In a xj12 1988 5.3 HE lucas amp... I have one direct question..

The cable that connect the amp with the ecu (pin 18) that is coaxial, in the same loom goes this cable and another one... So the loom at the end where it connects to the amplifier ends in two.. one goes to the amplifier, the coaxial one, and the other?

I have this one disconnected because I don't know if it just earth wire or goes somewhere...

Any help please?

 

If you mean the shielding on the outside of the inner wire, it is an earth and should only be earthed at the ECU end.

 

No no, I meant the following... inside the shielding cover where its go the coaxial cable, is also another cable...

So Inside the loom are two cables, one the coaxial one that goes to the amp and pin 18... And another cable, normal one...

 

What colour is the other one? It may well be a connection to the ECU for one of the sensors or TPS?

 

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Colour is black... And length is same as the coaxial one, so has to be connected somewhere close to the amp...

Car starts good... So engine is running... I have not tried the car yet, but after 5 minutes at idle, seems to run good (only problem detected by now is idle hight (1.300rpm)

 

Black on Jaguars of this era is always earth.
1300 rpm idle is a stuck AAV and/or butterfly valves on the throttle intake adjusted too far open.

 

​​​​​​Ok... And regarding idle, I will check it..

Right now I'm just testing the engine, and idle sometimes in the 10 minutes that I had run it, sometimes it goes to 700 and again to 1300, and then 700... In a fast move... I will do a video and postit..

 

This cycling is because the TPS says the throttle is closed but the revs are above 1400, so the ECU cuts the fuel until the revs drop, then fuel starts and revs rise, and so on.
This is called the over-run cutoff, it means that when you take your foot off the throttle to slow the car down, the fuel is cut to enhance fuel mileage and to enhance engine braking.
The AAV is stuck open and that is the cause, or a vac leak, or throttle butterfly adjustments.