AC fan runs too fast when blower speed is on full . Then the RPM begins to fluctuate a bit only on idle . Soon after the vehicle turns off all electronic consumers incl. ABS /ESP on its own . It has even displayed a red battery light once. This only happens with AC on . When the AC is off the car is perfect . Mech says to replace alternator as the voltage reaches 18V when this tends to occur . Otherwise the voltage reads 13-14V . Battery is 6mnths old . Any views would be highly appreciated!

In my opinion if the alternator was tested and found to be putting out 18v then I would replace the alternator, or replace the voltage regulator on the alternator which will be less expensive.

Your situation points strongly to an overcharging scenario that's commonly linked to a faulty alternator voltage regulator. When the voltage spikes to 18V, it can cause the electrical system to go into a protective mode, hence shutting down non-essential electronic consumers like ABS and ESP. This excessive voltage not only risks damaging these components but also leads to erratic behaviors such as RPM fluctuation and enhanced blower fan speed, which might be the HVAC system responding to electrical anomalies. While seemingly unrelated, an unlikely but potential cause could be a poor ground connection somewhere in the engine bay. A poor ground could lead to inconsistent feedback to the alternator, causing it to overcharge intermittently, especially under load from the AC.

To address this, I'd recommend checking fault codes from the Body Control Module and the Powertrain Control Module. Each may have recorded overvoltage events or communication issues. For thorough diagnostics, utilizing a full system scanner like youcanic can access the full breadth of control modules and offer a holistic view of what’s happening internally, which a basic scanner like the Autel MaxiScan MS300 might miss. Have you inspected or cleaned the main power-ground paths to ensure there's no corrosion or looseness that could exacerbate voltage irregularities?

Hey @jimyj

thanks so much for your input. Yes I got the car scanned using both Autel and Thinkcar . And it did record over voltage as well as undervoltage in many components . Will be getting the alternator changed at the earliest .
Any idea why this happens only with AC on ?

hey @TimC300 thanks so much for your input . Will get this checked A few fault codes that were recorded are as follows .

Hey guys ,
one technician also suggested this issue could be related to a bad ECM . How do I know if it’s the ECM or the alternator that’s playing up !?

Since this is only ac related i would suspect maybe a short circuit in the ac clutch circuit or the clutch itself. This is possible that it’s confusing the voltage reading and causing the alternator to act out. Just my opinion and something else to look at.