1994 Jaguar VDP No Spark

Robin you hit on the reason for my question about the thermal paste / grease (goo). My understanding is there are both conductive and non-conductive (electrically insulating in fact) types.

Hmm…purported to be good. Was it in a vehicle that was running up to the time the module was removed?

A high-grade thermal paste is 8.5 W/mK, and the heat conductivity of copper is 385 W/mK, or for aluminum 205 W/mK. … With that said, some thermal pastes can be conductive , but the majority of them are not. I believe the main reason for the paste on the ignition module is heat dissipation and not conductivity. That’s why it’s mounted to a large chunk of aluminum. However the question remains, in Test 2 (above in photo) how do you check if the ignition module is grounded?

To be honest I don’t know and that’s why I said “purported”. So unless you have the ability to see or remove the module from a running car yourself, you pretty much have to take the sellers word for it.

Agreed. It is unfortunate we Jag owners are relatively few and far between. Ideally there would be someone local with a compatible vehicle that we could swap parts with for test purposes.

Hi Robin, “Are you sure the ignition module is grounded?”, excellent question and one I am trying to get an answer to. For sure the aluminum plate it is attached to is grounded and the bolts that hold the module to the aluminum plate are also grounded but I really don’t know how to check the module itself. The module housing is plastic, as you know, and the metal (alum) plate under the housing that touches the large aluminum heat sink it is attached to is not accessible. that leaves the connector pins of the module. What else is there on the module to check for ground???

From memory the unit grounds through the base, not via any grounding wire hence the query about the paste, jus try cleaning off all the gunk and give that a try?

Metal ring under and inside where the mounting screw is the ground. Also a ground wire from alum plate to body. Conductive paste is not in the equation, conductive or otherwise.

Thanks exMBman, that makes the most sense to me also and that is definitely a good ground.

Robin, I agree, this has been cleaned and polished with new thermal past applied.

I believe this trouble shooting matrix eliminates the CPS as a possible culprit in the No Spark issue. The resistance measured 1.37k OHMS and Resistance to ground measured 0.L m OHMS. I wasn’t sure about what the 0.L m meant so I did an online search and found this: “When measuring between any terminal to Ground, you always want to see OL. This means that there is no path for current to flow to ground”.

No Spark Summary and Solution

Well it’s been a long thread to finally get to the heart of the issue. I can’t tell you how many hours I have spent reading post, checking components, running down wires and trying to decipher wiring schematics in order to resolve the No Spark issue on my ’94 VDP.

I would be completely remiss if I didn’t acknowledge all the support and suggestions received here on the JagLovers forum, what a great bunch of guys, really, thank you all. I also want to acknowledge a couple of forum guys who went above and beyond what would normally be expected to help resolve the issue. My heartfelt thanks to Grooveman and Mike Stone, and a special thanks to Mike’s brother Mark, who is not a forum member and doesn’t even own a Jag, but got involved at Mike’s request because of his expertise with wiring schematics. At my last post #184 I had really exhausted everything I could think of and was at a dead end, I really had no place else to look and I still had No Spark.

There are four major components that work together to produce spark, the CPS (Crankshaft Position Sensor, ECM (Engine Control Module), IA (Ignition Amplifier aka Ignition Module) and the IC (Ignition Coil). Although I had purchased a new coil and used but purported good replacements for the CPS, ECM & IA there was a lingering question as to the working condition of the used parts. In other words, were the used parts really in good working condition or was I just replacing one bad part with another bad part?

It was here that Grooveman sent me an email suggested that I might find someone local with a ’94 VDP that I could persuade to let me swap out each of my original components on their Jag to see which component was bad, if any. I replied to Grooveman that the thought had never crossed my mind, and while I thought it was a great idea, that was not something I would feel comfortable asking another Jag owner. You can imagine my surprise when I received an email from another forum member with a ’94 VDP offering to test my parts on his Jag if I would send them to him (he was out of state). Of course, I agreed and was extremely grateful for the offer (this had Grooveman’s fingerprints all over it). As it turns out testing each part individually and then in unison they all checked good.

After receiving my components back and installing them I still had no spark. This left me back at No Spark and out of ideas and places to look. It was then I got a call from Mike Stone and his brother Mark, they asked if I had a few minutes to run some test and of course I was delighted to hear from them and to run any test they wish. Mark had been studying the wire schematics and wanted me to pull the Ignition On relay and manually jump across 30/87 terminals in the base, in other words make the connection the relay would normally make. I did this and checked for spark at the HT tower of the coil, not only did I have spark, I had strong spark. I installed the HT lead back in the coil leaving the jumper wire in place at the relay base. Cranked the engine and SHAZAM it fired right up. BAD RELAY.

ISSUE RESOLVED: Long, long story short, it all boiled down to a BAD Ignition On RELAY. The problem was intermittent, early on I checked the relays and they checked good. As reported in this thread there was a couple of times the Jag started and ran fine for a couple of days, then just quit for no apparent reason.

Had I taken abercanadian’s (Larry) advice back at post #15 I would have saved myself and everyone else’s time. That said, I’m not a fan of replacing parts without knowing the part is bad. On the plus side I know more about my Jag and what makes it tick then ever before and made some really great friends in the process.

In closing, I can’t express my appreciation for this forum and its members enough, THANK YOU ALL.

Jag.Man, thanks for the kind words - I’m glad we were able to be of help and Mark (my brother), who was off work convalescing following knee surgery, was glad to have this challenge / learning experience to occupy his time. Through his perusal of the J-L forum whilst ‘boning up’ on the issue and researching potential problems, he was amazed at what a close community of like-minded folks we have here. Very special.

The main takeaway for me from this experience is that although a relay may seem to test good (i.e. you get voltage out of it when you should) the relay may not be able to pass the requisite amount of current for the task at hand. Mark explained to me he sees this occasionally with equipment he services (he is an electronics maintenance tech at a local hospital) and it is presumed to be because the relay contacts are burned and/or oxidized. Note the pictures Jag.Man provided from his ‘autopsy’ of his bad relay. At the end of the day, the mission was accomplished - keeping another XJ40 running and on the road!

Reading Groove’s “success” post yesterday with your ‘thanks for help with no-start issue’ bit, I wondered what had happened to your starting issue and if you ever got the damn thing running again …and indeed you did get going again!

Great news and BTW had to scroll way back to post 15 to see what the heck it was that I advised! :face_with_monocle:

Oh yeah, those damn Hella blue relays again …


Glad all is well (although I have to admit I missed reading the continuing BAFFLING saga) :wink:


Larry, good to hear from you and thanks again for your contribution. The Jag is running great again. The truth is I really needed to know for certain that the issue was fixed so I didn’t have to wonder if it was going to start. I can now take her out of town on a road trip with confidence knowing the issue is resolved. If you ever get to Arizona, look me up.

Let’s hear it for the Stone Brothers, an awesome dual.
Mike your take-away is exactly right. When testing relays there are three (3) test to run:
Apply power to terminal 86 and ground to 85, listen for “click”
With power applied check across terminals 30/87 with multi-meter for voltage
And most important, with power applied check across terminals 30/87 for resistance.

Time for Mark to get a Jaguar :grinning:

Joe, I’m so glad that you finally found the source of the problem. I KNEW :stuck_out_tongue_winking_eye: all along it’s gonna be and easy fix. So we learned something new about these relays. The clicking sound is not enough to determinate the overall condition of the relay. It’s strongly recommended to replace all relays with sealed ones. I would do that if I were you plus if I remember correctly I suggested to open up all the relay modules and fuse boxes and check their soldering joints and reflow them if necessary. Then the electrical system will be set for a long time.

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Thanks Joe77, I have replaced all the relays with the Hella Sealed units and in the process of trouble shooting I did open up the relay modules under the passenger side foot well and checked them. I have not removed the fuse boxes to check them. Thanks so much for your support and contribution to this thread, you are a valuable resource to this forum.

This is also very relevant to the relays in the widow packs, had me fooled for a while until I worked that one out.

This TSB has some good practical info. It occurs to me that to avoid problems resulting from excessive probe insertion (YIKES!):

Perhaps one should keep a couple of Lucar terminals with short pigtails handy. As suggested in the TSB they can be used to check the fit (tightness) of the female terminals in the relay base:

Once inserted the terminal would provide a contact point that would avoid, in fact prohibit, excessive probe insertion.