I'm new here and frankly new to jags, but I have always loved the xjs and I decided to buy a high mile 91 to get my feet wet, something I could drive and fix up. I recently did plugs and wires, cap and rotor bc it had a slight misfire at idle. Upon first startup, I had no spark, and the 12v power wires going to the coils from the ignition amps caught fire. I luckily was looking when it happened, and extinguished it quickly enough to save the loom. Upon further testing, neither coil has spark but the key ground on both coils is there and I'm getting 12v on both coil power wires. The coils ohm out okay so I'm really at a loss. I have no idea what the issue is and I'm unsure what the problem was to smoke the wires to begin with. Nothing is shorted according to my multimeter.

Any ideas?

 

The under bonnet wiring gets very crispy from heat & age, and disturbing the wiring looms and connectors during regular service always runs the risk of damaging something unknowingly. Wires will crack rather than bend when stressed. The loom most likely started smoking from excess load or resistance on a damaged conductor. A wire that's hanging on by a strand or two will show continuity on your meter, but when a load is applied it will overheat.

I would recheck all connections and condition of your wires, even if it means taking apart the wiring harness to inspect. If it wasn't damaged before, it will be now after catching fire.

 

I had this issue on my car. I bought new connectors (where available) and silicon high temperature cable and built a new loom.
The ‘93 has the engine and injector loom together, but earlier years the injector loom is available from SNG for a reasonable price. The lowest quote for the 93 was £2.5k to make a new loom (as no longer available). It took three evenings to unravel the old and replace old for new keeping the shape. Cost £250 for cables, connectors and fabric tape. The only tricky cable to find was the coaxial cable for the crank position sensor. This is a better solution either way to buying a like for like replacement which will do the same thing.

 

I've begun the process of disassembling the loom, and have started to replace the wire that caught fire, though thus far I can't find any spot on it that would've caused it. I'm really unsure how the ignition amps could drive enough current through that wire to melt it, I'd have thought they'd failed lo g before that. That's my only working theory right now is one decided to pick that moment to go and nuked both of them.

 

The problem is that the insulation drys and cracks. In combination with any oil / water it tracks down the wires. Mine was running, but in the dark you could see the small sparks. Though I guess mine included more than just the injector loom. The wires were like straw. Quite satisfying to rebuilt though with nice new and much better wires!

 

Hi jgremmoss

It does sound like you could have had what is known as a 'Marelli Fire' which is where there is a 'Short' underneath the distributor Cap, that can quickly burn right through and most usually caused by poor maintenance like for instance either bad wiring 'Plug Leads' etc or not changing the Spark Plugs underneath the 'Air Con Motor'

So the first thing to do, is have a look underneath the Distributor Cap and see if there are any signs of Burning

Whatever it was you were lucky you caught it in time

 

I was glad to have seen it when I did, if I hadn't been looking it would have taken the loom and possibly the whole car. The cap and rotor were both new, that's why I've been hesitant to suspect immediate failure from either. I'll put the old parts back on tomorrow and see if I have a change.

 

Have you renewed the HT leads and the pluigs? This is particularly important on a Marelli car.

 

yes, that was actually what I was doing when the fire started. 12 new plugs, wires, cap and rotor