Voltage spikes - bad alternator regulator
Our E63 645Ci (2004) ran into problems at about 60k miles, with intermittent module shutdowns (gearbox, DSC etc in limp-home mode). Fault codes pointed to voltage irregularities caused by a bad alternator regulator.
The whole alternator was replaced with what I believed was a new unit, the cost covered by an aftermarket warranty. The same symptoms reappeared within a week - the garage who fitted the alternator swore 'we've never had one back' - so an array of other components were investigated but proved faultless.
I hooked up a C110 BMW scanner which has a handy graphic interface showing alternator voltage in real time. While driving the spikes were clear as day, brief intermittent over-voltages but enough to cause modules to trip out.
The alternator was then pulled and tested, immediately showing a wildly fluctuating output on the bench; the regulator was kaput, and the unit not new but 'reconditioned'. The tester said the regulator itself was old, so not much of a recon at that.
The fault was possibly lessened by the intervention of the ECU when the alternator was on the car although I have no proof of this other than the voltage went wild during the bench test.
A new alternator has now been fitted and the problem is gone. I would recommend the C110 scanner, by the way, a comprehensive range of diagnostic outputs for less than £40.
Last edited by Queasy Driver; Today at 10:46 AM.