Hello all,
I have noticed that when the CTS is unplugged the car behaves very good (different): not stalling during cruising, smooth idle, great revving and throttle response.
Today checked on the sensor on the car while the car from cold, idling became hot, a cold CTS shows ~5500 Ohms and HOT is ~230 Ohms, an unplugged CTS logically is 0 Ohms....
Today removed the ECU and located the CTS pins on the red socket #14 & #31.
After a short ride, unplugged the CTS and checked the Ohms on the sensor, 300, plugged the CTS and checked the Ohms on the pins, 300...
Start the car and run, bad, too bad.
Stopped the car, unplugged the CTS and run, good, very good.


Really don't know how to proceed, any feed back is welcome.

 

The ECU is looking for a voltage rather than the actual resistance of the sensor. I would expect the voltage to vary based on the changing resistance in the sensor, but that would depend on the circuit being complete.

If you have checked the resistance at the ECU and that looks OK then I would suspect a Ground fault for the sensor.

I would check that Pin 31 is grounded at the ECU (with Plug in) and check the voltage between Pin 14 and Ground at the varying temperatures,

(unplugged CTS is not 0 ohms, it is infinite ohms)

 

panagiotis:
Are you suggesting you have no OBD codes with the sensor plugged in?

 

Hi panagiotis,

In case this is helpful at all, according to the AJ16 Engine Management System Dealer Training Manual, the engine coolant temperature sensor (ECTS in X300 terminology) is "a negative temperature coefficient (NTC) thermistor," (the most common kind of thermistor). The chart in the manual indicates that at -22F/-30C its resistance should be 26 kohms, so its resistance should probably never be infinite, and looking at the other end of the chart, the resistance should only approach 0 ohms at very high temperatures (250F/120C). Otherwise, its resistance should reflect the ambient temperature (e.g. 2.5 kohms at 68F/20C or 250 ohms at 193F/90C).

The ECM applies 5 volts to the sensor and monitors the voltage across the pins to detect the varying resistance.

Like Ross, I would expect running the engine with the ECTS disconnected to trigger one of the related codes (P0116, P0117, P0118 and P0125). Have you scanned for codes?

Cheers,

Don

 

Thank you Don,

I have to scan it for codes and come back for advices.
Till i find a scan for codes, should i unplug the ECTS and check on the cable the presens of 5V from the ECM?

 

Hello b1mcp,
How can i check that pin #31 is grounded at the ECU? Disassembly the ECU, "touch" pin #14 and "touch" somewhere ground?
I should read the Ohms at this certain moment, then assembly the ECU, run the engine hotter and recheck the above ? and then?
What should be a "ground fault" on the sensor?

 

No dis-assembly required. Leave the connectors on the ECU and back-probe the pins.

1. Test resistance between Pin 31 and known good ground - should be 0
2. Test voltage across Pin 31 and Pin 14 by back-probing the plug. The voltage should change as the temperature of the engine changes.