Hard Shifting and Venturi Switch

[Harry]

My 1966 230SL has been displaying a rather harsh downshift from 3rd to 2nd, sometimes quite harsh.  I serached this forum and read about the switch located on the venturi opposite the throttle linkage and decided to take a look.  On inspection, I really wasn't convinced that this might be the problem because if it was out of adjustment, it appeared to be imperceptible.  (The throttle plate in the venturi sure seemed to be fully closing.)  I decided to run through the procedure to properly set the adjustment screw on the venturi.  Then I checked continuity of the switch, expecting it to break the circuit at some point in the opening of the throttle  That never happened.  So, I put an ohm meter across the switch and watched as the throttle was opened.  It did show a progressive change in resistance - so I assumed the switch was likely good.

The test drive was remarkable.  The downshift from 3rd to 2nd was corrected and n hardly perceptible.  MUCH better.  I have to say, it sure seems that any adjustments to the position of the throttle had to have been very small, but obviously big enough to make a difference.  I post this only to tell others w this problem that this is absolutely worth checking/setting.  And - I'm still somewhat in the dark on this switch.  Based on what I have read, I would have expected it to basically function as on/off.

Thanks again to the forum for valuable information,
Harry

[stickandrudderman]

The switch is just that, a "switch". It is not a throttle pot (potentiometer).

[glenn]

Ah yes, the venturi s/w, 3 way tranny solenoid, etc.  We still need the correct wiring diagram and written description of the correct operation.

[wwheeler]

As Stick said, this is only a switch. It is either on or off and nothing in the middle.

The 3-way solenoid has three positions. low pressure, med pressure and high pressure.
- Low pressure position (idle) is achived when the venturi switch is closed (throttle at idle position) and is sending current to the solenoid.
- High pressure position (kickdown) is achived when the kick down switch is closed (throttle wide open) and is sending current to the solenoid.
- Medium pressure (normal driving) is achived when the throttle is off idle and not wide open. The solenoid receives no current and is automatically moved to the neutral postion by springs.

That is how it works.

[Harry]

Great feedback - thanks.  I guess I didn't have good contact when checking continuity because I didn't observe "On/Off" behavior across the switch as I moved the throttle from closed to wide open.  That's what I would have expected.  I may check again just for fun.  Either way, this adjustment is highly recommended.

[twistedtree]

Great feedback - thanks.  I guess I didn't have good contact when checking continuity because I didn't observe "On/Off" behavior across the switch as I moved the throttle from closed to wide open.  That's what I would have expected.  I may check again just for fun.  Either way, this adjustment is highly recommended.

To get a good reading, you should disconnect at least one of the wires from the switch.  Otherwise other stuff is part of the circuit and will impact your readings.

[glenn]

See above comment.... We still need the correct wiring diagram and the correct operational description.....