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Jaguar Repair/blown fuse/electrical

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Question
1977  xj6 4.2
35 amp fuse for map interior lights cigar lighter power antenna blows as soon as new one installed. Any past experience with this issue? Where do I start to resolve?
Thank you.

Answer
Hi David,

Any time you have a dead short that blows a fuse instantly you need to isolate the short to a smaller area to locate the short. It is from extremely rare to never, that a short will happen in the same place on two cars. I have been working on Jaguars since 1960 and I have never seen a short show up in the same place on two cars.  

There is a procedure to follow and that is to first take a 12v test light and note on the fuse box on the blown fuse, test which side of the fuse is the power side. The other side is of course is the shorted wire.

Next, use a large draw light bulb like a head light bulb as a test light. You can't use your standard 12v test light in this test because the normal circuits will light your standard test like but a head light bulb will burn dim on a antenna motor and on a series of interior light. When the head light bulb is connected to a shorted wire it will burn brightly.

Remove all the wires off of the dead side of the fuse box of the problem fuse. Then connect a jumper wire to each side of a filament on the head light bulb and connect one to the power side of the fuse position and with the other test lead just touch each one of the loose wires that you removed from the fuse position. (You are using the headlight bulb as a test light.) When you touch the wire leading to the power antenna the head light should either burn dimly or not at all. The same is true for the interior lights and the cigar lighter. But which ever one of those wires has the dead short that is blowing the 35 amp fuse is touched by your jumper from the head light tester, it will make the head light burn brightly.

If you don't have a extra head light bulb to use as a test light but do have a box of 35 amp fuses, you can just put a fuse in and one at a time touch the removed wires to the fuse position. You will soon arrive at the bad single wire that blows the fuse. Then you leave that wire disconnected and connect all the other wires to the fuse box and see that each item operates ok and the one item that don't operate is the shorted wire that you did not re-connect.

That only tells you the single circuit that has the short in it. So now you need to find the short in that one circuit.

There is a test procedure for that too, but I need to know which circuit that is. So run the tests and contact me with which circuit that is and I will give you the test procedure for that circuit.

Let me know.

Howard  

Jaguar Repair

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Howard M. Fitzcharles III

Expertise

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

Experience

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.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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