Hi all,
My neighbour has dropped his 2001 S-type at my house. I have what seems to be an ECU issue that I've never seen before. ECU is not connecting to my OBD reader - ebay ELM327 (never hooked to a jag before so unsure if it's a protocol problem) and after checking voltages and analysing the wiring diagrams I suspect the ECU has failed but hope to confirm before I dismantle the engine bay to extract it. I'm getting a reading of 12.2V at the input and output of powertrain control relay 1 but 3V at the battery side of fuse #5 and #6(engine compartment fuse box) with the fuses out. With the relay out I get 0.39V at the same side of the fuse connector. Has anyone else seen this sort of problem before?

 

Hello Dafikza,

Welcome to the Jaguar Forums!

If your app and ELM327 can connect to and communicate with another vehicle, it should be able to connect to the Jag.

Aside from the lack of communication with your scan tool, what are the symptoms that prompted your neighbor to bring the car to you? Crank/no-start? No-crank/no-start? Please give us more details.

Are you using the official Jaguar wiring schematics? You can find them at the link below, thanks to our member motorcarman, a former Jaguar technician who provided the file and our member Gus who hosts the jagrepair.com website:

Jaguar S-Type Electrical Guide 2001

Cheers,

Don

 

Hi Don,
Thanks for your reply, It is a crank / no start problem. The owner was informed by his local mechanic that his computer read a low voltage error code but also recommmended replacing the fuel pump, he bought a pump and new battery and I swapped those out for him, I checked the pump for priming at key on but it sits silent. After that I assumed the RECM wasn't receiving a signal to run the pump at all and went to the ECU to check for voltage and found it at 3V which is obviously unlikey to be enough to power the PCM but also appeared to me to be a brown-out of sorts(I assume also the reason my OBD reader gets no response). Removing fuses #5 and #6 for the most part eliminates sensors, servo's and motors from the circuit and according to the diagram leaves only the "Powertrain Control Module" hard wired to "Powertrain Control Relay #1". I'm about to dig into the engine bay to disconnect the PCM to see if the voltage at the fuse pin returns to 12V+ as i suspect it will. I didn't find anything similar while trawling the forum so I figured I'd post a thread in case someone else in the future stumbles upon the same issue. Any thoughts you and others have would always be appreciated,

Cheers,
Glen(Dafikza)

 

This is the section of circuit that reads 3V

 

Welcome to the forum.

I am viewing this on my phone, so can’t see the full PDF wiring diagrams. Hopefully tomorrow I'll have a bit of time for a better look.

A few quick thoughts:

Have you tried swapping relays? Pick one of the same size from a known-good circuit as a donor.

Have you tried a click test of the suspect relay? Place your finger on it, and have a helper cycle the key. You should feel the relay click. If good, this doesn’t confirm the relay is actually sending power, but it does prove the relay is receiving the command to energize. Make sure the click is coming from the relay being tested. Adjacent relays may also click at the same time, so be positive of the source.

Relay numbering: IIRC, this can be confusing. In the wiring diagram, a particular relay may be called “Circuit Name Relay #1”, but in the fuse panel the physical location may be #7 or some other number. Don’t let this possible mismatch throw you. There’s a relay location illustration near the front of the wiring diagrams.

 

Relay was clicking( I was on my own but I figured, with the key on, removing and replacing the in-line diode would have the same effect) but I decided to crack it open to double check and found ant carcasses between the contact points and the same with the relay I had tried in it's place. I'm going to seal the relays with some silicone to make sure nothing like that happens again. I knew the voltage drop was at the wrong end of the circuit but I didn't trust my initial suspicion. Thank you very much for your response,

Cheers
Glen(Dafikza)

 

The very last suspect is the "ECU" (i.e. PCM aka ECM).

Wiring etc - far more likely.

 

Wow, that’s quite the find. Never heard of that before.

Did that fix the problem?