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We do off the vehicle bench testing and ultrasonic cleaning of fuel injectors for all makes and models of Jaguar vehicles, that are equipped with fuel injectors.  We also test and clean injectors for Porsche, BMW, Ford, Audi, Buick Grand National, and multiple other vehicles.


This page is devoted to testing the CTS (coolant temperature sensor) and the CTS wiring harness on your Jaguar.  The CTS, the harness wiring, and the harness connector, are all prone to failure.  The CTS pin contacts, and the wiring harness contacts are also prone to failure due to corrosion.  Failure of any of these items, will cause a no start, a hard start, or if the engine starts, poor idle, or overfueling, and a possible shut down of the engine shortly thereafter.  The usual engine result is it will overfuel, and cause a no start.


Prior to discussing failure of the CTS, or the harness wiring, we need to understand the function of the CTS....


The CTS is an important primary engine sensor, and is typically threaded into a port on the B bank thermostat housing...(for the V12 engine). The CTS "measures" coolant temperature, as a heat resistance value, and transmits that value to the engine ECU,  The value is measured in ohms resistance. The mapping table within the ECU interprets the resistance value as engine coolant temperature.


A cold engine requires more fuel.  More fuel is required on a cold engine, because the fuel injected into a cold engine does not evaporate completely  (engine heat aids fuel evaporation), so additional fuel is added to aid in a cold start.  The added fuel is based upon the mapping table of the ECU, which is measuring the resistance value of the CTS, and is adding fuel accordingly.


Aditional fueling is likewise added by the ECU, based upon resistance values of the CTS, to overcome engine friction, which is another factor to consider on a cold engine.  The ECU has been mapped accordingly to provide an injector pulse width to provide enriched fueling (on a cold engine) to overcome engine friction, and the reduced atomization of fuel.


As the engine begins to reach warm up temperature, the CTS resistance (ohm) value decreases and the ECU begins to reduces injector pulse width accordingly....adding less fuel.  Once the engine is fully warm, there should be no pulse width correction added by the ECU.


NOTE !!  Prior to conducting any test shown below, it is recommended you read everything first, and then select the method(s) most suitable.  Also, try the "quick test" first, which is detailed further down this page.


The CTS can be tested with an ohm meter.  Do not start the engine.  Pull the connector from the CTS.  Touch one probe of the meter to one pin on the CTS.  Touch the other probe of the meter to the other pin.


The resistance values at various engine temperatures shown below should be within 10%.


  0  degrees C  (32F)      5.9 kohms

10  degrees C  (50F)     3.7  kohms

30  degrees C  (86F)     1.7 kohms

50  degrees  C (122F)   840 ohms

70  degrees  C (158F)   435 ohms

90  degrees  C (194F)   250 ohms


It is also possible to test the resistance values with the engine warmed, but NOT running.  Start and run the engine to full warm up.  Turn off the engine, and remove the connector from the CTS.  Test as per above.


Testing the CTS on the vehicle requires some method to measure the engine or coolant temperature. An infrared temp gun is useful, aimed at the thermostat housing on the B bank side.  See table above for engine temps...cold and hot.


If you don't have an infrared gun, you can remove the CTS and test it in a pan of water. will loose some coolant when removing the CTS, so something to plug the hole may be advisable.  Maybe a rubber plug, fishing cork, or similar.


On the stove, you'll need a cooking thermometer to measure water temps.  Pliers to hold the CTS...(an assistant would be helpful) and your meter.  Measure the initial temps and values without warming the water.  If you want to measure for values at the lower temperatures, add some ice to the water.  Begin heating the water, and check as per above.


The "Quick Test"....


If you suspect the CTS is causing a problem...try this first.  With the engine OFF, disconnect the connector from the CTS.  Use a bit of wire, paperclip, etc. and plug one end each into the harness connector sockets.  Doing this "fools" the ECU into thinking the engine is hot, since there is now no (or minimal resistance) measured.  The ECU will not add pulse width correction to the no additional fueling is supplied.  Now start the engine, and observe any difference in operation.  If there is a difference, probably the CTS is weak or bad, and needs replacement.    ***   If there is NO difference...there are TWO possibilities.  A)  The CTS is good, or...  B)  The CTS wiring harness is shorted or has a broken wire(s).  The "Quick Test" is over.  


If in the "Quick Test" you got NO difference, you now need your meter to check for item B above.  It may be that the wiring to the CTS is faulty, and the "jumper wire" didn't work because the wire was faulty.


To test the CTS wiring harness....on a 5.3 liter HE V12...


Leave the CTS wiring harness connector plugged into the CTS.  Locate pin 5 and 19 at the ECU.  Backprobe these two pins with your ohm meter.  Compare with the log provided above, for correct ohm readings at the current engine temp.  The ohm reading should be very close to to the ohm readings when just testing the CTS by itself.  Have an asst shake/wiggle the CTS wire harness, and observe the meter.  If the ohms vary from the ohm reading on the CTS, the harness is faulty, or the connector is corroded.  Clean the contacts and recheck.  If no difference, the CTS harness is faulty

Info provided by a series 3, 1986 Jaguar XJ6 owner, on April 2017, says the pins at the ECU are 13 and 17 (17 being ground).



The above testing procedures require a fundamental knowledge of using either a digital readout/or analog meter, and setting the meter dials on the correct position.   If you are not able to do so, I would suggest have a competent shop do these tests.


Alternately, if you like to spend money on parts that may not be faulty, buy a new CTS and install, and see if it cures your problem.  If it doesn't, the CTS wiring harness may still be suspect, and requires checking by a competent shop.


The CTS, and the wiring harness is prone to failure.  Some have labeled the CTS, as a Constant Trouble Sensor.


A very brief synopsis from a site which I frequent...


The owner of a XJ6 Jaguar had starting problems.  The engine would only start/run with the fuel pump relay unplugged.  If the fuel pump relay was installed, the engine would die out immediately.  The vehicle would actually drive around for 15-20 minutes with the fuel pump not running.  The owner had 45 days earlier, replaced the CTS.  After perhaps 75 posting for diagnosis (which included testing the entire fuel system, pressure testing of fuel delivery, testing the fuel return system, testing fuel non-return valves, installing a "test" ECU, o-scoping of injector pulse width,  I decided to thoroughly read back thru everything that had been done to remedy the crazy engine running with no fuel pump.


I suggested a retest of the CTS, and the wiring, even though the owner has replaced the CTS.  The CTS wiring harness proved to be faulty.,,and most likely the CTS wiring was broken during replacement of the CTS.  The harness was tested and proved to be faulty with the above testing procedures.


The summary conclusion was...


1)  The break in the CTS wire harness caused a "false signal"  to the ECU.  The ECU "thought" the engine was stone cold... and increased injector pulse width, adding more fuel.  When the fuel pump ran under this condition, the engine immediately died due to overfueling.

2)  When the fuel pump relay was "unplugged", the engine started because there was enough engine cylinder vacuum created to suck fuel from the tank, and maintain a running engine.  The engine ran under this condition, because one wire on the CTS wire harness was broke...the ECU thought the engine was "stone cold", and increased injector pulse width, which supplied additional fuel, which compensated for the lack of fuel pump pressure.


Hoping any portion of the above procedures may get your Jag...back on the road

This page last updated...4/13/2017.

SD Faircloth

phone 1-904-289-2020

Jacksonville, Florida   USA   Zip code  32234

Go to my home page at  HOMEPAGE