Voltmeter 14 to 12?

[Rubba]

Hello everyone,
My friend owns a B5 A4 and when he starts up the car the Voltmeter is on 14V but when the car is warm at 90C the Voltage is down to 13 or 13.5 and when that happens the engine seems to be missing some power and the car may stall at 12V and turn off, Especially when turning on the AC.
But if the car was driven cold 10-60C, it won't have any problems.
We suspect that it maybe the alternator or something related to battery but we can't confirm.
Any suggestions?

PS: we tried to turn on the engine and disconnect the battery to check if it was the alternator but the car didn't stall.

[Seerlah]

Are the contacts on your battery terminals clean? Also check your ground and wires on alternator.

[Rubba]

Are the contacts on your battery terminals clean? Also check your ground and wires on alternator.

Will try that in morning and feed back, because it's almost 1 am here :p

[Thumper3]

PS: we tried to turn on the engine and disconnect the battery to check if it was the alternator but the car didn't stall.

If the car ran after the battery was disconnected (and if it starts without needing a jump after driving it LOL), it's not the alternator.

Likely sounds like there may be a couple issues possibly. The car shouldn't stall from dropping to 12v. The fact that it stalls out when AC is turned on may point to an issue with the system recognizing the AC is on and bumping up the idle speed to compensate for the added load.

Same thing, when warmer the system is changing idle/fuels/etc, so something else is going on to when that extra RPMs/etc are reduced the car can't handle it.

Check all the battery terminals but it sounds like something else going on to me......

[elduderino200]

As a side note, I was under the impression it could be VERY bad to disconnect the battery while the car was running. Luckily for Rubba I think you know right then if you messed something up so it sounds like everything went ok this time.

[Rubba]

If the car ran after the battery was disconnected (and if it starts without needing a jump after driving it LOL), it's not the alternator.

Likely sounds like there may be a couple issues possibly. The car shouldn't stall from dropping to 12v. The fact that it stalls out when AC is turned on may point to an issue with the system recognizing the AC is on and bumping up the idle speed to compensate for the added load.

Same thing, when warmer the system is changing idle/fuels/etc, so something else is going on to when that extra RPMs/etc are reduced the car can't handle it.

Check all the battery terminals but it sounds like something else going on to me......

As a side note, I was under the impression it could be VERY bad to disconnect the battery while the car was running. Luckily for Rubba I think you know right then if you messed something up so it sounds like everything went ok this time.

Seems to be a non battery related, i checked every wire.
will see if it's something to do with ecu

[Thumper3]

As a side note, I was under the impression it could be VERY bad to disconnect the battery while the car was running. Luckily for Rubba I think you know right then if you messed something up so it sounds like everything went ok this time.

It's an old troubleshooting trick from the 60s/70s. It is not highly recommended on modern cars with minders and 17 million ECUs no. Easier just to go to O'Reilly and have the system tested for free.

It could be ECU related, like I said, it seems to be tied to the AC and the temp of the car, which are all things that the system changes fueling and timing, etc. So something is being changed wrong or not at all somewhere.