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Jaguar Repair/the forementioned short and...


QUESTION: 1977 XJ6L 4.2 again.
Hello Howard,
Just read that Lucas auto fuses and USA auto fuses are rated or function differently.
Simply put a Lucas 35 amp is rated to blow at 35 amps while USA is rated to run at 35 amps.
The article included a conversion chart stating a Lucas 35 converts to a USA 255.
Any insight to this?
Thank you,

ANSWER: Hi David,

Over the years I have used both in British cars and even though there may be some difference in the load that either one can stand. The difference must be slight as I never had a problem using either the US type or the Lucas wire fuse. For example I tested several US and Lucas 35 amp fuses  to see if a dead short would burn a 14 ga wire (most common size in British cars) rather then blow the fuse and I found that either fuse would blow before it melted a standard Lucas 14 ga wire. I have found a few finer wires then 14 ga in Jaguars (mainly in the XJ-S) and very few in the XJ-6 cars but never a 35 amp fuse on that circuit. Keep in mind that 35 amps is the same in England, Germany, Japan and the US.

You have to use amp size fuse to be well above the "Start" amp draw like a 12v electric motor. 12v electric motors do draw the most amperage when they start up then the load decreases as the motor runs. The application I found to be the most troublesome is the windshield wiper motors. When you stall any electric motor it draws much more amperage and especially on a dry windshield where the motor struggles to move the blades thus the amperage goes very high and can blow a fuse.

The amp meter in a Volt meter is not usually high enough to use to test a car circuits and very few mechanics in my day would go to the expense of buying an amp meter to test large amp draws.

That is why we often used a spare head light bulb as a test light on circuits that were known to blow fuses. By testing a suspected wire with a headlight bulb and put it in series with a power to the bulb and use a suspected circuit as it's ground and if it lights the headlight bulb brightly then that circuit has a dead short.

If you follow my instructions on the tests of the problem fuse you can not possibly fail to find which of the several wires is the shorted circuit and blowing the fuse. Once you ID that wire you connect up the other wires back onto the fuse and operate all the other things that will operate. This leaves you with the shorted wire off of the fuse box and also now you know what item don't operate. Then you take your own long jumper wires from a battery source and try to operate the failed unit (what ever that is) with your jumper wires thus you have bypassed the wiring harness. If the unit works with your jumpers then you know that the short is in the harness (worst case) However if your jumper wires spark a lot when you try to operate the failed unit then the unit is the fault.

Let me know.


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

QUESTION: Hello Howard,
Managed to isolate the short from the circuit and the fuse now holds. It was the purple and brown wire. Tidied up in order to drive to the laundromat as it is raining.
As I was pulling away from a curb gently the car jerked to a halt abruptly like either the brakes locked up or the tranny went into park. This condition lasted a few minutes and the car would finally move but deemed like one of the front brakes was still dragging as the car wanted to pull to the right.
Master cylinder?  Vacuum assist?
Thanks again,

Not, a master cylinder nor a vacuum booster because neither has the ability to lock up only one wheel

I would suspect either a sticking caliper piston or more likely a bad hydraulic brake line at the caliper.

I would try turning the steering wheel in the same direction as you did pulling away from the curb and see if you can duplicate the problem. The flex lines have a woven case so when the inside rubber swells up, it closes the line off on the inside especially when the wheel is turned sharply. If that is the problem be sure to replace all the flex lines. And be sure to use DOT 4 fluid not DOT 3 as some books state.


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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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