transmission jumps when ac on

[rmada]

Just got the ac unit working on my 1969 280sl.  However, when I turn it on the transmission jumps to lower gear and back and forth.  When I turn ac off, transmission goes back to behaving normal.  Any help appreciated...this is my first post.

[Peter]

Interesting issue.

I assume you have a 280SL with automatic and the solenoid to adjust the idle speed.

1. To be checked the electrical wiring of the air-conditioning.
    Two wire go/come from the “airco-box” under the dash:
     1.The current (+) from the fuse box, via an extra fuse. Only current when car is turned on.
     2.The wire to the compressor to switch the compressor on and off
Is this correct connected? Does this function as it should – fan in position 1, 2 and 3? Does the thermostat switch the compressor on/off when it should?

2. The solenoid is functioning well? This means that the idle speed is 750/800 and when you put the gear in 2,
    foot on the brake, the idle speed adjust back to 750/800.

^Peter

[rmada]

Thanks Peter but I believe I was wrong and the problem is transmission fluid low.  Now if I can find a way to get down to the fill stick without making a mess.  Thanks again

[ja17]

Sounds like the switch on the intake manifold needs adjusted. It probably is to sensitive to linkage change and is cycling on and off, raising and lowering the transmission modulator pressure. Check the adjustment and make sure it switches off around 1200 rpms. Loosen the two mounting allen screws and rotate it forward (clockwise) to adjust the switch to break contact at higher rpms (1200) if needed.