Is the transmission pressure switch adjustable?

[jedwards]

Gentlemen,
My 250SL is running really nicely after so much help from this forum, however there is one area I would like to improve.

My issue is that the pressure necessary to activate the transmission pressure switch and therefore the Constant Speed Solenoid, or Fast Idle Solenoid is set a little too low to provide a smooth transition when coming to a stop.
My  revs drop to round 500 rpm before the switch activates and Solenoid cuts in and pushes it back up to 750.  As a result, every stop results in RPM falling then bing pushed back up again. It is just not as smooth as I am sure Mercedes intended.

The pressure switch (if I have the right component) seems to have a knurled adjustment knob at one end.  Mine is not keen to move, although I have not tried to force it, so is this an adjustment? 

Does this sound right or am I barking up the wrong tree?

[WRe]

Hi,
have you read this: www.sl113.org/wiki/Automatic/Start ?

Corections may be:
1.) Venturi switch changes automatic transmission modulator pressure via 3 position solenoid.
Check if the lamp turns off at 1200 rpm.
Rough downshifting into the 2nd gear: Increase stepwise to 1400 rpm.

2.) Check 3 way solenoid if it works correct (through opening on the tunnel).

3.) Check modulation pressure
 - Basic pressure w/o vacuum 0,6 +/- 0,05 bar
 - 2,9 bar above 1200 rpm
 - 4,6 +/- 0,2 with kick-down

....WRe

[jedwards]

Thanks WRe
A little confusion here I believe.

I'm not referring to the 3 position solenoid that alters the transmission's change characteristics and kick-down by changing modulator pressure.  I am referring to transmission's low pressure switch that activates and deactivates the Constant Speed Solenoid that moves the throttle linkage to maintain engine RPM at idle.

[Larry & Norma]

There is no adjustment on these switches. They can corrode which may
cause problems.

[WRe]

Hi,
my point was: Before you start to correct the pressure (see picture, pressure switch is fix), which has to be handled with care, you should control points 1 and 2.
...WRe

see www.pagodentreff.de/diskussionsforum/t10466-elektroanschluss-automatic-230sl-4.html

[stickandrudderman]

The switches are not adjustable.
If you have problems with EITHER fwd or reverse activation of the idle speed solenoid, then try swapping the pressure switches left to right to see if the problem follows the switch. If so then you have a failed switch and it needs to be replaced. If the problem persists for the same gear selection then you either have a wiring issue or low pressure in the transmission.
I'm sure you know that the switch is only a ground and you can easily test the wiring by grounding the wire directly, either at the switch (undo the knurled knob, remove the wire and touch it to earth to see if the solenoid activates) (other required conditions being met of course) or you can disconnect the wire to the switch at the solenoid and do it that way.
Pressure switches were NLA for a while but they appear to be available again as I just bought two last week but they are very expensive.
I do have in mind an alternative that is much cheaper but will not satisfy the originalists.

[jedwards]

Thanks Colin,
That’s really helpful advice.  Being able to fine tune the transition from drive to stationary might  be handy, but as you say, the originalists may have a coronary.