on a picture for a bigger view
Typical 2-pin CTS.
CTS located on 2L 8v engine.
CTS located on 2L 16v engine.
CTS located on 2L 16v Ecotec engine.
Measure the resistance of the CTS.
Test the CTS in hot what.
is a 'Coolant Temperature Sensor''?
The coolant temperature
sensor (CTS) is commonly found on modern injection engines. It measures
the coolant (water) temperature in the engine itself, it is not to be
confused with the thermostat as found on car radiators. The Engine Control
Unit (ECU) uses the CTS value to calculate the ignition timing and injector
How does a CTS
For more information
on the CTS, how it works and where it is located on the engine, read the
How do I test the
These sensors are
thermistors, it transforms heat energy into a electronic signal that the
ECU can read. Its a variable resistor inside that changes value according
to temperature. These usually follow the NTC (Negative Temperature Coefficient)
principle. But other manufactures may use the PTC (Positive Temperature
A NTC decreases (negatively)
in resistance as the temperature increases, a PTC operates in the opposite
direction. The open circuit supply to the sensor is a 5v reference level
and this voltage reduces to a value dependent upon the resistance of the
CTS resistor. Normal operating temperature is between 80ºC to 100ºC.
A new CTS from a Vauxhall
dealer is about £20.
- Inspect the CTS
plug and socket for signs of corrosion. Clean contact pins with a solvent
to remove tarnish marks (this may cause a resistance).
- Check that the
connector on the loom connects to the sensor plug fully, that its not
lose and makes good contact with the pins.
Checking CTS voltage
- If possible roll
back the rubber boot on the back of the CTS loom connector.
- Identify the signal
and earth terminals on the back of the connector.
- Connect the Positive
probe of the DVM (Digital Volt-a-Meter) to the CTS signal wire.
- Connect a DVM negative
probe to the engine (for earth/GND contact).
- Switch off the
engine and allow the engine to become cold. Switch on the ignition only
(to supply power to the CTS) measure the voltage on the signal wire.
Depending on the temperature a voltage of approximately 2v to 3v should
be measured. See the chart below.
CTS voltage and
resistance values (typical):
||4800 to 6600
||4.00 to 4.50
||3.75 to 4.00
||2200 to 2800
||3.00 to 3.50
||1000 to 1200
||2.50 to 3.00
||2.00 to 2.50
||270 to 380
||1.00 to 1.30
= 5v ±0.1v
circuit to earth = Zero (0v)
- Check that the
voltage measured corresponds to the temperature of the CTS.
- Start the engine
and allow it to warm up to normal operating temperature. While its warming
up, measure the voltage from the CTS, it should also be slowly decreasing
(if its a NTS item).
- If the CTS varies
in voltage (or resistance) that's goes outside the normal range, the
CTS works but its faulty as its sending the wrong information to the
ECU. This will NOT result in the ECU warning light to come on, because
the CTS is still operating within its design parameters. This is a common
fault if the engine is being difficult to start warm or cold, yet there
is no ECU warning light on the dash. A new CTS should be fitted in this
Zero Volts obtained
on the CTS signal:
- Make the following
tests and checks if the CTS voltage signal was zero (supply is open-circuit
or short to earth/GND).
- Check that the
CTS signal wire/terminal is not shorted to earth anywhere. Check the
cable form too for splits or cuts etc.
- Check the continuity
along the loom from the sensor to the ECU.
- If the wire loom
appears to be OK and you get good continuity on the CTS signal back
to the ECU check all voltage supplies and earth/GND connections. If
they too appear OK, then suspect a fault ECU (very rare).
Constant 5v at
the CTS terminal:
This is the open-circuit
voltage fault, and will be obtained in the event of the following conditions.
- The signal pin
on the loom connector is not making contact with the CTS.
- The CTS has gone
open-circuit inside the sensor.
- The CTS earth/GND
connection is open-circuit (i.e. the pin is not making contact on the
CTS on the engine:
You can make a resistance
check on the CTS too. You can do this with the CTS in the engine or it
can be taken out and submerged in some hot water. When you check the resistance
with the CTS still in the engine, you do it with the engine/power OFF
and the CTS loom connector disconnected.
- Disconnect the
CTS loom connector.
- Place the Positive
probe on one pin and the Negative probe on the other pin on the CTS
- Set the DM to ohms
to measure resistance. With the engine cold, make a resistance check
against the chart above. With a cold engine (20ºC) the coolant
temperature should be within ±5ºC of that figure.
- An allowance should
be made for the temperature obtained by the probing the outside of the
CTS or coolant passage. This is because the actual temperature of the
coolant may be hotter than the surface temperature of the CTS.
CTS off the vehicle:
- Remove the CTS
from the engine.
- Place the CTS is
a container of water, do NOT submerge the contact pins in the water,
just the probe of the sensor only.
- Connect the Positive
probe to one pin and the Negative probe to the other. Set the DMM to
ohms (to measure resistance).
- Measure the CTS
resistance in cold water first. Compare this value against the chart
above. Then measure again but with hot water (or known temperature)
and again compare the result with the chart.
- If no change of
resistance or no resistance can be measured (i.e. 0 ohms or infinity
measured) then the CTS is faulty. Replace with new sensor.
based on text in the Haynes
Books series and peoples personal experiences.