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Discussion Starter · #1 ·
A couple of weeks back I had an issue with cooling on my GTV. Got stuck in the UKs biggest carpark aka M25, the temps began to rise but at just over 90C the fan didn't kick in like I would expect, kept climbing so I headed towards the hard shoulder by which time the temp gauge was indicating about 120C and no fans running.

A quick bit of diagnostics at the road side indicating pulling the temp sensor should start the fan but to no avail. I spent a while there on the hard shoulder until the motorway reopened before continuing home.

Post issue diagnostics have show the small thermal resistor in my slow speed fan has gone rendering the slow speed fan useless, have bridged the wires while I repair the resistor, so now have working fan but the high speed fan never came on. The low speed fan now comes on once the gauge has gone a small way passed 90C and with the high speed bring temps under control quickly.

Testing using Multiecu, this engages both the high and low speed fans via the ECU so everything seems correct there.

Any ideas why my high speed fan didn't spin up despite dangerous temp levels? Why doesn't the fan come on when the temp sensor is disconnected which seems to be what should happen according to various posts on here?
 

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These are NTC devices ( negative temperature coefficient ) so normally the resistance decreases as the water gets hotter. The ECU should switch the fans to on ( failsafe ) when it sees infinite resistance ( unplugged or a completely failed thermistor.

It is possible that your sensor has failed with a low resistance. The ECU then thinks the engine is running normal/cool and not hot.

You can confirm that by measuring the resistance across the temp sensor. With current ambient temperatures ( approx 10°C ) you could expect to see a resistance of about 20K Ohm and with the engine up to temperature the resistance ( with a working thermistor ) would be in the hundreds of Ohms. Ensure you measure across the ECU temp sensor and not the gauge sensor ( they're both in the same housing on a P2 ).

You could even take out the sensor and put in in a saucepan of water and bring the pan up to the boil. You should see a resistance across the sensor behave a bit like the first graph on this page* - https://www.maximintegrated.com/en/app-notes/index.mvp/id/817



* It won't be exactly the same as that graph won't relate specifically to the device in your car, but it's close enough for diagnostic purposes.

EDIT : I've made the assumption you have a method of measuring resistance - apologies if you haven't.
 

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Discussion Starter · #3 · (Edited)
I think there is only a single sensor on the P2. My high speed fan doesn't come on when I pulled the plug off the sensor, but the temp gauge dropped to zero when doing so. The high speed comes on when tell it do so via the ECU. I think the sensor is fine as the sensor drives the temp gauge which reads correctly and now I've bringed the low speed the fan comes on at the expected temp on the low speed side. Concerned why the high speed didn't kick in.
 

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Concerned why the high speed didn't kick in.
Maybe it wasn't hot enough.

I'm not sure how hot it needs to be to trigger the high speed fan, but it will be quite high as in normal circumstances the low speed will keep things under control.
 

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Discussion Starter · #5 ·
Maybe it wasn't hot enough.

I'm not sure how hot it needs to be to trigger the high speed fan, but it will be quite high as in normal circumstances the low speed will keep things under control.
I guess it could be that, the gauge was getting very high, the warning light hadn't come on but it was up too about 120C on the gauge which seems pretty hot to me!
 

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I have too have a suspect coolant temp sensor, the engine managemnt light comes on intermittently and scanning my ecu brings up error code p0115 coolant temp. sensor

I just happened to find the resistance readings from the repair manual for the sensor see picture below. I spent an hour or more looking for this the other day and found it by chance just now without even trying!

Have a look at the repair instructions volume 3 electrical diagnosis section

.:BerlinaSportivo - Alfa Romeo 155 Q4 and Sports Saloons on the net:.
 
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