I have an 83 w123 300DT and a strange acceleration problem that recently developed. When accelerating at 3/4 throttle there is an intermittent surge or pulsation that feels like the transmission is thinking about downshifting. The pulsation cycles are in harmony with a faint clicking, like that of a relay. I pulled the HVAC controls, shifter surround and glove box trying to find the noise, or associated relay, but it seems as though it may be coming from under the shifter. I tried disconnecting the kickdown solenoid at the pedal, but the pulsation and faint clicking continued. Thanks for your thoughts.

Have you checked your fuel filters and your primer pump? I have had that same "surging" and it was my primer pump. The clicking might be your cruise control.

Have you checked your flex disks?

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Since it is so simple to do. Check your transmission oil level properly. These cars can display some of your symptoms like surging with low transmission fluid levels. It does not account for the strange noise but check it anyways.

Ditto on the fluid. The clicking may be from your shifter linkage bushings being worn/missing.