A very interesting idea.

The trouble is that limp mode is different depending on the model. In the mid-80's cars (380/560SL), limp mode just means that lamba mixture control and maybe electronic idle are shut off, along with ABS, but I am not aware of a restriction on speed.

On the newer cars, you might be able to trigger an OBD fault that would accomplish your objective.

Suggest posting this on the main tech forum.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe