Burnt Marelli ingnition module

1994 V12 convertible. Big loss of power three months ago had it towed home and put repair on back burner.

Looked at it today and noted smoked driver side ignition amp and wiring. Was thinking the rotor and cap might be bad but have not looked at those.

Would it be okay to repair wiring and replace module then hook up an oscilloscope to test further, or should I visually inspect cap and rotor first?
I generally like to test one thing at a time to rule out possibly disturbing something else and possibly causing an additional problem. However if a defective part is causing the amp to burn out would like to rule that out beforehand.

Inspect the cap and rotor first! Make sure that it has not had to classic Marelli failure in the cap or rotor. If you are not aware of it take a look at Kibert section on it in his book for this car. (http://www.jag-lovers.org/xj-s/book/XJS_help.pdf). Sounds like it may have had this failure, It could also just be that wiring to that cylinder banks amp.

Thanks. Will have a look.

1 Like

I don’t know anything about the 6.0 V12 cars, but if this happened to a 1990 5.3L V12 like ours I would remove the spark plug leads to inspect their condition, the spark plugs to inspect their condition and gap, the distributor cap and rotor for condition, and the exhaust system to inspect the catalytic converters.


Thanks Paul. I just bought the car with several known mechanical problems that are within my abilities to repair. That car cruised 80 mph on the interstate for an hour like nothing. Pulled into get gas and before I could get to the pump it just fell on its face.

Pulled B lead from coil to distributor and it still ran bad. Plugged it back in, pulled A lead and it would not even start. So I knew there was a bank out.

Glad I did a lot of reading up on this and it happened while idling.

Pictures of the cap. Interestingly enough the previous picture of the ignition module with cracked insulation wiring is bank a and working.


My novice eyes have detected nothing unusual with these two items.

There seems to be a lack of silicone sealant in the bottom of that rotor. Otherwise, all looks OK.

The pic of the underside of the cap clearly shows the metal sleeve surrounding the carbon brush that I recommend be demanded of replacement caps.

Geez, some people still have oscilloscopes these days? I haven’t seen one of those since I was a teen … :open_mouth:

Speaking of which, which brands (popular) of caps have that feature, Palmdude, being that Marelli is N.L.A. (if it ever had it?)? :confused:

Thanks for taking a look. On to the other suspects; wiring, coil. ignition module, trigger wire from ECM. Hope I find something before that bit of expensive equipment.

Ohmed the coils. Both had .6 primary and about 5200 secondary resistance.

Also noticed gasket between distributor and cap is missing. Rather than spend $20 will cut my own. Anyone know the thickness of the gasket?

About as thick as a cereal box! (Raisin Bran works for me.)

Is it me or is that plug missing a wire? My Marelli system has 3 wires going to each coil not two.

Nevermind It looks like your model is different than the 1990-1991 Marelli systems. one coil does in fact get two wires and the other gets three. the wires are shot though so that might be your cause.

diagram: http://www.jagrepair.com/images/Electrical/Elect-Tech%20All2/90%20-%2091%20XJS%20Electrical.pdf

Thanks Dave. Just what the Dr. ordered, more fiber.

That was b bank. A has three, one is for the tach.

Saw one spot where the insulation was gone. Noticed a few more. Wire is shiny so may be recent. Plug wires seem very pliable and hand numbered. May be recent replacements and brittle insulation may have cracked in other places, wire break or coil wires shorted.

Wow … :open_mouth: I’d say guard that one with your life … it even looks to be in VERY good shape, Bret … :+1: