V12 Ignition Problems
 

Hi everyone, have re-posted in this section in the hope that someone will be able to help.

I have a question about the ignition system fitted to my V12. This engine is fitted in a kit car built in 1988/1989, so not a ‘standard’ installation, and is fitted with 6 x 40DCOE Weber carbs. I didn’t build the car myself so don’t have an in depth knowledge of the setup but it is primarily XJ12 based.

The car has stood idle (garaged) for approximately 14 months and I'm now trying to get it going again. The situation now is that the engine turns over but does not fire. Fuel is good, battery is good, oil & water all ok but I'm not getting a spark. I should point out that the engine was running ok – albeit in need of a serious tune – before it was left idle.

The engine has a Lucas AB14 system installed with single Ducellier coil (all mounted on top of the engine) and I'm not really familiar with diagnosing issues with this arrangement - or any setup for that matter, it’s been a few years! From what I gather reading various comments online (including on this forum) this Lucas arrangement was a standard fitting to the V12?

Also, when checking out the distributor I noticed that the HT leads to the spark plugs are not all fitted to the distributor as per the diagram in a Haynes manual for an XJ12. In some cases the actual lead isn’t long enough to reach the distributor connection indicated on the diagram which seems a bit odd to me – I would have thought they’re all standard. Is there a scenario where the firing sequence would be different to the manual with this type of system - different generations of the engine perhaps?

Can anyone give me any pointers on how to diagnose where the problem may be? For example, what voltages and resistances should I expect at various points around the coil/amp/distributor etc.

Thanks in advance.

You have the "HE" variant of the V12. The earlier "Pre-HE" *might* have a different firing order and perhaps that's what your manual is showing. Just a wild guess....and a stretch even at that.

Primary resistance of the coil should be about .8 ohm

Unplug the distributor wires from the amplifier. It'll be a two wire plug. The resistance between the terminals of the connector should be 2.2k to 4.8k ohms. If not the pick-up coil in the distributor is on the blink

I can't remember any other testing specs at the moment.

The "+" side of the coil should have 12v, obviously

The most common failure point is the module *inside* the amplifier, AC Delco part number D1906, GM part number 10482820 (which has a newer number that I can't remember. Just Google it)

I'll try to find more testing specs for you in my piles of books.


Cheers
DD

Thanks Doug

I'm hoping to have another crack at it on the weekend.

I've also read some info relating to some ignition issues being caused by the crank sensors - is this something worth checking out also?

Cheers

You don't have crank sensors. That was on the V12s with "Marelli" ignition

Cheers
DD

Thanks again Doug, that's saved me some scrabbling around under the car looking for something that doesn't exist!

Cheers

Firing order
 

The firing order is the same for all 5.3 litre 12s (dont know about the late 6 litre but from a balance point of view I would hazard a guess that it is the same). So check that the no 1A cylinder gives a spark when the timing mark is at or near the correct setting. As long as the dizzy cap says 1A that is fine, then fit the remaining HT cables as per book. If the engine was running the dizzy is not 180 degrees out of alignment. I assume you have the correct dizzy, not the Marelli one which is smaller in diameter and has two coil inputs. (It is in effect two 6 cylinder systems governed by a common ECU.)

I didn't notice any markings on the distributor cap, which is a Lucas part and has only one coil input top center.

I'm still a little puzzled why a couple of the HT leads fitted are not long enough to reach the indicated position on the distributor cap even though the cable is marked with the correct cylinder position - it would suggest to me that it was not fitted correctly from the start? I'll post back the differences when I can for further comment.

Thanks for the info though Per.

As I recall the dist cap is notched (or the screws off-set) to allow it being installed in only one position. However, the entire distrubutor might be rotated outside of the normal range....which might account for some of the wires not reaching their proper location.

Just a thought

Cheers
DD

1) are you sure all leads are V12, jag numbered 6 cyl leads too
2) find TDC for 1A and work from there if you suspect the dizzy has been moved. If only the HT cables have been messed with find the timing mark on the crank and check that it is 1A TDC. Then lift the lid on the dizzy and identify which connection is 1A.

Thanks guys

I can only assume the leads are V12 (I didn't install them originally), they're numbered 1A, 2A......1B, 2B etc. I should probably replace them anyway as they're over 20 years old! I'll double check the TDC positioning etc - thanks. I guess it is possible that the actual mounting of the dist itself may be rotated as you suggest Doug - Pers suggestion of TDC/1A check should confirm that hopefully.

As far as the dist cap fixing goes, it will only go on the dist in one position, there are three securing screws which only align with the cap in one position.

Cheers

One more thing: Some aftermarket rotor arms are notoriously bad quality. And if the car has been sitting that long you may have corrosion in the centre electrode in the cap. It can look perfectly good, while being shot. No manner of cleaning etc has made it come alive again for me at least. see if you get a spark connecting a plug directly to the coil.