fast idle on 1990 Mercedes-Benz 560SEC

560 sec when i start it up it runs at a very high speed and the abs light comes on at the same time when it decides to idle down the abs light will go off too i dont understand what the brake system has to do with the idle 2200 rpm

This idle and abs problem occuring simultaneously is often caused by an over voltage relay failure, or, the wiring / connections from the fused output from the relay to the idle control module and the abs system. Start by testing the easily accessible over voltage relay on the firewall near the brake booster. If output is good at the relay during too high idle, test for battery at the idle control module which is under the passenger front floor carpeting behind a large plastic cover.
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Possibly either the vacuum line to the power brake booster is leaking or the brake booster itself is leaking vacuum and causing the high RPM's due to vacuum leak and possibly triggering the ABS light to come on, have a shop check this
also check the fuse on top of the over voltage relay - usually under a red tinted plastic cover - relay part number is 201 540 37 45 or 201 540 38 45 - I have seen both. Is your idle smooth or rough?
Just got my 89 Mercedes 560SEL yesterday. It idles at 800 rpm when cold, 1800 rpm when warm. ABS light is on, an it idles a little rough, sounds like a vacuum leak. It has 128000 miles. Thanks!
What did you find out on the 89 Mercedes 560SEL idle problem