Couldn't think of a short enough description for a very complicated electrical question. The original problem was driving and random shutoffs killing the car. The solution I implemented was bypass key switch entirely and replace both Key On Relay behind radio (early 93) and replace EMS Main relay on firewall. Why? Since unplugging and replugging one or the other made the car start again. So the gremlin simply moved somewhere else. So the car doesn't die during driving anymore. Now it gets me where I'm going, and then has no power upon "door open" , "notice no dome lights" , key switch nothing. So my current workaround ended up to be a battery negative quick disconnect, Off, On, and power restored, dome lights up, car starts. I've spent hours, nay, days, weeks staring at the 93 schematics looking for devices to blame. Thought I hit the holy grail with "original shipping battery relay not removed by dealership..." no luck. Battery terminals are cleaned and tight, grounds are cleaned and tight, battery is fresh and holds a good charge when parked for over a week. Volt meter hard wired to center console fuse block shows full battery voltage which eliminates battery positive cable from battery to all loads.

I'm starting to "feeeeeeellll" an invisible 5 pin relay sticking "open" until +12 is killed causing the relay to snap closed (normally closed). Except there are no more relays in any path anywhere. All that's left are Controller Modules. And if this was any post 2000 GM , the Body Control Module (BCM) would be the obvious culprit. Which I guess Jag calls the "Central Processor".

Can anybody save me hours/days of scouring the boards and brief me on central processor failure modes and fixes? Or comments on battery disconnect completely restoring power, what devices might be resetting?

 

From Adrian Bowles in 2021 "This devise essentially distrubtes the power from the fuses to the various electrical functions. i.e central locking, turn signals etc."

Don points to "Figure 5" (which is logic black box), there seems to be a power supply section in the controller. What electrical components, transistors, on board relays, caps, coils, solder joints, anything that fails over time?

 

I don't have a spare to autopsy. Who's done it with photos?

 

Since Madam EMP lives in state of permanent disassembly anyway, it was fast enough to yank the central processor and do a biopsy. Here's the meat:



Surface mount, no visible burning or smoke.

This is a sophisticated board compared to most of the onboard controllers.

Only a few through hole components that can be repaired on the bench like the 220 cap, maybe the 7 gang Motorola darlingtons. I don't see anything here that can work most of the time and randomly stick wrongly once or twice a week.

That's a big ole custom TI processor, eprom flashed with the car specific info, 3 banks of high current darlington transistors. This is the heart and operation center of the car, clearly the most important component, bar none. And I don't have a spare....

 

Most of the time electrical problems on these cars are due to cracked or cold solder joints in the footwell fuse boxes or relay modules.

 

Madame EMP and her invisible failures. The power terminal on the L/H fuse block came completely unsoldered from the board but is trapped by design so it doesn't fall out. It just slops around. 3 years of random power failures and multiple tows home. An impossible to find 93 only replacement fuse block fixed the problem.
WARNING: 93 and 94 fuse blocks are visually identical and use the same connectors and physically interchange. But the pinouts are completely different. Who knew? And there are occasional 94 parts available whereas 93 only cars and parts are unobtanium. Resulting carnage from lighting up the 94 L/H fuse block in the 93 smoked most of the circuits controlled by the security module except the key fob remote signal circuit, which stills beeps normally. But the central locking, dome lights, wiper motor, starter motor, and who knows what the f@ck else, those trigger wires are all dark even though jumpering each relay works fine. So it might be a wounded Security module, a fried Central Processor Unit, or both. This has turned into a real Hate-Hate relationship with this 93.

So let me save you my agony.

92 and earlier fuse blocks have different connectors. Can't screw that up.
93 only R/H fuse block is DBC12078, L/H is DBC12079
94 only R/H is LMD2827BA, L/H is LMD2827AA

I have a 94 pair if anybody's interested.

Remember, anybody having the random power goes dark issue, gently wiggle the power stud on the L/H block with your fingertip (gently!) , any movement at all is not repairable as best I can tell.
Any 93 owners out there who want to form a support group called 93 anonymous, DM me for a shoulder to cry on...

 

I'm pretty familiar with the 94 Jaguar fuse boxes and if you say the 93 boxes are physically identical and the harness plugs in with no issue, then I'd assume that if you match the fuse map for the 93 to the face of the 94, it should work? But you found there were internal differences?