|Vehicle details:||BMW i3|
While the future may be Electric, and technology marches on at a relentless pace, the following case study highlights a typical fault we have seen on numerous vehicles for many years. It is as relevant today as it was all those years ago.
The customer reported that when they tried to switch on and power the vehicle to Ready mode, the instrument panel displayed a “Drivetrain error warning” and the vehicle could not be driven.
Verifying the customer complaint is an essential step in the diagnostic process but it is quite often a time-consuming task without success. On this occasion, the customer complaint performed right on cue where the instrument panel and the iDrive displays both indicated a Drivetrain error accompanied with the message “It is not possible to continue journey”.
Note: It was not possible to shift the vehicle from “Park” with the transmission select switch and while the ignition could be turned on to illuminate the instrument panel and associated warning lights, the high-voltage (HV) contactors were not energised (i.e., the HV system appeared inoperative). In addition to the symptoms above, the vehicle would not allow charging to commence when connected to an EVSE point (Electric vehicle supply equipment).
The following video demonstrates the symptoms described above:
With the customer complaint verified, we confirmed the Vehicle’s ID and Specification.
Confirmation of specification is of the utmost importance when it comes to diagnosis as there is often a temptation for customers to modify their vehicle with fashionable accessories that lack the fundamental quality control and engineering that was originally intended for the vehicle. The customer confirmed that no such accessories had been installed.
The customer interview followed the 4 targeted open questions principle below to establish facts from fiction:
Quite often the customer interview leads to a probable diagnosis but I think you will agree when reading the responses above, that there is literally nothing to go on!
The basic inspection confirmed no fluid leakages, no visible signs of damage to hoses, connections, wiring harnesses, or that favourite discovery: accident repair.
A vehicle scan of all onboard control units revealed 17 fault codes, which we saved and erased. We cycled the ignition from on, to off, to on and made an attempt to power the vehicle to Ready mode. Once again, the Drivetrain error message was displayed. Using this technique ensures that the remaining codes are relevant to the fault condition and not codes introduced by any previous diagnostic work.
A rescan of the vehicle revealed the following six faults codes:
HV batt.charge control 4.0:
Informat. electronics 4.2:
To interpret the fault codes we obtained during the vehicle scan, we had to decode the terminology as universal scan tools often use differing descriptions for OE system components! This is a typical hurdle to overcome, which we have demonstrated and discussed here.
While this may not seem relevant at this stage of the diagnosis, we needed to interpret the abbreviations used in the BMW wiring diagrams against the component descriptions, fault codes and data we obtained from the scan tool. I guess you could liken this to the translation of languages, i.e., the language used by the scan tool and the language used in BMW technical documentation.
Before I move on, for those new to EV diagnosis and repair, please take a look at the following forum post on how HEVRA (Hybrid and Electric Vehicle Repair Alliance Ltd) can help with the translation scenario above.
So which fault code do we follow and why?
Given that we were using a generic scan tool, we did not have access to the occurrence order of the fault codes. You can find some example of occurrence orders here.
Based on the fault code descriptions, the following was my interpretation:
SME fault code 21E687 Voltage Terminal 30C: Crash detected would be the primary fault code to pursue as “Crash” takes priority over everything else!
With a “Crash detected” fault code it is no surprise that the EDME is returning 21E96F Ready for starting signal: Signal missing. With a crash detected the HV system would have been shut down immediately.
Likewise, the SME is also returning:
CE5402 Message: High-voltage battery 2 data: Missing message
CE5403 Communication with HV battery control unit: Missing message
CE5406 Message: High-voltage battery 1 data: Missing message
Informat. electronics 4.2:
B7F805 L-band antenna: Open circuit in wiring is not relevant to our diagnosis.
Let’s be honest here. How difficult would it be to chase “Missing messages” compared to the Terminal 30 C voltage at the SME? For this reason, we left the CE5402, CE5403 and CE54006 codes alone for now. At this stage, my mind is running riot thinking this could be a crash repaired vehicle even though the basic inspection revealed no accident repair work!
Before diving in at this point, it is paramount to take a step back and check for technical bulletins (recalls and campaigns, etc.). The best possible way to qualify the presence of known issues and fixes is to use the manufacturer’s technical portal combined with the vehicle chassis number.
While this may seem obvious, there are a number of avenues to obtain free technical bulletins and so-called “fixes”! I have experienced that while the entire description of such obtained data matches the exact symptom and fault code you have obtained, the bulletin may well be for other markets i.e., not applicable to the UK. Please don’t fall into this trap as I have done with other vehicles in the past.
In this case, there were no such bulletins to take into consideration.
The action plan was, as usual, predominately governed by accessibility, probability and cost. Based on the acquired scan data, the fault code 21E687 was very descriptive about the voltage level at Terminal 30 C of the SME.
The action plan, therefore, focused on the integrity of Terminal 30 C and the associated circuit.
Circuit 30 C can be seen below in the wiring diagram (supplied by ALLDATA). I have added a number of annotations to help with my interpretation of the BMW terminology.
N.B. The High-voltage safety connector in the image above is responsible for switching off the HV electrical system to carry out repair and maintenance work. Please refer to the workshop manual and make sure that the relevant procedure is followed by suitably qualified personnel.
If the High-voltage safety connector is open, the 12 V supply to Terminal 30 C at the SME and OBC will be removed (30 C now @ 0 V) and so the HV system is switched off until the power supply to Terminal 30 C is re-established.
The High-voltage safety connector is linked directly to the 12 V battery positive post via a Safety Battery Terminal which includes a pyrotechnic device designed to break the connection between the 12 V battery and the High-voltage safety connector (triggered by the Airbag crash module) in the event of an accident.
Below you can see the voltage present at Terminal 30 C of the SME, as reported by the scan tool.
We can immediately see a concerning issue with the voltage at Terminal 30 C of the SME when we applied electrical load in the form of attempting to power the vehicle to Ready mode.
The load applied would be in the form of the HV battery contactors attempting to connect the HV battery to the vehicle HV system. However, as soon as Terminal 30 C voltage at the SME drops below a predetermined threshold, the contactors would be de-energised immediately.
Let me share a tip on using the scope to monitor and capture voltage dropout: Select a slow time frame (2 s/div) where-by the waveform is drawn across the screen in “real-time” which can be interpreted by the human eye during events such as ignition on/off or wiggle testing.
While the scope can also be used to capture these events at ultra-speed with triggers, it is difficult to interpret and link waveforms to events, such as wiggle tests that are only displayed on-screen for a split second! In such cases, the user would then have to scroll back through the waveform buffer to find an event that may or may not be linked to ignition on/off or wiggle testing.
The PicoScope capture provided sufficient evidence to further test the integrity of the crash sensing circuit (30 C) unlike the value displayed by the scan tool that may lead you to believe Terminal 30 C at the SME is OK. (The refresh rate of the scan tool is too slow to capture the voltage drop.)
So far we had confirmed that the Terminal 30 C voltage at the SME is unstable under load. Based on this knowledge we needed to qualify the integrity of the 30 C circuit at the High-voltage safety connector.
As most workshops know only too well, having dead cars on the ramps is not good for profitability and so on arrival, the BMW i3 had been dragged off the ramp to a workshop bay (the relevance of which will become apparent later).
Re-measuring the Terminal 30 C voltage at the SME, OBC and High-voltage safety connector revealed the fault had moved from intermittent and while under load to permanent! This was great news as permanent faults lend themselves to speedier diagnosis. (See the scope connection diagram below.)
This confirmed that both the High-voltage safety connector and the pyrotechnic device (at the 12 V battery positive post) are intact because we have 12 V exiting the High-voltage safety connector. The 570 mV that was present at the SME and OBC was most likely to be a residual voltage from within these components via their respective ignition supplies. (We made measurements with the ignition on).
When we introduced a fused jump wire from the 12 V battery positive post to SME pin 1, the result was a successful power-up of the vehicle to Ready mode with forward/reverse drive established.
The conclusion, therefore, was that we had an open circuit (30 C) between the High-voltage safety connector and the SME/OBC. The question now was where is the open circuit, as the High-voltage safety connector was at the front of the vehicle and the SME was at the rear!
Before I move on, let’s assume we don’t have an open circuit (even though we do) but a high resistance instead. I want to discuss and qualify the voltage drop in circuit 30 C (the High-voltage safety connector to SME pin 1), which in theory should be equal to Channel B minus Channel A. Or should it?
Because we were using the 4823 scope (which utilises common ground inputs) to measure the voltage drop, we created the math channel B-A (see below).
However, if we had a scope with floating inputs (4x25 or 4x25A) we could have obtained the voltage drop using only a single channel. The following training article explains the difference between common ground, floating inputs and conducting voltage drop measurements.
The image below demonstrates the connection required to measure voltage drop in circuit 30 C (the High-voltage safety connector to SME pin 1) via Channel A on a 4425A scope.
How can we have such different voltage drop values using PicoScopes with different architectures (common and floating ground)? The answer is that both scopes are correct, the difference comes down to Ohm’s law!
Voltage drop can only be present where there is current flow (V = I x R) and with an open circuit cable, there is no flowing current. (Had there been a high resistance instead, these values would have been near equal)
While the additional voltage drop tests above were not truly relevant to the diagnosis (we had enough evidence with the capture from the 4823 scope) the information contained within provided further evidence that circuit 30 C was “open” and delivered an explanation as to why these measurement techniques returned different values.
Finding the open circuit
Tracing an open circuit within a wiring harness can be challenging at best and a nightmare at worst. Non-intrusive measurement techniques are always the best approach, as they minimise disturbance to vehicle components and hopefully reduce labour time. Because the wiring harness was routed along the outer chassis, we drove the vehicle on the ramp to improve accessibility.
The PicoScope 6 Automotive software in conjunction with masks and alarms provided the perfect hands-free tool to capture and warn us of “make and break” connections while we wiggle tested the wiring harness.
The Mask feature in PicoScope can be likened to a waveform “trap”, where the user specifies an area of the graph where they are notified (with an alarm) if the waveform encroaches or touches the mask.
In the following screenshot, I created a mask around Channel A at SME pin 1, which allowed approximately 2.8 V deviation from 0 V (-1 V + 1.8 V) before the waveform/signal touched the mask and PicoScope sounded the alarm.
The plan was to wiggle test the wiring harness from the front to the rear of the vehicle in an attempt to momentarily “fix” the break in circuit 30 C. If we were successful, the voltage should increase from 0 V at the SME to 12 V and the mask would capture the event and set off the alarm.
Unfortunately, no amount of harness wiggling resulted in a mask failure! However, the mask failure in the capture above occurred while we were raising the vehicle on the ramp! My initial thoughts were that flexing of the vehicle had resulted in a momentary connection of circuit 30 C.
Looking at the zoomed section on the right-hand side of the screenshot above, we had captured a disturbance in the electromagnetic field (EMF) emitted by the contactors of the ramp when raising the vehicle!
This event was also captured by Channel B at the High-voltage safety connector but with reduced amplitude.
This was further proof that circuit 30 C was open and behaving like an antenna. It received the disturbance of the EMF from the ramp contactors, because the cable was not terminated but open to atmosphere.
Where to now?
When non-intrusive techniques failed us, we had to go old-school and perform continuity testing of the wiring harness. But where should we start? Looking at the wiring diagram, X45*1 was a sealed joint located at the rear on the left-hand side of the chassis and it served as a possible failure point. When we cut into the harness at this location, we confirmed the sealed joint to be in perfect condition so we needed to make further incisions.
After five strategic cuts into the wiring harness, we located the open circuit approximately 50 cm away from the High-voltage safety connector, directly beneath the 12 V battery carrier.
The following video describes the procedure we followed:
With the wiring harness repaired, the fault codes erased and the trims refitted, we could charge the vehicle via an EVSE point and drive it as expected. The screenshot below shows the power up stage to Ready mode after locking and unlocking the vehicle. Channel D captured the current flow to and from the 12 V battery, Channels E and F captured the same events from the HV battery.
None required, purely a harness repair.
It never ceases to amaze me how faults occur and how they also change in nature with an intrusion. The customer confirmed in the interview that there had been no previous issues with the vehicle “one day this vehicle was fine, the next day it would not power up!” How does a wire go open circuit overnight?
Once the crash circuit (30 C) had been triggered the vehicle could not power up to Ready mode until a stable 12 V had been re-established. This brings me to the different fault conditions of 30 C during Day 1 and 2 of the diagnosis.
On day 1, the Terminal 30 C voltage failed “intermittently and while under load”, but on day 2, there was no voltage present because the vehicle had been dragged off the ramp and relocated to a workshop bay. My theory here is the movement and flexing of the vehicle/harness resulted in a total failure of the “weak” crash wire beneath the 12 V battery carrier.
Regarding the location of the broken wire, during the Mask test, note how the amplitudes differ between Channels A and C (SME and OBC) and Channel B at the High-voltage safety connector. Could this be a clue to the location of the fault as the SME open wire (30 C ) was as long as the vehicle itself, hence lending itself to improved reception of EMF by comparison to the 50 cm length of open wire from the High-voltage safety connector?
When reading through the case study and thinking about the time spent to diagnose the vehicle (20 hours including research) I started to think about how such labour content is charged and who should pay for it. Without any manufacturer warranty, the cost falls to the customer and 20 hours at £100 per (as an example) is £2000 for labour! (Parts Darts is even more expensive).
Aftermarket extended warranties will not pay for diagnosis, the repairing garage most certainly should not pay (only benefit) which leaves the tab with the customer.
Having been fortunate enough to travel with Pico and learn how other countries manage such diagnostic situations, vehicle insurance policies often include diagnostic support. Vehicles such as those above where the diagnosis has the potential to “run out of control” are commandeered by the insurance company who will then hand the challenge onto a “centre of excellence” or write the vehicle off as beyond economic repair.
I am not sure if such policies are in place here in the UK, but if nothing else, its food for thought and worthy of discussion.
Many thanks to Steve and Jane Winn at Steve Winn Autocare Ltd and to Pete Melville of HEVRA for their support and shoulder to cry on during this case.
March 19 2021
Good read thanks for taking the time to post this one Steve. Interesting antenna length theory. I might try and replicate something like that when next tackling an open. Although in my experience the open is always at the furthest point from where you start looking.