Coolant temperature sensors - how do they work?

[RustyNuts]

Hey all!

 

So the safrane has the cooling fault that means the fans no longer work via the ECU. I've bodged up an arduino override but that has the shit effect of mysteriously killing the dash temp gauge while causing other things to start blinking.

 

The arduino is reading voltages from the temp sensor of about 4.9V to 5.3 and it fluctuates very quickly with nothing precise being available. Today I started it from cold and it was still measurng 5.13 on a cold start - which is a voltage it reads when the engine is hot! Naturally the fans spun up straight away wasting battery power...

 

So.. can anyone please tell me what a bosch motronic ECU is actually doing with this sensor? - Is it measuring the current flow, voltage (unreliable), planetary orbits, hyperspace windows... ?

 

I'm seriously stumped with this.

 

Thanks,

 

Rusty

 

 

[Kiltox]

My understanding is that the voltage changes with temperature due to a change in resistance - being a Renault anything is possible though!

[dollywobbler]

Don't forget there are usually two sensors on the cooling system. Usually one on the rad for the fans, and one on the block for the ECU (for fuelling) and temp gauge. Mind you, the XM had only the engine sensor - and would often sound an overheat warning and put the fans on for no reason.

 

So yeah, French.

[castros_bro]

My understanding is that the voltage changes with temperature due to a change in resistance - being a Renault anything is possible though!

 

As yer man says, remove the sensor and connect to a £2 multimeter and put sensor in a saucepan of cold water wiv a thermometer on yer nearest gas hob and take readings from 20C to 100C. Post up here the plot of temp vs resistance and the part number of the sensor.

[scaryoldcortina]

Normally the temp sensor on a motronic car will be the one on the top of the engine somewhere, and it changes resistance with heat with a big swing from cold to hot (3k cold, 2.5k at 20 deg, 300 ohms at 60 deg for example). Because one side is connected to ground the ECU can use the voltage drop to read the resistance. Seeing as this doesn't seem to be happening I wonder if your arduino input is messing up the ecu, try putting a big resistor (100k at least) between the sensor and your input.

 

Other than that, what voltage do you get on the temp sensor wire when it isn't plugged in, and have you tried reading the resistance of the sensor at different temperatures?

[RustyNuts]

Don't forget there are usually two sensors on the cooling system. Usually one on the rad for the fans, and one on the block for the ECU (for fuelling) and temp gauge

 

A point I forgot to mention!

 

Yes this engine has two sensors - one at the coolant outlet / top pipe which goes to the radiator and another at the side of the block. Renault called that one the oil temp sensor.. but if you unscrew it you get coolant out. We think they named it such to avoid confusion between the two (and i say we because it had the local renault dealership stumped to see it named an oil sensor when it's a coolant sensor). Nothing on the rad though.

 

Anyhow, it's the 'oil' coolant temp sensor i've taken the voltage readings from and this supplies the dashboard temp gauge and air con unit - so it seemed like an ideal choice. The other one supplies the ECU directly and is in a pain in the arse location to access and I have no spare connectors to make up a patch that i could get the voltage from.

 

So.. what I don't understand is why the 'oil' coolant sensor is only changing very mildly and why it has the same voltages when cold as it does when hot unless it's actually putting out a digital signal.. but that would be an overkill.

[Asimo]

Try Bosch for a data sheet on your sensor:

 

   http://rb-aa.bosch.com/boaasocs/;jsessionid=30A5E58CEA6A94AF739D0BC0C475D7F0.sundoro2?ccat_id=130