Intermittent ignition failure on XJR-S

GlennH · Postby **GlennH** » Wed Nov 23, 2005 11:50 pm

Hi all,
My newly acquired '89 XJR-S has developed a stalling habit!
It runs fine when cold, runs fine at speed, idles OK on the driveway with the bonnet open but the engine dies after a few minutes when idling with the bonnet closed or when driving at low speed.

Once it has died, it will refuse to start until left for a minute or so, then fires up without complaint. I thought I had cured the problem by cleaning up all the connectors on the ignition amp and the coil, but the problem's still there. Having trawled the Jag-lovers forums, I suspect the ignition amp is overheating and cutting out. It eventually gets too hot to touch when the engine is idling. Once it has cooled a little the engine can be fired up. Sadly, I've only had the car a few days and don't yet have all the service manuals for it, nor have I been able to put a scope on the ignition to see if the problem is ECU, amp, coil or the connections in between.
I am picking up a new amp in the morning, but I wondered if anyone here has had similar problems?
Also, can anyone tell me the function of the two wires that run from the loom to the connector on the end of the ignition amp. The connector has a 1/4" and a 3/16" spade terminal, the smaller of the two I suspect carries the ignition pulses from the ECU (disconnecting it kills the engine), but connecting or disconnecting the larger one seems to make no difference to the running of the engine. It works either way.
Thanks and regards,

GlennH · Postby **GlennH** » Thu Nov 24, 2005 11:39 pm

Folks,
A quick update. I fitted the new ignition amp at lunchtime. Engine started without trouble. The car made it all of 4 miles before the new amp died completely. Had to put the old one back on and limp home.
Fortunately it made it all the way home so I can really begin to trace the problem tomorrow with the aid of 'scope.
Regards,

GlennH · Postby **GlennH** » Tue Nov 29, 2005 10:51 pm

It's fixed !

Although I appear to be talking to myself on this thread, here's the fix to add to the pool of knowledge here.

The fault was in the wiring loom adjacent to the ignition amp. The coax cable that runs to the amp from the ECU has a braided shield. At the amp end, the shield stops about 3" short and is connected to the amp via a short piece of wire and a ring connector under one of the amp mounting bolts. The cable manufacturer quite rightly tinned the end of the braid and similarly tinned the end of the wire, quite inexplicably they decided not to solder the two together, but instead just laid the ends together and held them with a bit of heatshrink sleeve. As the temp under the bonnet got too hot, the sleeve 'relaxed' a bit and the ends separated killing the engine. A quick clean up and solder job put an end to this madness.
As a side issue, I had fitted another new ignition amp from Lucas shortly after fixing the fault to see if it ran any less hot than the old one. This one died after 25 miles and I had to put the old one back on. Both new amps have gone back to Lucas for investigation. In the meantime, if anyone needs to swap out their AB14 ignition amp, you are better off getting an old unit from a breakers yard. The old ones can take the heat much better than the new ones!
It seems old Joe Lucas is not only the Prince of Darkness, he's also now the Prince of Silence...

Regards,
Glenn.

hyperxjs · Postby **hyperxjs** » Sun Jan 01, 2006 10:35 pm

Dear glen

only just got your problem glad you sorted it the biggest problem with the 6ltr engines are the coils you shold buy a spare and keep it with you the standard 5.3 will do

best regards paul.

GlennH · Postby **GlennH** » Mon Jan 02, 2006 9:30 pm

Hi Paul,
Thanks for the tip. I hope the coils last longer than the amps

Regards,
Glenn.

nigel1 · Postby **nigel1** » Mon Jan 02, 2006 9:40 pm

Glenn, did you apply the heat transfer gel to the ignition amp before refitting?.

i got a replacement amp from the USA, 1/4 of the price of the best deal I could find in the UK

GlennH · Postby **GlennH** » Tue Jan 03, 2006 9:43 pm

Hi Nigel,
I did it the lazy expensive way. I bought a new Lucas AB14 unit from Jaguar parts (all 200 quid of it!). All I had to do was bolt it onto the manifold and plug it in. Fortunately I got a full refund. Both units went back to Lucas for investigation, but I don't expect to hear anything of the results.
I did take the back plate off the units before I took them back, the HEI module has changed design over the years, the new one has a much softer plastic cover compared to the old units. I'm not sure why they are less reliable.
Regards,
Glenn.