Technical Coolant Temperature Sensor

[yydelilah]

Hi - I have a 2001 Fiat Ducato 2.8JTD where the temperature gauge is only moving a quarter way up the scale. I've changed the thermostat, I've grounded both pins on the coolant sensor and the gauge/warning light seems OK. The sensor seems ok too.
I've also measured the voltage at the the two pins on the connector to the coolant sensor. The one which feeds the warning light reads 12.xx volts. The one that feeds the temperature gauge reads just 7.5v! Anyone know what voltages to expect at these terminals?
thanks

 

[DucatoMiltonKeynes]

The temp sensor is wired straight into the ECU, which then controls the cooling fans and sends a voltage to the temp gauge to show the coolant temp. First change the temp sensor. If no better, check for bad connections, including re-seating the ECU connectors (battery disconnected). If no better, you're looking at a bad wiring loom, faulty ECU or temp gauge.

 

[T14086]

On mk2 ducatos (cant remember engine size) but there are 2 sensors, one for temp gauge and other for ECU.

 

[yydelilah]

The temp sensor is wired straight into the ECU, which then controls the cooling fans and sends a voltage to the temp gauge to show the coolant temp. First change the temp sensor. If no better, check for bad connections, including re-seating the ECU connectors (battery disconnected). If no better, you're looking at a bad wiring loom, faulty ECU or temp gauge.

Hi - thanks for the reply - you were spot on with your analysis. Just after I posted here, I changed the coolant temperature sensor in the thermostat housing (easy job) and now everything is fine. I'd already changed the thermostat prior to changing the thermistor and the problem persisted. What threw me was the 12v/7.5v (to ground) disparity at the sensor terminal. The 12v feeds the over-temp switch in the sensor which then switches on the instrument panel light if the engine overheats, and the 7.5v feeds the thermistor side. Seemed a bit odd to have the 7.5v there and 12v on the other terminal! Without a circuit diagram I was working in the dark. Anyway, I whipped out the old sensor to see if the thermistor side resistance responded to being dunked in hot water, and surprisingly enough it did. But I decided to change the sensor anyway and I'm pleased I did because there was obviously something not right with the old one.
Anyway, thanks again for your advice. Problem solved.
regards

 

[DucatoMiltonKeynes]

Hi, Glad I was able to help. The wiring diagrams are included in the workshop manual available in the downloads section of this site. If you select Ducato, then the ELearn manual, you can download it. It's a bit tricky to install on your computer, I found. What you download is an ISO file, which is an image of the original service manual CD. You then burn this ISO file to a blank CD, using CD burning software. When the CD is burnt, open it under My Computer and you should find a Setup file (as I recall). Double click it to install the program. You will have to keep the CD in your CD drive to run the workshop manual program as it pulls the data from the CD. To use the manual, click on the shortcut the installation put on your desktop, use the top left boxes to select Language and engine type, then click on the Ducato Logo. Now you can select Da (from) 03/2005 or Fino (ending)02/2005 and away you go.