I've been having problems with wiper not working, won't start, headlights won't work, etc. Those are the classic symptoms of a bad Body Processor Unit. I ordered a used unit from Ebay, and the car will start, but the next time the mechanic tries to start it, the same thing happens, nothing works. I'm sending the bad part back and a friend is letting me try one from his working XJ6. If his BPU does the same thing, could this be a problem with the Immobiliser?

 

Try the 'bad' part on your friend's car before you return it. If it works OK you can also rule that out W/O risk to his part.

 

The BPU is the common factor here but lights and wipers would not be affected by the immobiliser as this will only inhibit the starter motor and prevent cranking. The fact that a replacement BPU did not make much difference would indicate that the problem lies elsewhere - possibly with wiring or the power and ground connections to the BPU. Diagnosis of non cranking will require a methodical approach checking the BPU inputs and outputs when the car will not crank.

For a 1996 4.0L NA spec car the wiring diagram (Manual Fig 03.1) shows these inputs are required to enable the BPU cranking output i.e FC1-33 is at 0V (gnd) when cranking -
1. FC2-7 is at 0V (gnd) - from ECU check engine output
2. FC2-20 is at 0V (gnd) - from decoder module output (P N signal from gearbox)
3. FC2-5 encoded communication - from Security & Locking Control Module immobiliser output (this is a short burst of serial communication that you can't measure)
In addition -
+12V supply is required on FC3-1 and a good ground is required on FC3-5 & FC3-6
The BPU supply is via a 10A fuse #5 in the boot (trunk)

When the ignition key is held in the start position the ignition switch brings FC2-41 on the BPU to 0V (gnd) and if all the other inputs are correct then the BPU cranking output on FC1-33 goes to 0V (gnd)

The cranking output from the BPU is then routed through the P N selector switch to pin 1 on the starter relay which should also be at 0V to activate the relay when the ignition key is held in the start position. When activated this relay supplies +12V to the starter solenoid to crank.

 

Thank you so much for that information. Just an update to this ongoing saga. I picked my car up from the mechanic today. The basically gave up and ran out of ideas... after 9 weeks and several hundred dollars. I was able to get it home because they installed a push button start that will start the car, but headlights, wipers, tail lights, turns signals, etc. don't work. I was messing around with it in my garage..and found that if I remove the positive battery cable for 30 seconds to a min. and then putting it back on, it will start with everything working as it should and runs great. It runs as long as I leave it started, without any issues at all. Once I turn it off, I can start it again as many times as I want, but only within a min. or two, then have to remove the positive battery cable again and start over. It's actually not that huge of an inconvenience, but of course I want the issue fixed. I'm wondering now if it sounds like a ground issue or something wrong with the positive cable from the battery cable to the firewall?
I can't seem to find a good mechanic around here that really knows Jaguars and are guessing, at best.