1981 XJ12 Early distributor diagnosis

(Bard Th Hesvik) #1

A club member with ignition issues has approached me. He has dismantled the upper part of the distributor as he suspected the pickup to be faulty.

I want to measure the pickup and the check the ignition amplifier (the aluminium finned type).

Can anyone tell me about the correct procedures for this, please?

Kind regards,

(Ed Sowell) #2

There are a couple of write-ups on the PreHE distributor at my website. One describes a complete overhaul, and the other my eventual conversion to the Crane XR700 aftermarket ignition.

The bottom line is the amplifier is the weakest link, and is no longer available. The Crane system is now marketed by FAST www.fuelairspark.com, and the kit no longer includes a 12-slot shutter disk. There might be other aftermarket ignition systems for 12 cylinder engines.

BTW, the Repair Operations Manual (ROM) has diagnostic procedures for the OE system.


(Bard Th Hesvik) #3

Hi Ed,

Cheers for the useful information, but this isn’t a pre HE engine. It’s a very early HE, but with the early type of distributor…


(Ed Sowell) #4

Just to me sure which ignition system you have, where is the amplifier? If it’s mounted on the top cover, down in the vee, it’s like mine. If it’s mounted high, on the left intake manifold, I’ll let someone else handle it.

One last thing, be very, very sure you’ve got a good battery, well charged. If it’s weak, it may crake just fine but the voltage to the amplifier might drop too low.


(Bard Th Hesvik) #5

It is sort of an inbetweener; as mentioned, it is a very early HE engine, but still one where the ignition amplifier had been moved to the top rail above the radiator.
It is definitely sporting the plastic plate with ferrite inserts to trigger the ignition.

In my understanding, the first symptom was that the engine wouldn’t rev above idle speed without choking/flooding and dying.


(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #6

Being an “early HE”, I presume the car has Digital P EFI? There is no trigger board inside the distributor? I didn’t realize the Opus ignition system hung around that long.

You’re describing the Opus ignition system. They were never good, and at this age they’re likely to prove more trouble than they’re worth. As Ed suggested, updating to an aftermarket ignition system is one possibility. Another is a replacement scheme utilizing the later Lucas CEI ignition packaged in an amp that LOOKS like the Opus amp, thereby maintaining the original underhood appearance. Finally, the car could be simply upgraded to the Lucas CEI by replacing the distributor with one from a scrapped car. The Lucas CEI is, in fact, an excellent ignition system, its one noteworthy shortcoming being that the centrifugal advance mechanism tends to seize up – but that’s true in the Opus distributor as well.

Or one could go completely overboard and go with a modern aftermarket EFI/ignition controller like a megasquirt.

(Ed Sowell) #7

The box on top of the radiator is NOT the ignition amplifier. That’s the EFI power amplifier. Unless someone has moved the ignition amplifier up there too. did you look down in the vee to see if there’s a finned box just forward of the dizzy? I’m pretty sure it is the original preHE Opus ignition system.

If I were you I’d put the dizzy ack together and start over with the classic diagnostic procedures.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #8

Jaguar issued a relocation kit to move the finned amp from within the V to the top of the radiator support. I suppose that’d mean later cars came with it there from the factory – or perhaps this one had it moved using the kit. Either way, if it’s a finned block it’s either an Opus or the Opus lookalike. The CEI amp is not finned.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #9

In regard to the original question: In the Opus distributor, the pickup looks like a tiny transformer with an E-shaped core. There are primary windings on the two outer legs and a secondary on the center leg. When a 600 KHz signal is fed to the primaries, they cancel one another and there is no signal generated in the secondary. The plastic rotor contains 12 little pieces of ferrite, and when one of those little ferrite pieces lines up with the pickup, it “closes” the magnetic loop between the center leg and one of the outer legs. That gives one primary far more influence than the other, so they no longer cancel and a signal is generated in the secondary. The amp responds to this signal by generating a spark.

Hopefully that will help with your diagnosis. Obviously, the height of the rotor must be correct so the ferrite pieces line up with one side of the pickup, not centered. I dunno what the clearance between rotor and pickup needs to be, but it’s probably not critical. Problems have arisen from the wires between pickup and amp having trouble handling that 600 KHz. The wires themselves can generate a signal in the output signal line by simple crosstalk. Lengthening the wires to relocate the amp can therefore cause problems. Keeping the wires separated could help with the crosstalk, but increases the chances of picking up interference from other sources.

(Ed Sowell) #10

The gap should be 0.020" to 0.022"

I don’t see how the symptoms suggest an ignition problem. Did you ask the owner how he got there?

(Bard Th Hesvik) #11

Actually Ed, the it is the ignition amplifier I am referring to. It was moved there already on pre-HE Series II XJ12. Please see link to pics.
How he arrived at his diagnosis, I really can’t say, but I have suggested that he may have started his trouble shooting at the wrong end… I don’t have the car here. He came to see me and only brought the distributor pick up and the plastic disc.

I guess the initial questions should be: Is the pickup prone to fail? If so, how do I determine if it indeed has failed?



(Bard Th Hesvik) #12

Hi Kirbert,
Thank you for your input. Please see the link I sent to Ed for pictures of the pickup. You are bang on with regards to the type of distributor; plastic disc with ferrite insets…


(Mike Eck) #13

The resistance between the yellow and black pickup wires should be about 2.5 Ohms, while the resistance between the red and black wires should be about 1 Ohm. If those readings are correct then the pickup is probably OK.

(Ed Sowell) #14

Bard, I don’t recall ever having to replace the pickup or the disk. The reason for ditching the OPUS was the amplifier. One other part that often fails is the ballast resistor pack.

BTW, I have a pickup among my spare parts. Let me know if you want to try it.

(Bard Th Hesvik) #15

Well, these are my readings:

Yellow and black: 7 Ohms (0,7 with meter set to 200 Ohms)
Red and black: 3-4 Ohms (0,3 alternating with 4)
Red and yellow: 10 Ohms (1,0)

Seems a bit off…

Do you have similar expected values for the ign. amp.?


(Bard Th Hesvik) #16

Thanks, Ed! What do you think about Mike’s values and my measuring results? What does your pickup read?


(Ed Sowell) #17

Y-B: 2.7
R-B: 1.2
R-Y: 3.7

These values are pretty close to what Mike has indicated. BTW, Mike’s values are clos to what the ROM says, 2.5 & 0.9 ± 10%

The ROM doesn’t give any static measurements, probably because they usually don’t have much significance for active electronics. It does give procedures for testing things while things are assembled & the engine cranking etc. That’s why I suggested putting it back together and starting over.

Let me know if you want to try the pickup. I also have the rotor.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #18

Yeah, those readings seem off. The next question is how reliable are the readings? VOM’s are challenged when reading ohmage values this low. I’d hate to see a lot of effort expended only to find that the VOM used didn’t give accurate readings in the single digits. Frankly, when something like this fails, I expect to see an open circuit, but we have had reports of crank sensors with high ohmages so I guess it’s not unheard of.

(Mike Eck) #19

A reading of 0.7 on the 200 ohms scale is 0.7 ohms, not 7 ohms, so that value is reasonable considering the accuracy of inexpensive digital ohmmeters. If the red and black resistance really reads 0.3 ohms (and not 3.0 ohms) then that doesn’t sound good. The reading of the red and yellow wires, which is simply both coils in series, shows that your readings are consistent. If you double-check your readings and they are the same I’d be tempted to try another pickup, although I’ve never heard of one going bad.

(Ed Sowell) #20

I used a TPI 133 which I believe to be a pretty good instrument.