F4i engine uses a fan motor relay switch on the radiator, which connects linearly with the signal from the ECU to the fan (to switch it on or off). I had assumed that the reading from the ECT Sensor was what decided if the fan should be on or off, but it seems not so.

In the case of the RR engine, there is no fan motor switch. The fan I'm guessing is then directly operated by the ECT values. Also, the working of the PE ECU suggests the same, since it has upper limit and lower limit values of ECT output by which the fan is operated.

This led to a heated discussion in our workshop as everyone debated how the ECT sensor even worked.

My question - does the ECT sensor absolutely control the working of the radiator fan? It has uses in fuel injection pulse width too, but the PE ECU working suggests it is the prime controller for the fan.

In either case, my 2nd question - what kind of fluctuations does the ECT sensor sense? I mean, we figured that once the engine was up and running properly, the temperature would always remain on the higher side (higher side meaning above the temperature set for bypass circuit to be closed by the thermostat).

But that would make the ECT sensor redundant, if the temperature didn't fluctuate much. So, does the temperature fluctuate? And if so, what kind of variations are normally seen?

[jaca]

the PE controls the fan relay based entirely on the ECT sensor input, and can also take other arbitrary action based on most sensor values

not sure what/how the stock sensor works - we're running an F4i, we run our own ECT sensor on the radiator and leave the one on the engine disconnected

[Nishant Jain]

do you use your ECT sensor only for running the fan then? i suppose you're not running the stock ecu.

[Mikey Antonakakis]

Originally posted by Nishant Jain:
do you use your ECT sensor only for running the fan then? i suppose you're not running the stock ecu.
We (I'm on jaca's team) use the ECT sensor to measure the coolant temp at the radiator. The ECU uses this as an input for various things, for instance, as an on-off switch for the fan. It is set to turn on at a particular temperature, and turn off at another (or the same, if you choose) temp, same as the PE ECU. It is also used for warmup compensation, i.e. richening the mixture because the engine is cold (I think you referred to that in your post). I am assuming you are using the ECT sensor that PE suggests (probably a GM one).

We used the PE last year, we're using Megasquirt this year. In both instances, to the best of my knowledge, the ECT does completely control the fan. Its use beyond that is significant, as I mentioned for warmup compensation, but also just as an overheating alarm, too. With the PE, I'm pretty sure you can set one of your outputs to turn on at a set temperature (useful in a dash warning light). You're correct that once the engine is warmed up, the coolant temp will stay above the temp that the thermostat opens (although I don't know off the top of my head what that temp is). The temperature certainly does fluctuate. With our Megasquirt, so far this year we haven't run the engine under load, and the temperature doesn't exceed about 165F (~74C). Last year, when we drove the car around, the temperature was higher than that. And if you improperly bleed the cooling system, the ECT is VERY useful. You will see very quick spikes in the temperature, to over 210F (100C). This happened to us last year on the dyno. After one pull we noticed the temperature was alarmingly high, so we shut the engine off. It cooled very quickly, so we turned the engine back on, and it rose to a dangerous level again very quickly. We re-bled the system and the temperature was much more stable after that. To summarize, make sure your system is bled properly!

As far as the magnitude of the temperature fluctuations, I can't remember exactly off the top of my head, but I'm sure it classifies as "significant." It depends on your cooling system, as last year's engine seemed to idle a bit hotter than this year's, but it would rise maybe 20-30F when we were actualyl driving it.

[Nishant Jain]

We're getting to be doing our first few dyno runs end of this month. I'm trying to run our old F4i radiator with the new RR engine, so I think I'll try placing the ECT sensor on the radiator itself.

I think the thermostat opens at around 84C. If this also means that water coming into the radiator is above 84C at all times during the running of the engine, I'm guessing my set temp for turning on the fan should be somewhere more than 84C. Right? Or else, my fan would be running all the time!