From the "Model Year 1990 Model 129 Introduction into Service"

#1
Should there be an open or short circuit of the idle speed air valve, or an interruption in the data exchange between the CIS-E and the EZL/AKR units, the power to the idle speed air valve in shut off. At that point the ignition timing is extremely retarded, so the idle speed increases only insignificantly.

#2
If the idle speed contact is closed and there is a malfunction in the idle speed air valve (increased idle RPM), a fixed operating mode will be chosen by the EZL/AKR unit. Thus the most retarded ignition timing possible will occur, preventing an increase in idle speed.
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Based on this and the experience with the car, I think #2 is happening. But I am at a loss explain why. The car is shooting itself in the foot. The ECU is causing the increase in idle speed by opening the idle valve. Then it reaches a point where it notices that the idle speed is too high. So then it kills the power to the valve. WHY?

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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