I was preparing hands on training about troubleshooting various faults caused by intermittent loose automotive connectors and I wanted to simulate it on real vehicle. I could not create such poor connection reliable faulty even it sounds very easy, the result was usually one of two stages, good connection, or open circuit, not between as what I wanted. (Who tried already knows)
The real poor connection is mostly caused by surface oxidation and once the connector is unplugged and connected back, it’s often temporarily fixed. This can be ECU socked, fuse in fusebox or component connector. In such scenarios the vehicle system may work normally for several hours or days and then suddenly return to workshop with exactly same DTC again. The vehicle CM may not register the problem immediately, but it is usually visible with help of Picoscope.
My first idea with live simulation was going nowhere so there is plan B, instead of simulation I will use existing real cases waveforms with explanation and pictures, which is basically a case study, so here we are.
1). Background: This Bentley W12 had history of misfires, replaced parts as coils and plugs, swapped injectors… However, the vehicle has always returned with intermittent DTCs of multiple cylinder misfires, Cyl 9 misfire and Inj. 9 circuit.
(Please ignore the repair process as there is many ways how it could be approached, my point is to share how such poor connection can be identified in Picoscope waveform).
2). Planning probe locations: Previously, all involved components were replaced or swapped and as the problem is intermittent, means unlikely mechanical. The question was Engine ECU or wiring from ECU to component? First target is to get the injector waveform with DTC Injector 9 circuit.
Here is one advantage, the engine has 12 injectors so not all are expected to be bad. There are two fuses, each supply 6 injectors. Fuse 1 for bank 1 and fuse 2 for Bank 2 injectors. The ECU is opening each injector individually by switching each to ground according to map timing.
3). Connection: Fuses 1 and 2 got installed 0.1 Ohm resistors with fuse in the loop which will represent current flow via each fuse. The Fuse 2 power supply side connected also. Injector 9 is under intake manifold, but it was possible to probe the ground switching wire leading from injector to ECU. (F1“B”, F2“A”, Inj. 9“D”), then F2 pw supply or Inj. 10 ground wire channel “C” as per waveforms channel labelling.
4). Waveforms: First you can see the waveform where nothing is faulty, hot engine, first idle and accelerating stationary to high rev. This is the condition most of the time and it will fail depends on temperature, vibrations, or humidity.
Second waveform is already missing two injectors, the problematic area has been identified by moving and pulling wire harness on every reachable location while monitoring the live waveform on the screen. This capture luckily captured the condition causing the misfire and DTC.
Third waveform is detail where the connection is loose but still able to open the injector, there is visible sparking which is happening under the current load at injector energising phase. Here the DTC for injector may not be logged yet as the ECU needs several full open circuit phases to log in. However, this will cause lean combustion on affected cylinder and rich in other cylinders of the affected bank while the fuel trim will be OK (As the O2 sensor can read only average for entire bank). This condition may cause misfire of good cylinders also.
I hope this case study will help to motivate at least one workshop to invest in oscilloscope and the current Picoscope users to add their interesting cases to this forum and upload more to waveform library where I have also uploaded all the waveforms from this article.
Thank you for reading.
Last edited by Rfmotors1
on Wed Oct 04, 2023 5:25 am, edited 2 times in total.