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Jaguar Repair/86 xj6 wont start


QUESTION: hi i have got a 86 xj6 it wont start had mechanic look at it he put new computer in it then ignition amplifier then the pick up for dis its got no fuel injector pulse he back prob the computer its got 12v main feed and all it earths and the wire from coil to computer is pulsing but the injector wire ant even at the computer its not pulsing the cold start injector got power and temp sensor as far as he see its it should be going any help wood be nice thank you for your time


Normally the ECU just needs to see that ignition pulse signal and have the powers and grounds at the ECU plug (several) and it then will ground the injectors in groups of two at a time and two times per cylinder firing. It is very rare to find a bad ECU on an XJ-6.

If you tested the injector pulse with a Noid light you need to first confirm that the one wire on each injector from the resister pack (on the fender well) is supplying power to the injector. The ECU only grounds the injectors to operate them.

The cold start injector is independent of the ECU and is operated by power from the start position of the ignition key and grounded by the Thero-time switch in the coolant rail. This would work and fire up the engine even if the ECU were not even in the car.

There is an odd thing about the XJ-6 that is not in any book or normally known. It is easy to test though.

With the battery fully charged take a second fully charged battery and connect it up just like you were giving someone a "Jump Start" that had a dead battery. Now you have two known good batteries connected to the car in parallel and still 12v. Try to start the car.

If the car then starts, remove the existing battery (even though it may be good or even new) and replace it with a "Deep Charge" battery or a "Diesel/Marine" battery.

This is a common ailment of the series III XJ-6. Most of the time when a battery goes bad in a series III Jag, you can put that battery in a different brand car and use it for several more years.

The last resort since you have done a lot of the tests is to test every wire at the ECU to see that it goes where it is suppose to go. If you don't have a good diagram of the injection system let me know and I will post one on my web site for you to copy.

Let me know,

---------- FOLLOW-UP ----------

QUESTION: hi howard thank you so much for your help this car is geting the better off me and the mechanic his had it for like 3 months now we put a new battary in it two day after i printed out the info from you and put jumpleads with a big 4wd battary 770cca on the other end still no go there is power on both sides off injector wires and the mechanic said at the computer pin number 5/16/17/25 pins are earth 10 is main 12v power and 30/31/14/32/15/33 are the injector with no pulse out off computer he like to know number 1 computer pin is coil signal it blinks when trying to start is that right and pin 4 is cranking signal and has 12v when turn on is that right thanks for your help we have talk to people about this and no one likes works on then in Tasmania its hard finding good help now thank you for your time Ron

Hi Ron,

All of the pin numbers you quote are correct. The #1 pin is the ignition pulse signal and it is the single one wire connector laying on top of the intake manifold about halfway back.

You did not tell me if the car starts for a second or two using the cold start valve. It still should start to run while in the start position as that has nothing to do with the computer.

Another test is to just start the car using spray "Starting fluid" by spraying into the air intake to get it running. This test proves that the engine itself is good and the ignition system is good. You should be able to start and run the engine with the spray alone and keep it running by continue spraying.

When you say he gets power indicating on both wires of the injectors, you must have all the injectors unpluged to test for volatage. The reason is that the injectors are ganged so if you only un-plug one injector the power to the other injector is feeding through it to both wires. Un-plug all the injectors then test for power and you should see only one side powered.

Run the engine test with starting spray to see if the engine runs and 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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