Guys
Interesting condition yesterday. My car, having been performing perfectly recently, flatly refuse to start yesterday morning when I tried to take it out of the garage. The car usually starts instantly, therefore tested for the following:
Injectors clicking: yes
Fuel: yes
Power to coil positive: yes

Therefore deduced something to do with coil switching system, ie Lucas ignition amplifier or Lucas dizzy electronics on my car. I had a couple of years ago replaced the HEI module in the amp as a precautionary matter, so I was fairly sure it was not that. I knew that the shielded wire to the ECU was 100% for the same reason.
The WoOZ (aka The Great XJS Prophet Grant Francis) has long held that the condenser inside the Lucas amp goes home and if so, car will not start. I remember this fact and decided to remove it from the amp, which is very easy to do as it just unscrews from the amp chassis and unclips from the HEI module once the amp is opened up. (We will gloss over at this point that I had brainfade and clipped the zener diode wire first by mistake, so that was reconnected ASAP - it has been a hard week of partying...)
The pic below shows the Lucas amp interior, it is part labelled C that is condenser that is removed, A is the zener diode that apparently is an over-voltage protection device for the ignition electronics in the event of a plug or HT lead failing.
Anyway, replaced amp, car started right up. Interesting, huh?
Greg

 

Very Interesting and very useful to know but as I have got Marelli ignition, as I know you pointed out to me sometime ago.

This should be of mega interest to all the Lucas 'bods'

 

i have always said Jags are from another alien planet, they sometimes do things without any rhyme or reason!

they for sure can be possesive of there owners!, or the other way around,LOL.

great you got it fixed.

what do you think that condenser is for?

 

The Zener Diode is there to suppress the high transients generated (large voltage spikes), the zener acts like a voltage regulator.

The condenser is a capacitor its primary purpose is to prevent arcing as the points open, if this fails (it will usually short out) and prevent the primary of the coil discharging.

 

Warrjon
Re: arcing. I knew that is what condensers did on a points ignition, I did not know it was needed for that purpose on the HEI, and presumed it was there for radio interference or some such. Do you recommend replacing the faulty one with a new one then?
Greg

 

NOPE.

Save the money for spouses 51st.

10 years now, NO condensor, NO noise, NO issues.

I dont even run that silly "Shielded wire" from the amp, until that wire gets inside the cabin (complicated to explain to those that have not delved that deep into the XJ-S, and Greg has I know), and my amp is NOT in the engine bay anyway.

Less weight, so more SPEED, whoopee.

 

Thanks Grant. Will (not) do!
Greg

 

DOH I'm stupid sometimes I realize you have HEI. The condenser on the HEI does the same thing but as there are no point its main purpose is noise suppression and smoothing spikes, which the Zener diode will take care of to a point.

If you get ignition noise in the stereo put a 1000uf 24v electrolytic with a small 0.1uf in parallel across the radio power lead.

 

Interesting ...
What is the white rubber thing (part "B" in your picture) ?

 

That is "The Blob".

It contains 2 resistors.

One is for the tacho.

One is for the ECU shielded wire to pin #18 of said ECU.

First one dies, NO tacho.

Other one dies, NO ignition pulse to the ECU, dead engine.

Neither has any input to spark etc.