I have a USA Jaguar, live in NC. I bought a 1996 XJ6 in Oct of 2012 and it started up and ran great. Within a month, the engine wouldn't turn over, no clicking - just nothing happened when I turned the key forward, minus the dash lights and the engine fan turning on, I believe the radio turned on too. The headlights, wiper, starter, tail lights, interior lights, trunk light, license plate light and prob some other things didn't work either. So mechanic 'tested' lots of things, whatever that means. I really don't think they knew how to work on a Jag at all, but the main mechanic was a master Mercedes Benz tech, so I thought he would be great. Anyway, the starter was good - so then he put a new Neutral Safety Switch on - no luck. He then said he could bypass the Neutral Safety Switch and put a push button starter on.. but the car could start in any gear..which was ok with me. Once they did that, the car would start, but still no headlights, wiper, etc. So then, he said it was the Body Processor Unit. Tried two of those, including one that was taken off of a working 96 XJ6 from a friend just to see if it worked in mine and that didn't change anything, same thing happened. They supposedly tested the grounds and connections leading to the BPU and were ok, but I don't really know that they did that. At this point, after 9 weeks of having my car, they decided they didn't know what else to do, so I took it home this week on Tuesday. That evening, I was messing around with my car and decided to take the positive battery cable off for a min or two and try to start it - it started right up and everything worked properly - lights and everything. I can drive the car as long as I want and after turning it off.. can restart it within a couple of min. with no problem. After a couple of min., if I try to restart it, the same thing happens, no headlights, won't start, etc. If I remove the positive battery cable and put it back on.. everything works properly again until I turn it off again. Battery is brand new as of Oct. 2012. Any idea's of what I should test or try next?

 

There is a relay known to go wonkey located under the hood at the right hand side fuse box. If it's the original ignition relay, it'll be the only blue one with the rest being black. I'd lay good money on that relay failing. A schematic for the fuses and relays can be found above in the FAQ's.

Also, did they test your ignition switch? There is a sensor that senses when you insert the key, which can also be problematic.

Good luck

 

This "suggests" a problem with a solid state module. There are three shown as playing a role in the starting circuit. The body module has been replaced already. That leaves the engine ECU and Security module.

I'm not entirely sure how the three modules talk to each other. If you wanted to guess you could try replacing them. Often these modules go cheap on Ebay.....at least the security module.

If you have a test meter/test light we could walk you thru some basic checks that would eliminate the ignition switch and starter relay.

Cheers
DD

 

Thank you Doug! The starter relay and ignition switch have been tested, at least that's what I was told. I'll see if I can get my hands on a security module first, then try ECU module.

 

All modules are reset by removing +12V so it's difficult to determine which of these are at fault.

As a quick check when you turn the ignition to the on position all the dash lights should initially go on and then go out leaving the Check Engine light on at the bottom left. This is the Check Engine output signal from the ECU that I mentioned in my last reply. It is one of the inputs that is required by the BPU to enable cranking. If this light is on then the ECU is producing the correct output. It is possible that the problem lies with the Security and Locking Control Module (SLCM) which sends a burst of encoded communication to the BPU when the immobiliser is disabled. You cannot measure this communication without specialised equipment. If the SLCM is replaced it may need to be configured using dealer level diagnostics (certainly true in the UK).
As menioned in Suede's post the ignition switch contains a set of contacts that are activated when the key is inserted into the ignition switch (i.e. before it is turned).
The excellent Steve Sparrow has documented the problems this caused for him in the jag-lovers.org forum (JagFORUM Logon). Bear in mind that his work here is on a UK spec car which has the added complexity of a security transponder chip in the ignition key that I believe the US spec cars don't have.

To summarise what Steve found - the contacts that are activated when the key is inserted were stuck so that the ECU relay behind the headlights was active even with the key out of the ignition (this also led to running down the battery prematurely). Because this part of the ignition system was not turning off,
the ECU didn't know the engine had been shut down, so it failed the
engine start sequence, disabling the engine. (Your method of removing and restoring +12V forces the ECU to reset along with all the other modules so that the ECU does know that the engine has been shut down).

A clue to this problem is that you also can't lock the car using the key fob.
Steve found -
When the engine is disabled in this way (at least by this ignition
switch lateral movement fault) the doors and boot cannot be locked
with the fob for the same reasons that they cannot be locked if the
boot lid or doors are actually left open. The locking procedure
only works when the ECU relay is off (not powered) but won't work
if the ECU relay is powered when a request to lock the doors is put
made by the fob. In this case, you will hear a refusal 'peep' from
the security horn.
Removal of the ignition barrel to free up the igntion switch is
done by putting the key in and turning it to position 'I', and then
pushing in the little detent pin on top of the outer barrel with a
bent pointed tool. The lock barrel can then be pulled out, exposing
the laterally moving part of the ignition switch. Naturally the
column shrouding will have to be removed to do this. An alternative
method would be to remove the ignition switch itself and free it up
or fit a new one.

Failing all this it's down to systematic checking at the BPU to ensure the correct input signals are present before cranking.

 

Thank you very much for taking the time to respond in such detail, I really appreciate it!

 

Brad ... When the car was in the shop being tested by the Mercedes guy you mentioned that a part from a friend's car was swapped over to see if that made a difference. Did the mechanic also try YOUR part in the friend's car to see if it worked properly?
If not, why not. Seems an obvious thing to try to me.
Cheers,
Steve

 

I'm not sure why he didn't. Either way, I took the car back to the place I bought it yesterday.. I've had enough with the issues. There is no reason a $60,000 car should have so many problems, not only mine, but Jag's in general. They are beautiful cars, but I will never buy one again.

 

You spent $60K on a 96 Jag?


It is a 17 year old car. Do you expect everything to last forever? If so, I expect a lot of disappointment in your future.

 

SOOO funny. A car this expensive NEW shouldn't have so many problems...yes, this car is 17 years old..if you take a look at the new Jag's..they have just as many strange electrical problems too.

 

As a general rule, the more systems installed, the more to go wrong. And these cars have about 3 times the number of systems of most cars. Heck, when was the last time you saw a car with 5 separate fuse boxes?

It's kinda like that new Boeing Dreamliner. A dream WHEN everything, works. But that dream experience requires constant maintenance and isn't for everyone. I've always joked that Jags are for "PC & Android" people who like to tinker, and a Lexus is for "MAC & Iphone" people who just like to get in and go.

 

Try wrestling with a 2002 BMW X5!!
That thing has more electronics than a Boeing 747.

Thanks guys for the valuable input.

Bernie