1986 XJS V12 has been difficult to start over the past few weeks try's to fire but just will not catch.

Fuel is good pressure is ok timing is spot on plugs are gapped ok.

Everything was new a few years back all I can find is a weak spark not a blue angry crack just a weak yellow ping.

Anyone have any ideas on what would cause a weak spark? coil?, plug wires?, amp? or pickup maybe any thoughts would be much appreciated

Thanks

 

Amp and pick up, NO. They simply supply the means for the coil to supply a spark, as in a switch.

The leads, yes.
Coil/s yes.

A quick test. Remove the HT lead from the centre of the cap, the one FROM the coil, and insert a spark plug in the end. Any spark plug will do here. Crank the engine and observe that spark. Is it better than you have at the plugs, or the same???

If its better, then the 12 leads are suspect. Also the carbon brush contact inside the cap centre post, that contacts the rotor, may be worn away, it does happen.

If that spark is basically the same, then the coil/s are suspect.

AND

One item that is proving problematic now, is the condenser cylinder thingy INSIDE the amp. Open the amp, it is fastened in one corner with a small screw, and has a single wire attached to the module. Remove it, and close the amp. It was/is a noise suppressor, and age now is causing many of them to "leak" to earth. Usual symptom is NO or WEAK spark, but not always.

Next item that is age related. The ignition switch electrical section. Connect a Volt meter to the +ve post of the coil, and the other lead to earth. Switch ON the ignition, note the reading, 12++ volts should be seen. Have a helper crank the engine, and watch the readings. They will drop a bit, but a drop to less than 11v or lower, is a sure sign that the electrical section is dropping out during cranking. Easily removed, haha, and easily cleaned, and refurbished.

Once the saprk is returned to acceptable, you shoould be fine.

However,

Whilst you are stooging around under the bonnet, locate the EFI resistor pack, usually screwed to tje inner guard panel, RH side, front corner of the engine bay. Remove the multi ping plug from the bottom face, and spend some serious time cleaning that plug, AND the related socket of that resistor pack. Makes a HUGE difference to Injector firing quality.

 

UM, is this the V12 in the truck, or you got a new toy we dont know about?????

 

Thanks Grant for the input, just helping out a friend who is having issues he does not do the internet or computers, will take a look at the HT side and see what we have.

Any recommendations on plug wires?

cheers

 

Well done.

I do that here for a Jag collector also, so far 4 laptops have bounced of walls, his fuse is SHORT.

Nothing fancy on the leads.

I have always used Magnecor standard thickness.

Whatever you get, you will need 90deg AT the plug for 1 A&B, that a/c compressor is in the way for straights, and maybe 6 A&B. 90deg at the cap for all except the coil.

Dont go to the super thick leads, they simply will not fit is the allowable space, and look crap.

 

V12 in a truck !!! Fascinating!!!


1. While fussing with Jeep no run issue, son and I did the spark plug thing. Six available leftover from my thrash on daughter's Passat. Jeep easily provided that snap and crackle blue spark. Hmm, did the Passat really need new plugs? Hmm, glad I did not toss the old ones!!! Issue resolved, spark good, work on fueling issue. Resolved that one as well. Empty tank!!! Added fuel, Jeep fired right up.


2. One of my easies first on weak or no spark detection, is to jump 12 v to coil + .
if yes, go elsewhere, if no, trace back. Lamp or VOM works.


Carl

 

Magnecor, the brand Grant mentioned, is very high grade stuff and very popular among the V12 crowd.

However, because I am slightly contrarian and (nowadays more than ever) always questioning cost-vs-benefit, I'll add that on my previous V12 I used OE "Rists" spark plug wires with perfectly satisfactory results over a number of years. These were about $45 a dozen years ago.

On my present V12 I'm using "Bosch" (or was it "Denso"?) wires. They're 7mm or 8mm, silicone, and are doing the job perfectly well by my estimation; the engine runs like a watch. About $40 from most sources.

I mention this because, if you're on a budget, using the highest-grade stuff isn't an actual requirement for satisfactory results.....and the Magnecor wires will run about $100 to $180 depending where you buy and which variant you choose.

Cheers
DD

 

I just replaced my plug wires yesterday. I went with Magnecor ELECTROSPORT 8mm wires (blue). Custom made for V-12's. Each wire is marked with the wire #, 1A, 3B, etc... and cut to the correct length and proper plug boot, i.e. 90 degree or semi-angled.

Cost: $145.....and well worth IMO. The Magnecor KV 85 8.5mm wires (red) are over $220 and the 7mm OEM style (black) had a lot of blue arc bleed thru. Just not enough insulation for my comfort with the fuel and electricity in such close proximity.

 

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}-->Nice! The information I got through this blog has really helped me in understanding this topic.

 

Nice! The information I got through this blog has really helped me in understanding this topic.

 

Goodo, glad to help.

I suggest editing your LOCATION, to just suburb and state, this is a public Forum.

When you get a minute, please do an Intro in the New Members Area.

Good luck.

 

I'm not sure I completely agree with this. My '88 had starting issues due to weak spark and I traced it to an excessive air gap on the pickup inside the distributor. This was a Lucas system. Don't remember what the air gap is supposed to be but once I set it correctly it started easily every time.

 

I'll second that, I have had a failing module that would partially work intermittently, and it wasn't heat related. It would just feel like the the car was running on 4 cylinders for a bit and then come back to life on all 12. A replacement aftermarket module didn't change anything which resulted in more troubleshooting and then a new GM module cured the problem completely.