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Jaguar Repair/87 xj6 no start


QUESTION: stored my xj6 in the garage for winter now wont start. I have checked the spark and all the grounds. I have replaced the ecu,temp senser,ignition module,all three relays and the fuel pump and filter. I have 45 lbs at the rail.The noid light pulses but very weak. could it be the ignition amplifier.I am beginning to run out of ideas. thanks for your time fred

ANSWER: Hi Fred,

Try this before you do anything else. Charge the battery up and then AFTER you charge the battery connect a known good jumper battery to the battery in the car as though you are jumping off a car with a dead battery.

If it fires off and runs, put a new deep charge battery in the car. (you can use the existing battery in some other brand car possibly for several more years.)

This was an odd thing about the Series III Jags.

The function of the Ignition amp is to ground the coil when it is told to do so by the pick-up in the distributor. A pulse signal is sent to the ECU to tell it when to ground the injectors. (two times per cylinder firing) The temp sensor and the airflow meter are the main input signals to the ECU to tell the ECU how long in milliseconds to leave the injectors open. (mixture control) The O-2 sensor and the air temp sensor are minor mixture control. With intake vacuum on the pressure regulator the pressure is about 32 PSI and when you open the throttle the manifold vacuum drops and the fuel pressure then climbs to about 42 PSI as you tested already.

Unlike most cars, the series III Jag fails to start and run when there is not sufficient amperage to run everything. This is why a deep charge or diesel battery is needed. The simple test is to have two good batteries connected in parallel.

When we got a ser III Jag in and not starting into our dealership we did the above test first.

If that don't start it you must first test compression (throttle open#, then intake manifold vacuum #throttle closed#. If that is correct and only then do we spray starting fluid into the intake to try to start it. If that don't start it, you for sure have no ignition at the spark plugs. If you had not so great compression and vacuum, remove the plugs on the exhaust manifold and adapt a low pressure gauge to the port and spin the engine over with the throttle open and see if you get any pressure after spinning the engine over for a good time. The series III Jag will not run at all with pressures above 1.5 PSI. If you have high pressure, either one of the two CATs are stopped up or the mufflers. #not uncommon on Jags that have sat for some time)

let me know,


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QUESTION: Thanks Howard I have done the bat. test and it didn't work. The car starts and runs with starting fluid. I don't think it is a compression issue. The noid light takes a few revolutions maybe ten or so before it starts to pulse then it pulses almost on everyone with a week flash is there a ground wire that I cant find or a broken wire some place I have been on this for a month and still cant figure it out Thanks again Fred

Yes, there are several grounds on the ECU so you need to test each. Also voltage to the power wire at the ECU. (Pin #10)

There are several grounds that need to be tested and the worst case is to test each wire in the ECU plug to see that it goes where it is suppose to. I always test grounds and powers first though.

Go to my web site and copy the pages of the injection system diagram as it shows the inside diagrams of all the external components that operate the ECU. Thus it makes it easier to run tests.

Http://  then change the suffix to XJ-6Inj21x1.jpg and XJ-6Inj31x1.jpg and finally XJ-6Inj41x1.jpg

I have used this diagram and component locator for years since the 80's and worked on many ser III Jags and I still own two of them. The ECU is a Bosch unit and over the years I have only found one failed ECU and it had been under water. But have lost count of the replaced components that feed info to the ECU.

Keep in mind that there are major and minor influences to the ECU and the major ones are the coolant temp sensor, the airflow meter and the ignition pulse signal on Pin #1.
The O-2 sensor and air temp sensor are minor influences. The cold start system has nothing to do with the ECU. Fuel pump has nothing to do with the ECU. The Bosch throttle position sensor is nothing but a full throttle switch and only for full throttle enrichment.

I hope this helps, let me know,


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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.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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