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Jaguar Repair/1985 XJ6 cold start issue



Tested white/green wire at thermal/time switch tonight, no voltage to ground - same with W2 white/yellow wires at large relay on firewall. Cold start injector also does not spray fuel, even when connected directly to 12v + source. I can still feel and hear solenoid in the injector however, when connected to 12v source.

However, car starts on first try now, every time, ambient starting temp was just over 40 degrees tonight. This is an entirely new development that has come about since I have been performing these tests and makes it tempting to forget all of it, but I cannot stand knowing things aren't right, even if I am getting the result I was after.


ANSWER: If you have no power on W-2 of the start relay while the key is in the "Start" position then it is impossible for the starter to operate. Unless someone has rewired the whole car.


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I am just now back around to the cold start issue on my Jaguar and need to say, to your other question, yes, I had just over 30 psi of fuel at rail and at injector. Now for the electrical testing, I changed the batteries in my meter just to be sure, and pulled the coil wire so I could repeat tests several times without the car starting and the verdict is that I am in fact getting almost exactly 10 volts at the thermal time switch wires as well as the W2 terminal on the relay as a helper spins the starter. This is with a deep cell battery showing 12.71 volts by itself.

Where does this lead?


Knowing how the system operates is the key to making a diagnosis. And it needs to be done in order.

This system is the Bosch "L" jetronic and the cold start system is independent from the ECU. It has it's own injector and fuel supply and it operates even if the injection system is not operating.

Unlike the engine injectors that pulse and are powered through a resistor pack and grounded by the ECU in a two pulse per cylinder firing and not timed to the movement of the crank and controls the mixture by the time the injectors are open (called pulse width).

The cold start injector is a 12v injector that is powered by the "Start" position of the key. Nothing happens though unless the injector is grounded. It is grounded by the "Thermo Time Switch". The reason it is called a "Thermo Time" switch is because it only grounds the injector if the coolant is cold. However to prevent flooding if the engine does not start quickly the switch has a electric heater in it so if the starter supplies power through the injector too long the switch will break the connection to ground even though the coolant is cold.

So thus you can run tests to be sure it is working as designed. First it must have fuel pressure (42 PSI) when starting (not 32 PSI) 42 PSI is applied to the system as soon as the flap in the air flow meter opens as air rushes in. As soon as intake manifold vacuum is established, then the pressure will drop to 32 PSI.

To properly test the system you should remove the cold start injector which requires a fuel hose added so you can put the tip of the injector in a jar and watch that the injector sprays in a cone shaped pattern (not a thin stream) This is important because a stream of fuel into a cold manifold will not vaporize very well and cause hard starting even though the injector is opening and trying to spray fuel.

Then the grounding of the thermo time switch needs to be tested cold and warm and repeatedly when cold to see that the electric heater is working.

In the dealerships I worked in we would do a simple preliminary test when we had a complaint about cold starting before we did all of the above. We just sprayed a little starting fluid into the intake when it was cold and if it started right up we would run the above tests. If it didn't start right up we would have to test the engine with a compression test and a ignition timing and ignition test first.

Testing for voltage at the injector is good info after you have established that the injector is NOT operating. The function of the injector is what is important first and then if it is not spraying in a cone shaped pattern, then start testing why it is not operating.

I seen a mechanic in a hurry and short cutting by putting his finger on a cold start injector and triggering it and he said it is ok because he felt it "Click" when activated. After wasting a lot of time on other items it still did not start cold so he finally pulled the injector and found it to just have a thin stream of fuel off to one side due to carbon and varnish on the tip of the injector.

I always worked on commission and any "Come Back" had to be done "Free" so I used as many diagnostic sequences as possible as they are a "100% fix rate". Diagnosis by symptoms and spot checking costs time in the long run even though a mechanic can "luck up" and fix a car by short cutting. I got far better results by running the full tests and in order which resulted in less hassle and a far better fix rate then other mechanics in the dealerships I worked in.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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Import Car magazine

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