250 SL Slipping Gear when A/C is on

[Challenger_7]

Hi all
I bet this is a new one.
My gear starts to slip seconds after I switch on the AC.
This was happening before I had the gearbox completely renewed where I found many discs worn out and seals hardened etc, and I was sure that the problem would go, but it stayed.
I checked battery status when AC is on, and indeed the charging does go down a bit to about 12.8 v when in motion and a bit lower if on idle to about 12 or 12.2 v.

Any suggestions as to how to cure this problem?
Thanks for your contribution.

[Shvegel]

As someone who used to teach electrical diagnosis to BMW mechanics when there is an odd problem it usually originates with the vehicle grounds.  check the fat ground wire that goes between the chassis and the engine at the back of the engine compartment.  Also Check the main battery negative cable which should go from the battery to the chassis then on to the engine.

[Challenger_7]

Thanks much appreciated.
I did check one but not the other, will do

Cheers

[ja17]

Sounds like you could have a low voltage problem. Check to make sure your alternator and regulator are performing up to specs. The extra current drain from the AC might be causing the three position solenoid on the transmission to mal-function. The three position solenoid must increase the modulator pressure when the pedal is depressed, or slipping can occur.

[Shvegel]

Alternator belt is piggybacked off the AC belts.  If the AC belts are slipping the voltage output of the alternator might be dropping.