BMW F20 LCI sporadic no crank issue

[Radiantecon]

Greetings everyone,

Recently had an issue with the car failing to start, initially seen a few weeks ago. Due to my lack of initiative, I assumed the battery was not retaining charge and changed it, resulting in a £200 error.


After extricating myself from bed in the evening to do some diagnostics, I believe I have identified a power supply problem.

Here is a summary of the symptoms I am experiencing.

The car began the other day, remained idle overnight, and successfully started in the morning. After a brief drive, it failed to start five minutes after being turned off, with the battery reading 12.45 volts.

The vehicle begins operation with support from an auxiliary battery via the front jump terminals.
The alternator measures 14.5 volts. Current drain when the vehicle is off (not in sleep mode). 70 milliamperes
The push-to-start mechanism produces an audible relay click. The IBS sensor has been removed, still the engine fails to start.

The voltage measurements taken under load and straight from the battery in the trunk yielded the following results:
12.6 volts while turned off, 12.5 volts with ignition on, and 12.2 volts when cranking.

When measured from the front jump ports: 11.6V with ignition on, 0.4V while cranking.

Relocating the battery from the rear compartment to the engine block and trying to initiate the start using jumper cables produced a repeating clicking sound reminiscent of a machine cannon.

I am led to assume that the positive wire from the battery to the engine compartment is defective. Conversely, I believe that the battery connected to the front jumps should have started the automobile.

Any recommendations or additional aspects to evaluate would be highly appreciated.

I like a challenging problem, but this one is overwhelming me.

[Radiantecon]

It may be prudent to include the codes retrieved from the vehicle; I sense that they are likely irrelevant and only indicative of improper power distribution to the automobile.


| Identifier | Current Status | Detailed Description |-------------------------------------------------------------|

| 1A2002 | Intermittent | Electric fan activation: Short circuit to ground | | 1A2004 | Intermittent | Electric fan activation: Open circuit | | 1A200D | Intermittent | Electric fan function: Communication fault | | CD8E12 | Intermittent | LIN message: Alternator missing | | 0DA451 | Intermittent | No message received (actual steering torque driver actuator 10.0.0) |

I conducted a physical test of the electric fan at full AC load. The fan operated well, and I noted that the fan activation percentage on the reader fluctuated in line with the readings.

[Liveek]

There is either a deficient connection from the battery's positive terminal to the jump point or an inadequate ground connection.
At the jump point, where did you connect the multimeter lead to ground?

[Radiantecon]

Measured from the positive terminal to the engine bay grounding point on the left side.

I believe a weak connection between the rear battery and the engine compartment. I also investigated potential grounding issues but only examined the engine to chassis ground strap owing to the onset of darkness and cold weather.

[Liveek]

The issue likely resides in the main positive connection, since the voltage drops significantly under heavy load when cranking. Inspect the area where the cable emerges near the brake servo and attaches to the junction terminal. It is necessary to detach the scuttle trim part.
Have you examined the transfer points in the boot where the positive battery terminal links to the wire leading to the front of the vehicle, as corrosion may be present?

[Radiantecon]

The back cable seems OK; however, the bolted connection inside the battery compartment was rather soiled, which I have since cleaned. The engine bay side seems acceptable. Is there a method to test the cable for potential faults?

[Inspirli]

Ensure the jump point connection is secure.

[Liveek]

Not unless you own a megger, since a multimeter continuity test will not provide conclusive results.
The only action you can take is to do a voltage test at the junction where the cable connects to the jump point; if the voltage continues to decline while cranking, the problem lies further upstream. I have heard of the jump point terminal fracturing before, indicating a deficient connection between the main power wire and the jump point/starter cable.