Hi, new poster on here but after searching the net for answers and finding some similar but not identical symptoms, i thought i'd post this to try to help others. The car is a Mondeo mkV1 on a 2010 plate.

 

My Mondeo 2.0 TDCI Auto Tit developed a problem where the "Immobiliser active" would come on every now and then, this then would clear after trying several times and car would run as normal.This could happen regularly or days / weeks apart. A couple weeks ago, i was driving along around 60mph when the car juddered, like a stall, the engine management light and also the traction control light came on, after 2-3 seconds the engine light went out leaving the TC still lit. I put my code reader,(cheap), on it and it found no codes. Turned ignition off, back on to start it and the light had cleared, it briefly showed "low battery" but car started fine. Again this happened another couple times with the final straw being one night driving home, the car displayed "Engine malfunction" and dropped to around 5mph, limp mode? As i was on a dark road, i kept it moving until i could stop in a better lit position, after driving along for say another minute, the car picked up, all warning lights went out and the car drove home with no issues. Having got info from others about spurious electrical faults, i changed the battery which was now around 7 years old. This failed to fix my problem.

 

Assuming others would have had the same or similar issues, i checked on here and other sites to see if i could get a hint of the problem. I downloaded a copy of Forscan, got myself an elmconfig adapter and plugged the laptop into it, took me a while to understand how to read the codes but i eventually found a couple of DTC's in the PCM, the BCM and also the ABS modules, all communications! Again resorting to the net i found if the BCM had caused it, the interior lights, Instrument cluster etc would all have shown no power, i was able to simulate this fault by removing the 2 feed wires to the BCM, (under glove box, yellow and red wires in a black plug at the front), the dash and interior lights went out. Decided it wasn't the BCM. Assuming, again, that if it were a bad wheel sensor, which i had initially thought when the fault first occurred with the TC light on, it would more than likely have displayed an error to which wheel sensor and not just a comms issue. Now it looked like a bad PCM but having checked that others have fitted this and it hadn't cured their fault, i was hesitant to spend £300-400 quid plus the cost to re-configure it on a guess.

I cleared all the DTC's as the car was running fine and waited until the fault raised it's head again, it didn't take long. The next day i jumped into it, tried to start it and  "LOW BATTERY" appeared and the engine cranked but wouldn't start. I went to engine bay fuse box to test voltages and found although a little low at 12.2v, it should have been more than enough to start the engine, i then heard a click and what i can only describe as a "whine" like something electrical had been left on near a speaker? Tried to start the car and it started again. Hoping my issue now lay around the fuse box and i had somehow disturbed it and the wiring around the area i left the ignition on but car engine off and wiggled all the wires in the engine bay. Sure enough, the clicking and whining would appear when i touched the wiring leading down from the fuse box towards the PCM.

Removed the air box and further fiddled with the cables, with ignition on and moving the cables i definitely had some issue with the wiring, though not going to the PCM, it was leading down to below the radiator. Removed battery negative and proceeded to strip off the insulation and wiring shroud, getting down to the "Y" just below the radiator i found a yellow / red striped wire connected by a joint to a red wire, ( i have no wiring diagrams so i don't actually know what these do) the joint wasn't jointed! The joint had become corroded over time, moisture getting into the loom etc and had snapped off, the only reason they had still been touching at all was the fact that they were wrapped inside the wiring shroud and i assume with the vibration of the engine or bumps etc they would briefly lose contact and throw up the warning lights, once the shroud was removed, the joint fell apart. I put the battery neg back on and checked to see if the dash showed a fault, yip exact same issues, "Engine malfunction", TC, EML all on. Removed the neg lead again and remade the connection. Started first time with no issues and have now run the car a week so far without anything untoward.

Hopefully this will save someone a few quid replacing items that aren't the issue, it may also be the same on the focus / fiesta etc if someone can further determine what the 2 wires connect?

That made a very interesting read :-). Glad you got it sorted!

Well done 👍  Did it look like a factory joint or a bodged home lashup as going from green/yellow to a red wire sounds like a bodge. 

It looked like the original loom tape and it the shroud didn't appear to be out of shape, as it is now lol. But really not sure if it was a proper joint as it was so corroded, however there is info on the net detailing known stress points in these looms and one of them isn't too far from one of these points.