Automatic Gear Box on my 1969 280 SL

[Michael Siebel]

Hi fellow members, My name is Mike Siebel from Melbourne Australia, and I am a relatively new member , having acquired my car last October. I have had a major head refurbishment done on and a new radiator amongst other repairs, and the car drives well except for a very frustrating problem with the automatic gear box. I need to reach around 65 KMH before it will engage the fourth gear, and then, once it has selected 4th gear, any slight acceleration immediately changes down back to 3rd gear. I have been told by the 3 garages in Melbourne including two who are authorised Mercedes repairers, that the fault is due to the fact that I am missing the Throttle Position Switch located on the Throttle body , which sends a message to the double acting solenoid  attached to the Transmission.  For some reason, this switch has been removed but the Throttle Body still has the flange fitting for this part , however the spindle which houses the Throttle butterfly appears to have been modified and/ or cut off.
Firstly, I am having lots of problems in obtaining this part but just yesterday, a fellow member in Australia has informed me that the advice I have been given from the garages is incorrect and the Throtte Position Switch is NOT the cause of my problem as it has a different function !
I shall be most grateful for any advice that can assist me with this query
Thanks

[ja17]

The switch is critical to the smooth shifting of the transmission.  It may or may not be the late shifting problem you have. It depends what they did with the wires from the switch. If they connected the wires together, the tranmission might shift too early or slip. Left apart, the transmission would downshift abruptly and shifting could be late.

Yes, find the parts you need, and read up on the "venturi switch" and "the linkage tour".  I am sure there are members on this site who have all the parts  you need.

Try finding the wires that went to the switch. If they are wired together, separate them and go for a drive.

Keep up up to date!

[Michael Siebel]

Hi Joe, sincere thanks for your valuable input. Yes Joe the two wires coming from the transmission have been taped, but are NOT connected.The mechanic has also tested the double acting solenoid, and this is working. So you then also believe, as the garages have told me, that the missing Venturi Switch IS the probable cause of the problem. I am having heaps of problems though in locating a suitable solution as I would probably require a Throttle body as well because at the flange where the switch connects , the throttle butterfly spindle is flush with this flange connection, so how will the switch connect ? Maybe I could get a new spindle? do you know how this works ?
Many thanks Joe
Kind regards, Mike

[ja17]

Hello Mike,

With the wires disconnected and apart, the solenoid and transmission linkage return to the middle position, which is between idle and kickdown. The transmission linkage is in the basic driving position and the up and down shifts are regulated mostly by engine vacuum in this position.   At little or partial throttle your transmission should be shifting up at around 45 kph. even with the venturi switch disconnected or missing as long as the wires are apart and not taped together.  Try taping the two wires togethe as a test. This is the idle position and the shifting should occur earlie sincer the modulator pressure will be lowered. If slipping occurs, untape the wires again.  

You should also check the engine ignition timing at 3,000 rpms. It should be around 30 BTDC. Engine timing will effect engine vacuum drastically. As a consequence the tranmission vacuum modulator will respond to incorrect engine vacuum with bad shifting characteristics.

My thoughts are...........your transmission will never shift correctly with the switch disconnected and missing. However, it appears that there may be another issue causing the severe late shifts. The other causes could be engine vacuume, incorrectly adjusted tranmission vacuume modulator, incorrectly adjusted tranmission linkage, or an internal tranmission issue.

There is a removable access panel on the inside of your car on the right side of the transmission tunnel. It removes to expose the tranmission modulator linkage which is adjustable. Maybe you could take a look to see if the linkage has been tampered with?

The venturi and switch on your engine is the same as many MB fuel injected six cyl.sedns  of the era. These parts are quite common, used, here in the US. Many on  this group would be happy to sell you what you need. Shipping to AU may be a little pricey.

Try the test first and get bvack with us.

Good luck,
Joe Alexander (ja17)
Blacklick, Ohio

[Michael Siebel]

Dear Joe , Thank you very much for your valuable information.I have been recommended to a mechanic whom I am told is quite an expert on this motor and I shall pass on this information, it all makes very good sense, and thanks again sincerely for the time you have taken.I shall let you know how I go with this.
Regards
Mike