Bit of a public service announcement....

Today I revived a 2001 S-Type which had a heap of bizarre electrical problems and was about 1 day away from going to the wreckers... only 113'000kms on the clock (70'000 miles).



The problem started when the owner pulled into his garage one night, went to turn off the engine using the key, but the engine kept running, with the key removed from the ignition, it was just sitting there idling away...

He managed to shut it down by pulling a fuse in the engine compartment fuse box, but after putting it back in, the car would not start.

STEERING COLUMN LOCKED, and ENGINE SYSTEMS FAULT on the dash, PATS light flashing away like crazy... the engine ECU and throttle body were powered up, along with the Rear Electronic Module, which drives the fuel pump. Hence it kept running even with the key removed.

After several auto electricians, automotive locksmiths and even an automotive instrument repairer spent hours getting nowhere with it, eventually it ended up with me...

After a lot of head scratching and swearing, I worked out what the fault was. The giveaway was battery power at one of the wires in the back of the ignition barrel, when it should have been 0V

What caused this was a short between the two wires of the Transit Isolation Relay harness, which fed battery voltage back through the circuit to the key barrel, and powered up a heap of other circuits in the car as well.

Wiring diagrams explain a lot.


You see the Transit Isolation Relay in the drawing, this is not there in your car, the Relay is removed during the Pre-Delivery Inspection. But the wiring harness remains, connector CA16 is a small white 2-pin connector, next to the battery. The one that heaps of people ask "what is this for???" well as you can see one of the wires is pretty much connected straight up to the battery, via Fuse 12, in the rear fuse box. So one of those wires is PERMANENTLY LIVE.

Look at the rest of the wiring diagrams that include this circuit (use the INERTIA SWITCH as a reference to get your bearings).


If for whatever reason you manage to get Battery Supply Circuit 9, and Ignition-Switched Supply Circuit 10 shorted together (i.e. the two wires on that white "unused" plug), that puts Battery Voltage onto Ignition-Switched circuits 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 & 20, also right through to one of the switches in the ignition barrel... so a fair portion of the car is powered up without the key even in the ignition...

If this happens when the engine is running, the engine cannot be turned off with the key. If it happens when the engine is not running, it will not start, because of PATS faults...

THE KICKER

If the dealer techs had simply removed Fuse 12 from the rear fusebox during the PDI, this would never have happened...

So please, everyone, remove Fuse 12 from your rear fusebox... it is not needed, and could save you a big headache...

EDIT

Fuse 12 for X200 1999-2001
Fuse 33 for X202, X204, X206 2002-2007

I wonder how many electrical nightmares have been cause by this simple stupid problem...

 

Wow . Nice save Cambo . did you find the point of short or just isolate/separate it with the fuse ?

also this cable is on the 03 onwards revision s type's . so does mean its also applicable to these revision models as well - along with this fuse location ?

 

Ouch!!

Good catch.

We now have a better answer to "what is this for" - it's for a nightmare scenario

 

hmm... on the diagram for my car it looks to be called F50, a 5A in the rear power distribution fuse box.

 

The harness was coiled up and stuff behind some trim, when I pulled it out straight the systems which were powered up shut down... but pulling fuse was a certainty to avoid the short...

The cable is there in all late model Jags. But you're right it's a different fuse in the later cars.

Actually it's F33 in the facelift cars.
X202, X204 and X206, Fuse 33 in the rear fuse box.

Circuit numbers in a Square are Battery, the ones in Circles are Ignition-Switched.

 

Wow, some great troubleshooting! A tip of my orthopedic hat to ya...

Just curious, did the damaged wires show evidence of an external cause? For example, was a piece of trim pressing on the harness, squeezing the wires together?

 

Ah. I had a choice of F33 & F50 and it looked more likely to be F50. Sorry.