Avopaul

Got a q5 battery light /message came up on the dash rac came out said the alternator wasn’t charging so needed to be replaced took it off had it checked and it came back as working fine had it reconditioned anyway so as new condition again battery is a couple of years old but tests ok and is coded to the car but I’m still only getting 13.1 volts any ideas what it could be as I’m now lost thanks for any reply’s

SMac770

The J533 is the control module. It has two slave devices on a LIN bus to manage the charging system. The voltage regulator (VR) at the alternator and the battery diagnostic manager (BDM) at the battery negative post. The BDM directly measures the battery voltage and reports that back to the J533.

I'd start with the vehicle running. Use VCDS to look at the voltage values in blocks 14 and 18 of the J533. Block 14 will show you the voltage the J533 is commanding the VR to output. Block 18 will show you the voltage at the battery that the BDM is reporting. Then use a voltmeter to measure the voltage between the alternator B+ output post and the alternator housing. Then measure the voltage between the jump start posts under the hood. Then measure the voltage across the battery posts. While you're at it, take the cover off the passenger end of the dash and measure the voltage at a fuse in the brown and black columns (use the test contact pad on the top of the fuse for the + probe, find a good metal bolt for the - probe).

Between the five measurements and two scan tool values, there should be a hint to where your issue lies.