ECU fuse blowing - can anyone save my 75? by mat2504

Hi, I wonder if anyone has had a similar issue, or can offer any suggestions to help save my 75 from the breakers.

In short, 2 weeks ago, my 75 left broke down (failure 1) and ended up being recovered by the RAC who suggested that there was a crank sensor failure - the diags were apparently also showing errors with the VIS (Variable Intake System) (Variable Intake System), lambda, and other sensors. The car was recovered to my local garage who quickly identified that the 'number 1' 15A fuse (engine bay fuse box) had blown. This was replaced, the car started, and I drove the couple of miles home. A few days later, about 6 miles into a longer journey, the car died again - the same fuse had blown (failure 2). I replaced the fuse at the roadside and drove back to the garage. Several days later (I'd been working away) the garage returned the car to me having apparently found an electrical short in the engine bay and repaired it. I was told that they had been able to replicate the fault by changing from R to D and then accelerating - every time, the fuse blew immediately. After their work, this was no longer the case and a road test had been a success.

Feeling optimistic I took the car out for a drive and suffered failure 3, 4 and 5 within the space of about 5 miles. After the final failure I noted that it had also developed a new 'feature' - when started, the engine ticked over over as normal. If the throttle was touched, the revs did not decay and would actually build to circa 3k RPM; I assumed that something had gone away with the TPS too.

The car is now back with the garage and they have not been able to diagnose the root cause of the repeated failures, or what seems to be a cascade of failures/errors/issues. Does anyone have any thoughts/ideas? I'd hate my 75 to be heading to the breakers (especially since it only has 55k on the clock, I'd just had it serviced and had the belts/tensioners were done last year too!).

Thanks in advance

1st failure
Turning right out of of a t-junction, full throttle (15A 'number 1 ECU (Engine Control Unit) (Engine Control Unit)' fuse blown)

2nd failure
Turning right out of of a t-junction, full throttle (15A 'number 1 ECU' fuse blown)

3rd failure
Went under braking - the ABS (Antilock Braking System) (Antilock Braking System) triggered on some mud and the engine went at exactly the same time. (15A 'number 1 ECU' fuse blown)

4th failure
Went under acceleration at about 2/3 throttle with at 4k RPM (15A 'number 1 ECU' fuse blown)

5th failure
Went under little/no throttle having turned right from one road to another (20A fuse blown - I'd run out of 15A by this stage)

Posted 12 Apr 2014, 14:02 #1 

User avatar
(Site Admin)
Could possibly be the ABS connectors in the wheel arches. Try disconnecting each and cleaning the contacts then reconnect. These are the blue connectors. The rear connectors are located under the rear seat cushion, again blue connectors.


Posted 12 Apr 2014, 14:26 #2 

User avatar
It will probably be the wiring to the lambda sensor in the exhaust. I helped someone (I think it was Greeners on another forum) diagnose this I think. Let me check out the fuses to confirm it's the same one.

Posted 12 Apr 2014, 20:32 #3 

User avatar
OK. its not the same failure, as his was the cam sensor supply and the rear Lambda (O2) sensor. My guess would be yours is the front one. The cable runs past some pretty hot bits and would easily melt if the clipping was out of place. The fuse goes when the engine moves about and the wire touches something.

It might be another wire, but I'd still be checking for wires that might have chafed and be touching as the engine moves about.

Posted 12 Apr 2014, 20:40 #4 

Thanks guys, very much appreciate the time you have taken to reply. I've fed everything back to my garage to make sure they have checked your suggestions and, if they haven't, that they do now. I'll keep you posted. Still got my fingers crossed; hate to think of my 75 off to the breakers when I'm sure there was another 100k miles left in reserve still.

Posted 14 Apr 2014, 08:45 #5 

Well the 75 is back and, with a little trepidation, I went for a 50ish mile drive today and it did not miss a beat. Phew - now, date I try the usual 500 miles as of tomorrow? ;o)

Spot on with the diagnosis Duncan - it was the rear Lambda that was shorting on one of the heat shields. Wiring and sensor replaced and the 75 is back on the road :o) :o)

Posted 04 May 2014, 17:19 #6 

User avatar
Good news! Its always good when we save another from the non-inevitable!

We actually talked about your case today at the Nano, briefly.

Posted 04 May 2014, 20:01 #7 

Over a year on and 15k miles and still running well - thanks again guys

Posted 27 Jul 2015, 20:42 #8 

User avatar
That's good news. Good to get the feedback, too!

Posted 28 Jul 2015, 19:24 #9 

I was wondering if you could help my abs light is on got a diagnostic test to find out that there was no fault found but wheels keep locking when brakes applied hard

Sent from my LG-E430 using Tapatalk

Posted 09 Oct 2015, 14:00 #10 

User avatar
What tester did they use. If not a T4 (Testbook version 4.Computer Diagnostic System) it wouldn't find the fault. It's most likely a sensor or the sensor ring on the bearing.

You could always come to next weekends Nanomeet if you are in the Midlands and it might help point in the right direction (assuming the diagnostic you had wasn't a T4).

Posted 10 Oct 2015, 15:28 #11