* Need a little more information. Is your '89 300SEL (W126.025) a California or Federal car? Look at the plate/decal under the hood next to the radiator. I think black is Federal and green is California.
* You read the code 4 from the rectangular 8-pin data plug (X11/4) over by the battery. Which pin were you using (i.e., which system is the fault code in)? Fortunately, on an '89 you don't have many systems to choose from.
* Before you try to check duty cycle (sweep) at the round diagnostic plug on the left fender well (X11), check for a malfunction with the on-off ratio. Using pins 2 and 3 turn the ignition on, leave the (warmed up) engine off, and note the reading. Compare to an on-off ratio malfunction table.
* After you've recorded all the codes, it's a good idea to reset all the codes (disconnect the battery briefly), and use the car through several drive cycles. Then retest for fault codes. Some of the old codes may not reappear.
* These tests assume a few conditions. Engine at operating temperature, over-voltage protection relay and fuse in good shape, and climate control off.
* This might help you a little. I would recommend doing a search for basic theory of lambda, duty cycle, on-off ratio, Mercedes on-board diagnostics, both analog and digital. As long a task as it sounds, it might actually be quicker than picking through dozens of posts for information that's useful.

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536