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Old 05-13-2019, 11:40 PM
Joe
 
Join Date: May 2019
Location: Connecticut
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280SL downshift 'clunk' when slowing to a stop

My '69 SL automatic makes a rear end clunk sound when I slow down to a stop...it's much less after I adjusted a switch at the air flow butterfly near the air cleaner...I read there is an adjustment that is often mistaken for an idle screw that is used only for adjusting the shiftdown switch to the tranny...I had a hell of a time readjusting all the linkages and timing to get a smooth idle again and good performance, but there is still some clunking going on. So my question is, could there still be a trans adjustment required, or could I have too much rear axle lash now? I've heard this is a common thing on early MB automatics...
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Old 05-14-2019, 08:23 AM
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Quote:
Originally Posted by PreferVintage View Post
My '69 SL automatic makes a rear end clunk sound when I slow down to a stop...it's much less after I adjusted a switch at the air flow butterfly near the air cleaner...I read there is an adjustment that is often mistaken for an idle screw that is used only for adjusting the shiftdown switch to the tranny...I had a hell of a time readjusting all the linkages and timing to get a smooth idle again and good performance, but there is still some clunking going on. So my question is, could there still be a trans adjustment required, or could I have too much rear axle lash now? I've heard this is a common thing on early MB automatics...
Sorry to mention the FSM again - but the throttle linkages need to be measured and adjusted as specified in the book.


It is also worth checking all of the resilient drive line bushings and engine, gearbox, subframe / differential mounts for excessive play as well
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Old 05-14-2019, 10:28 AM
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There are a couple of bushings on the rear axle that can make noise but I would be looking at your kick down solenoid on top of your transmission. This solenoid controls modulator pressure and the control rod attached to it needs to be able to move freely.
Without starting your engine, turn your ignition on and then press your gas pedal right to the floor. You should hear a loud click as the linkage is pushed forward. If you don't hear anything check to see if you have power going to the switch. If you have power and the linkage isn't moving the solenoid is probably seized up. It can be removed and freed up.
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Old 05-14-2019, 01:13 PM
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This sounds exactly like what my roommate's 1965 W111 220S does when slowing to a stop. It was converted from Zenith to Weber carburetors, and the Weber carburetors don't have a space for a throttle position sensor, which unlocks the transmission so it doesn't clunk when slowing to a stop.

All the automatic cars that were converted to Webers have this problem. In your case, it may just be that the throttle position sensor went bad. It attaches to one carburetor by the linkage and the wires go down to the transmission.
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