XJ12 SIII Despair

[David27]

Help! I need cheering up. The non-starter still refuses to budge. Fortunes have been spent, my family think I'm mad, the bl**y thing turns over beautifully, catches but just will not keep running. I've tried: new battery, new ht leads, new ignition coil, new spark plugs, fresh petrol, new distributor cap, new rotor arm, new air filters, new water pump (hasn't started it since we changed it), replacement ECU (2nd hand), new connectors on some wiring, new thermostatic switch on electric cooling fan and the list goes on. I've had it looked at by a mobile mechanic who initially thought the head gasket had gone and persuaded me to have it recovered from Diss to his workshop in Ipswich; it turned out to be the ECU. So he eventually gave up and I had to pay for another recovery truck back from Ipswich. Then I sent the ECU off to a company in Poole for testing (cost £60), they said it was faulty and wanted £450 to repair it, I said s*d that and got it posted back. I eventually tracked down a guy in Surrey with the correct second hand ECU for £100. This seems to work ok, so I might have had some good luck there, for a change! So I carried on trying various things but still to no avail. A friend from work is hopefully going to come and have a look because he thinks the timing could be out. Does anyone think that could be feasible, especially as another amateur mechanic changed the water pump for me?

[piman]

Hello David,

the symptoms you describe match a faulty ballast resistor, as the coil is fed from the starter solenoid while cranking and the engine starts. Release the key and the coil is now fed from the ignition switch via the ballast resistor. If the ballast resistor is open circuit the car won't run when you stop cranking.
I don't know if that system is fitted to the XJ12 but what you describe is the classic example of a failed ballast?

Alec

[David27]

Thanks Alec, I'll look into that

[almcl]

David

What are the actual symptoms? I remember something about 'starts but won't keep running', but not the exact circumstances.

I don't think you'll find a ballast resistor, AFAIK they are only on the 6 cylinder cars and not all of those. Roger Bywater of AJ6 engineering operates an ECU check service. I think it's £50 to check and this gets taken off the repair price, if you have them repair it. http://www.jagweb.com/aj6eng/prices.php

Why did the mechanic suspect the ECU? They are quite reliable normally. Did he eliminate the starter relay and the ignition switch? Both or either of these can cause the engine to stop when the key is released.

[David27]

I suspect the mechanic was the one who blew my original ECU by using a booster charger with the battery still connected to the engine! I had the old one tested and they said it had been burnt out by a power surge of some kind - hence my suspicion. I'm pretty sure the starter relay and ignition switch are ok, but of course there is no guarantee that the 2nd hand ECU I've put on the car is fully operational. The engine is definitely getting fuel ok, so I think I've narrowed it down to something to with the injection system or ignition system which isn't exactly narrowing it down much! My current plan is to lie low for a bit to avoid divorce proceedings then maybe in my long Christmas holidays look at getting it recovered to a Jag specialist like the Jag centre in Spixworth, Norwich for an estimate on the cost to get her back on the road. In the meantime I'm advertising it for reasonable offers to a good home if anyone fancies taking on a project!