mjz3348

Recently my one year old ML400 engine idle would climb 200 rpm to 800 rpm when the AC is turned on and then drops back to 600 RPM when AC is turned off. It never did this before - the engine would aways idle at the same rpm regardless of AC action.

The extra 200 rpm is annoying in that the car lurches into gear and provides a rougher launch.

Is there a way to change the setting?

Koenig1

If I understand your problem, it would be the same as the old USA carbed V8's, which were equipped with a solenoid to up the idle when AC is on. Going with that thought, my guess is there might be something off with your fuel management.... Your computer is reading something is lean(example), and is upping the rpm's to compensate! Maybe a visit to your dealer to discuss/resolve.

mjz3348

I called the dealer and they don't know if this is "normal" or not - sort of like the old days with carbs (that I remember all too well). It's just that on the new cars, the computers do the job keeping the engine idling properly regardless of load. That's why I was surprised when this started to happen. For all I know it could be a battery thing where the car needs the extra rpm to manage the load. There are two batteries on this car and perhaps it's the short hops my wife does with this car that requires more engine rpm at idle. Interestingly - the rpm did drop back down again after a longer trip with the AC on.