Gavvygavgav

Hi,

Been having serious trouble lately. The iDrive screen has been flicking off, accompanied by error messages such as Braking/Driving Stab warnings, Transmission fault resulting in being stuck in whatever gear I'm in, ABS faults etc. all the way to the engine cutting out. Mechanic replaced the alternator, it drove fine for a week or so. Then the problems recurred. He then replaced the alternator again last week, at no charge to me this time. And now the problems seem to be happening again!! My head is melted. I've gone into the hidden OBC menu and checked the voltage - with engine off it sits at 12v, but when I turn on the engine instead of jumping to 14v it drops to about 11.8v. When I turn on the headlights, AC etc. it drops further down.

Having had the alternator replaced twice I can't see this as being the problem. The mechanic said that the voltage regulator was part of the replacement so I can't see the VR as being the problem either. The battery was replaced last year so, unless that was defective, I don't know if that is the issue either.

Does anyone have any pointers before I bring it to a specialist who will look to maximise his return on my desperation/lack of knowledge?

Thanks!

Gavvygavgav

Update: after I posted the initial post I accessed the OBC again and took it on a drive, when I turned the ignition the voltage jumped to 14v and stayed there or thereabouts for the whole trip. Great! However, when I turned on the ignition for the return trip it went back to 11.4-11.6v and stayed that way. What could cause such intermittent voltage variations?

BimmerFan52

If you are measuring the voltage accurately, then the problem is almost assuredly the alternator voltage regulator.

How are you measuring the voltage? If you are measuring the voltage with a plug-in cigarette lighter socket style volt meter, then make sure it is plugged in all the way and that it is accurate. I originally bought a cheap one that was inaccurate when compared to measuring the voltage under the hood at the jump terminal and ground lug on the driver's side (left side US) strut tower with a digital multimeter.

You can also take the car to most auto parts stores who will check the alternator and battery for free.

Sounds like your mechanic needs to find another source for rebuilt alternators.

Gavvygavgav

Hi Bimmerfan52,

I was measuring the voltage via the hidden OBC display while driving. Have driven it a few more times checking the voltage as I go. Most of the times it stayed at 14v-14.3v. But on a couple of occasions it would start fine for a minute or so and then the voltage would drop to below 12v, and would drop further if I turned on headlights, wipers, radio etc. down as afar as 11.3v. I have recreated the situation where the warning indicators come on and the transmission packs in yet.

This is the strange part, why would it be so intermittent? Is it a wire or a connection somewhere? The mechanic told me that the voltage regulator was replaced twice along with the alternator on both occasions? Most frustrating.

What are the symptoms of a faulty IBS cable?

BimmerFan52

I would think that the system voltage measured by the hidden menu display is accurate, but you might check the accuracy with a multimeter at the jump terminal and ground lug in the engine bay. You might also try a plug-in voltmeter to monitor system voltage while moving. The system voltage should not be varying that much, and you can almost be guaranteed that the voltage regulator is failing.

The system voltage is set by alternator current output . If more accessories are turned on the alternator increases its current output to keep the system voltage stabilized. As accessories are turned off current output is reduced to maintain system voltage.

When my alternator (voltage regulator) failed, the voltage would ramp up on startup to 17V. Sensing the system overvoltage or undervoltage, the ECM and CAS can shut down the terminal 30g relay, which powers the dynamic drive system, transmission control module and a number of other systems, generating a dash full of warning lights. After turning the engine off and then restarting it, voltage would stabilize somewhat, but during the drive bounce between 11.5V to 14V. This intermittent output is a fairly common characteristic of a failing voltage regulator, causing them to sometimes be hard to diagnose.

The function of the IBS is to gather data regarding battery charge level, net current flow into the battery and the negative terminal temperature (to insure the battery acid is not getting too hot, which can lead to excessive loss of water through gassing). A common failure mode of early E60 IBS was damage from moisture, which could cause the IBS to measure a false-positive excessive battery discharge and wake up the CAS to have it shut down the terminal 30g relay in order to shed load (while the car is not moving). The wake up function is designed to happen only once, but the damaged IBS can repeatedly wake up the computers, causing an excessive parasitic draw from the battery. It will repeatedly run down the battery, especially if the car sits a few days. But this type of failure can’t dramatically affect the operating system voltage which is primarily set by the alternator output. If the IBS is removed or dies completely and is not sensed by the ECM, the ECM will set a fixed 14.2V system voltage target.

I see you are in Dublin. Not sure if that is Ireland or Ohio. In the US all auto parts stores will test your battery and alternator for free with a hand-held computer.