Just to re-state the problem. The car starts and idles at about 1500 rpm, hot or cold. Then at some random time, it will drop to 650-700. It will almost die when the AC compressor engages or the steering wheel is turned.

Today's update:

The DTC 17 from the ignition module turned out to be my bad. This needs to be checked with the engine at idle. Thanks to Bob Terry for pointing that out. So did that, and no DTC.

Yesterday, I re-checked all continuity and operation of the various sensors and switches that connect to the ECU, along with the operation of the idle and decel switches, all of which were OK. Also checked all power and ground to the ECU - all good.

So then I farted around with the throttle linkage. If you carefully move the linkage with the air cleaner removed, you can see that there is maybe an inch of movement in the cable before the throttle plate actually moves and cancels the "idle" signal to the ECU. But the way the linkage is arranged, the decel microswitch releases immediately.

I discovered that if I moved the linkage just enough to release the decel switch but not move the throttle, the idle would drop. After a bit of experimenting with the accelerator pedal, I was able to start the car like this. When I did this, the car would start and idle at 650-700.

So something about both switches closed is telling the ECU to raise the idle after starting. When the decel alone opens, the ECU decides to cut all power to the idle valve. David Poole tells me that this is a "limp" mode and suggests there is a rev limiter not working correctly.

Remember, doing this with two different ECUs.

His suggestion is to check the O2 sensor output to make sure that the lambda reading reflects the mixture, and to check the engine and chassis grounds.

Anybody got anything else?

__________________
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