Long story short: I accidentally shorted the B+ lead that goes to the transmission rotary switch.

I had backprobed the rotary switch for diagnosis, and was moving the car for access to the rotary switch for removal. In the process of moving, the "hot" (W/R) probe touched a ground for a split second; it sparked, and the car shut off.

Now I have Stability / Transmission / Engine faults, the OBD scanner cannot communicate with the ECU, and of course the engine will not start. For what it's worth, I've got 1.5v on the W/R if that matters.

I checked all fuses, they're all good. I suspect I fried something on the ECU. I did remove the ECU and inspect the insides, it all looks clean (and smells clean, I don't smell any burnt electronics for what that's worth.)

Is there any fusible link or other safety mechanism for the ECU when a short occurs like this, other than the 5a fuse in the ECU fuse block (which tested good?)

As I said, I'm afraid I may have somehow ruined the ECU. Really hoping I haven't, after all the work I've put into this car. I understand the ECU's are married to the car, so if this is the case, it may be an unfortunate part-out situation. I really hope not.

 

they’re not

 

You could try a hard reset on the battery.

 

Right. If you do swap out the ECM, I think you'll get an odometer inconsistency with the Instrument Pack and the TCM.

 

Don't despair, there are plenty of folks out there who rebuild ECMs. Try Google.

 

I would probably 'chime-in' but since there are at least 3 iterations of X100, I have NO IDEA what car you have. (I'm out)

 

For what it's worth, I have a spare ECU for my 2000 XKR. I ran the car with it installed for a couple of weeks without issue before reinstalling my original.
l can't speak for any other vehicles.

 

@motorcarman It is the 1997 XK8 convertible. Sorry for neglecting the detail in my post.

I understand you're out, but I surely would appreciate your wisdom here. I would just say "it's just a car" and chalk it up to a learning experience & sell / part-out, but it's not just a car. I love this Jag and I did truly enjoy driving it (albeit briefly for less than a month) more than any car I've owned.

 

I did that by holding the cables together for 30 seconds, to no avail. I have it disconnected now, with the cables bridged, I'll leave it overnight but I don't expect any different result tomorrow.

 

Well, unless someone else has a better idea, that's likely that simple fix ruled out. Like I said earlier, there are plenty of ECM rebuild folks out there. No need to scrap your baby.

 

Your other option is to replace the ECM with a used one. Without knowing your VIN, the used price could range from $100 to $400.

 

I'm confused by a couple of things.
First l can't see how shorting a normally battery voltage live would blow the ECU, l would expect it to simply blow the #10 fuse in the engine management fuse box.
The AJ26 ECU is looking for battery voltage and a short to ground is basically just shutting off the current, unlike the AJ27 which relies more on switching to ground. Shorting that to battery might be cause for concern.
Second; If it did blow the fuse you shouldn't have any voltage on the W/R now and if it hasn't blown you should have battery voltage.
My immediate thought is that a dead short would blow a 5 amp and the 1.5 volts is feed back from ECU.
Do you get the same reading out of P&N as in?

 

As Baxtor says, its difficult to see how shorting an ECU input that is expecting 12v to ground would damage the ECU. If I were you I would check all the fuses and check continuity of that W/R wire all the way back to the positive terminal on the battery.

If you really want to hunt for a damaged module, I think its more likely the TCM if you had a loose 12v wire in that rotary switch.

 

Are you checking that voltage with transmission NOT in park or neutral? In any of the other gears that W/R is isolated from the ECU and if not reading battery voltage it can only be a fuse or supply problem.

 

Are you sure you checked all the fuses?

I'd expect a short on the R/W wire to take out fuse #10 (5A) in the engine management fusebox, as noted by Baxtor. Agree with the others that it shouldn't affect the ECM.

Note that it's the ECM controlling the outputs from the management fusebox:- including sending some power to sections of itself via that fuse, so you might see some odd things...

 

I couldn't agree more.
I can no longer shift out of P after this incident. Last time I turned the key, the radio reads "Err" and several faults appear on the dash (engine, transmission, stability, traction.) Including a new one: "Incorrect part fitted."

 

Excuse me if l'm overstaying welcome here Jim but it's a bit like one of those logic puzzles and l quite like them, when they work out.
l really think the answer is somewhere in that EMS fusebox. Have you checked for battery voltage at the battery bus and the ignition switched bus of that fuse box. Fuse #4 is also directly involved as well as the relay which controls all the ignition switched circuits. Swap out the relay maybe?

 

Never - your help is greatly appreciated & please accept my gratitude.

@michaelh "Are you sure you checked all the fuses?" Let me say, there are some days I wonder if I should even be allowed to change my own oil. I doubt anyone will believe me, but I swear to you I checked #10 5a twice last night, though it is true I was back-probing all the fuses instead of pulling them out.

This morning, I used a different (old) multimeter, removed the fuse, and checked the blades instead of the back probes, and it's reading bad. So I popped a new fuse in, and voila - this issue is resolved.

I do sincerely apologize for wasting everyone's time with such a trivial mistake & thanks again to all of you great people.

 

All good then Jim.

 

It's amazing how something fairly simple can lead to a rabbit's hole of problems and that using sophisticated diagnostics can be a distraction. A few years back, I had a non-start issue with the x-type which involved a non-communicating ECM. Turned out to be corrision on the ground connections near the headlights. Live and learn. Thank goodness there are a bunch of folks on this forum who really know their stuff. Glad you got your car sorted.